NASA







2022 Astrophysics Strategic Technology Gaps

Gap Name Description Current State-of-the-Art TRL Performance Goals and Objectives Scientific, Engineering, and/or Programmatic Benefits Applications and Potential Relevant Astrophysics Missions Urgency
Cryogenic Readouts for Large-format Far-IR Detectors Readout schemes including cryogenic multiplexing for arrays of large-format Far-IR detectors need to be developed. Readout schemes using HEMT or SiGe amplifiers and frequency-multiplexed resonant circuits are in development. A few hundred channels per 1 mW HEMT amplifier have been demonstrated. Low power dissipation at 4 K is required. For TES-based detectors a microwave SQUID multiplexer using frequency division, time division or code division multiplexing is needed. Frequency division multiplexing is well advanced and can meet the needs of a far-IR flagship when scaled to 2000 pixels (resonators) per 4 GHz channel. 3 Near-term, this scheme should result in 2000 pixels per amplifier channel (enabling), 3000 pixels/channel (enhancing).HEMT amplifiers from Low Noise Factory can achieve 10 dB with 0.38 mW of dissipation at 4 K. Sensitivity reduces observing times from many hours to a few minutes (≈ 100× faster), while array format increases areal coverage by ×10-100. Overall mapping speed can increase by factors of thousands.
Sensitivity enables measurement of low-surface-brightness debris disks and protogalaxies with an interferometer. This is enabling technology.
Suborbital and ground-based platforms can be used to validate technologies and advance TRL of new detectors.
Far-IR Flagship
Far-IR Probe
FIR detector technology is an enabling aspect of all future FIR mission concepts, and is essential for future progress.
This technology can improve science capability at a fixed cost much more rapidly than larger telescope sizes.
This development serves Astrophysics almost exclusively (with some impact on planetary and Earth studies).
Many synergies exist with similar developments for x-ray microcalorimeters
Required TRL 6 by mission PDR.
Extreme stretch
Single tech
Large-format, High-resolution Focal Plane Arrays The High Definition Imager UVIS-channel and the Multi-Object Spectrograph near-UV channels baseline a 2x3 = 6 mosaic of large format (8k x 8k) Si detectors with small pixels (e.g., <7 µm). High-speed readout over programmable regions-of-interest are desired to enable fine guiding and high-speed science objectives.
High dynnamic range Si devices with CCD and CMOS readouts are commercially available. They require thinning and other backside treatments to achieve UV sensitivity. Customizable bandpasses are possible with direct multilayer filter deposition. CCDs can offer photon counting via Electron Multiplying readout. Smaller pixel format, large format, and high readout rates need to be developed. Commercial off-the-shelf CMOS arrays with small pixels and large formats exist, however, they have not been adopted for spaceflight, nor do they exhibit optimal noise performance for scientific operations. Development challenges include optimizing detector and readout electronics for low noise performance, developing science grade and flight qualified small pixel and large-format detectors in buttable format. Reference C. Updates to Ultraviolet Technology Gaps Relevant to LUVOIR (K. France, U of Colorado): submitted to Program Offices in response to current call for input. 4 Array Format: 8k x 8k, three-side buttable
Pixel Size: < ~7 µm
Read Noise: ~1 e- per read
Dark Current:~1x10-4 e-/pixel/s ~ 200 cnts cm^-2 s^-1 for 7-micron pixel
Operating Temperature: > 150 K
Radiation hard and Solar-Blind a plus
Design and fabricate an 8k x 8k x 6 µm CMOS or CCD detector and associated readout electronics in a three-side-buttable package. Verify detector sensitivity and noise performance. Complete functional, performance, and radiation testing to achieve TRL 5.
Fabricate additional sensors and electronics and integrate into a single focal-plane array. Complete functional, performance, and environmental qualification testing to achieve TRL 6.
(see LUVOIR final report for more detailed plan, schedule, and cost)
Many missions would benefit from large format, high efficiency, high-resolution, low noise focal plane arrays, with small pixels. IR/O/UV Flagship UVIS channel on High Definition Imager (HDI) and NUV channel on Multi-Object Spectrograph (MOS) and any mission with UV science , including planetary and earth observation missions. Per LUVOIR Final Report, TRL 6 prior to Phase A start in 2025 is required. See LUVOIR Final Report for detailed development plan recommendations, schedule, and cost estimate.
Level of complexity: system of techs
Level of difficulty: straightforward (7 micron pixels for CCD might be stretch) xxx Extreme stretch
Large-Format, Low-Noise and Ultralow-Noise Far-IR Direct Detectors The most important technology for the FIR/submillimeter is large-format detectors that operate with high efficiency (≥ 80%), low noise, and relatively fast time constant.
Arrays containing thousands of pixels are needed to take full advantage of spectral information content. Arrays containing tens of thousands of pixels are needed to take full advantage of the focal plane available on a large, cryogenic telescope.
Detector sensitivity is required to achieve background-limited performance, using direct (incoherent) detectors to avoid quantum-limited sensitivity.
Kilopixel arrays at lower sensitivity are at TRL ~5, but demonstrated array architectures are lagging at TRL ~3. (Staghun, 2018)
Sensitive (noise-equivalent power ,NEP, of low 10-19 W/√Hz), fast detectors (TES bolometers, and MKIDs in kilo pixel arrays) are at TRL 3.(Suzuki, 2015), (Baselmans, 2017)
3 Detector format of at least 104 pixels with high fill-factor and sensitivity (NEP) of ~1x10-19 W/√Hz are needed for wide-band photometry. (enabling)
Detector sensitivities with NEP of ≈ 3×10-20 W/√Hz are needed for spectroscopy (enabling), available in a close-packed configuration in at least one direction. NEPs of 3x10-21 W/√Hz would enable background-limited sensitivity(Echternacht, 2018) (enhancing)
The detector system should be scalable to enable ~million-pixel total format (10k~50k pixels per sensor) in a large mission.
Array size of 1x104 is enabling and 5x104 is enhancing
Fast detector time constant (~200 µs) is needed for Fourier-transform spectroscopy.
Sensitivity reduces observing times from many hours to a few minutes (≈ 100× faster), while array format increases areal coverage by ×10-100. Overall mapping speed can increase by factors of thousands.
Sensitivity enables measurement of low-surface-brightness debris disks and protogalaxies with an interferometer. This is enabling technology.
Suborbital and ground-based platforms can be used to validate technologies and advance TRL of new detectors.
Far-IR Flagship
Far-IR Probe
FIR detector technology is an enabling aspect of all future FIR mission concepts, and is essential for future progress.
This technology can improve science capability at a fixed cost much more rapidly than larger telescope sizes.
This development serves Astrophysics almost exclusively (with some impact on planetary and Earth studies).
Need to demonstrate TRL 6 by mission PDR
Single tech
Extreme stretch
Large-Format, Low Darkrate, High Efficiency, Photon-Counting, Solar-blind, Far- and Near-UV Detectors Large-format (200mm)^2, low darkrate < 1 cnt cm^2 s^-1) photon counting detectors that operate in the Far-UV (~100-200 nm) are required to enable the spectroscopic and imaging science goals of the Multi-Object Spectrograph for the IR/O/UV 6m. Micro-Channel Plate (MCP) detectors have been baselined as the lowest-risk Far-UV detectors because extreme cooling is required for Si detectors to acheive low darkrate, which further requires extreme cleanliness for operation in windowless vacuum environments to avoid sensitivity loss due to ice build-up.
MCP detectors are a system of anode readout, MCP stack and photocathode. Large-format, low noise borosilicate MCPs are being developed as part of numerous sounding rocket experiments.
They are robust against gain sag in comparison to previous lead-glass MCPs and the Photocathodes are Solar-blind.
Remaining development challenges include the demonstration of GaN photocathodes on MCP to enable the redder end of the far-UV detector, demonstrating a tiled micro-channel plate focal plane array, and the development of low power readouts
New funnel-style micro-channels have also been shown to improve quantum efficiency by 50% (Matoba et al. 2014) and their incorporation into the baseline MCP architecture should be explored.
SISTINE, FORTIS, CHESS: Suborbital sounding rocket missions that have incrementally matured aspects of MCP detector technology, including array size, photocathode sensitivity, anode read-out electronics, and high count rates (TRL 9), however, low power consumption remains an issue.
Reference C. Updates to Ultraviolet Technology Gaps Relevant to LUVOIR (K. France, U of Colorado): submitted to Program Offices in response to current call for input.
4 Tile Size: at least 100 x 100 mm formats with < 25 micron resolution elements
Flat-field uniformity < 10% across face.
Low power consumption anode readout electronics.
Immunity to gain sag.
High photon counting rates (> 10^7 Hz)
Low dark (<< 1 cnt cm^-2 s^-1)
Quantum Efficiency: >30% between 100-200 nm.
Solar blindness appropriate to the mission
(see LUVOIR final report for more detailed plan, schedule, and cost)
This technology is required to realize the promise of MOS for answering the science questions outline in Table I.1 of the Decadal 2020 report Pathways to Discovery – under the Themes of Cosmic Ecosystems, and Worlds & Suns in Context. MCPs have long been the “gold standard” for far-UV science observations. Continued improvement of these or competing technology detectors would have science and programmatic benefits for any mission operating in the UV. Small resel detector offer the means to reduce the size of spectroscopic designs at a given spectral resolution. Low dark offers access to the faintest objects given a particular telescope effective area. Solar bindness provides immunity to scattered visible photons in MOS applications where faint UV objects share the same field with bright visible objects. IR/O/UV Flagship MOS and any other mission with UV science. Per LUVOIR Final Report, TRL 6 prior to Phase A start is required. See LUVOIR Final Report for detailed development plan recommendations, schedule, and cost estimate.

Level of complexity: system of techs

Level of difficulty: Extreme stretch
High Throughput Large Format Object Selection Technologies for Multi-Object and Integral Field Spectroscopy Future NASA mission devoted to spatially multiplexed spectroscopy require the capability to select multiple targets over an extended field-of-view in the in a variety of bandpasses from the far-UV to the near-IR, allowing spectral and spatial resolution of continuum emitting objects without overlapping spectral confusion produced by by objective prism/grism surveys. These targets may be barely separated as in the case of Multi Object spectroscopy (MOS), or dissect entire contiguous field as in the case of Integral Field Spectrscopy (IFU).
Upon completion of the existing development effort, design and fabricate a large format assemblies with accompanying control electronics.
Complete functional, performance, and environmental qualification testing to bring the programable shutter technology to TRL 6. (see LUVOIR final report for more detailed plan, schedule, and cost)
Objective prisms/grisms are effective at acquiring spectra of multiple objects in an extended field, and offer a simple solution to MOS when uniform spatial and spectral resolution are not required.
Programmable MOS technologies that overcome spatial/spectral confusion include:
Large format programmable slit arrays (0.1 mm X 0.2 mm pitch) exist in the form of 1st generation microshutter arrays (MSA-1G – magnetic aided actuation) currently waiting commission on JWST/NIRSpec. A 2nd gen MSA (MSA-2G electro-dynamic actuation) in proto-type format (128x64) has flown successfully on a the FORTIS sounding rocket 36.312. Large format (736x384) and proto-type format MSA-3G (quasi-electrostatic actuation) are underdevelopment; TRL 4.
Digital Micromirror Devices (DMDs) are commercially available reflective slit arrays that have a typical pitches of (13.7microns)^2 and come in a sealed hermetic package with a 1024 X 768 format. Use in the far-UV requires repackaging and/or customized solutions , which are being pursued under APRA. It is possible that commercial devices could be employed in the near-UV with little reconfiguration. These devices are being incorporated into groundbased MOS, but have yet to fly; TRL 3.
IFU technologies include:
An APRA sounding rocket instrument is in development to bring UV image slicers to TRL 6+ by 2023.
Other IFS-enabling technologies, such as reconfigurable analog mirror arrays or UV-transmissive fibers based on hollow core fiber optics, are TRL 2 or less.
3-4


MOS & IFU:
High fill factor > 50%, large area > (75 mm)^2, programmable targeting and actuation system, high shutter yield > 90% functional, two-side buttable if necessary.
Sufficient contrast for mission needs
Able to simultaneously measure a large enough number of sources for mission needs (e.g., ~50 - 100 or more)
Sufficient lifetime for mission needs (e.g., 10^6 or more actuations)
Sensitivity over the spectral interval 0.10-1.7 µm, including compatability with high-reflectivity mirror coatings (e.g., for DMDs)
Space telescopes today can obtain slit spectra of a single object or slit-less spectra of a field, but not slit spectra of multiple objects in a field, nor can they reconfigure the focal plane (like a fiber fed spectrograph) for integral field spectroscopy in the far-UV.
High-performance multi-object spectroscopy increase the multiplexing advantage of missions by orders of magnitude.
Slits eliminate spectral confusion in MOS and IFU applications, enabling efficient spectroscopic mapping of extended sources in the far-UV.
These technologies are required to realize the promise of multiplex spectroscopy for answering the science questions outline in Table I.1 of the Decadal 2020 report Pathways to Discovery – under the Themes of Cosmic Ecosystems, and Worlds & Suns in Context.
Enabling technology for small, wide-field telescopes appropriate for an Explorer mission.
IR/O/UV Flagship - Relevant Multi-object Spectrograph, and IFU in Coronagraph.

Also, these technologies are relevant to COR, PCOS and ExEP.

They are also relevant to non-strategic missions, which can be brought to bear in support of their maturation for IR/O/UV 6m. The programable slit technology could be employed in LYNX and OST type strategic missions.
Per LUVOIR Final Report, TRL 6 prior to Phase A start in 2025 is required. See LUVOIR Final Report for detailed development plan recommendations, schedule, and cost estimate.


Five years of support through a combination of a directed and entrepreneurial proof of concept (in small flight programs) will push most of these techs to 6 or higher.


Level of complexity: single tech / system of techs

Level of difficulty: extreme stretch
Heterodyne Far-IR Detector Systems 0.5-5 THz heterodyne focal plane arrays are needed for high-sensitivity, spectrally resolved mapping of interstellar clouds, starforming regions, and solar system objects including comets. These arrays require mixers with low noise-temperature and wide intermediate frequency (IF) bandwidth, local oscillators (LOs) that are tunable but which can be phaselocked, and accompanying system technology including optics, low-noise far-IR preamplifiers, low-power-dissipating IF amplifiers, and low-cost, low-power digital spectrometers. 7-pixel receivers have been developed for flight (SOFIA/upGREAT); arrays of 64 pixels (in development) are approaching TRL ~ 4. LOs above 2 THz are at TRL 4. Heterodyne arrays are used/being developed for infrared observations on platforms including SOFIA and balloons (GUSTO, ASTHROS). The sensitivity is still at the level ×20 QL (quantum limit) above 1 THz. Existing systems have good performance but power requirements far exceed what is available in spacecraft, especially if there are more pixels in arrays than currently employed. Ambient temperature LOs deliver < 100 µW around 1.9THz, cryogenically cooled LOs (SOFIA/upGREAT) have an output power < 2.5 mW at 4.7 THz.
TRL details:
3-4, except for the THz amplifiers (1), nearly QL mixers are available at ≤ 0.6 THz.
1-4 • Tunable-bandwidth array receivers for operation at frequencies of 0.5-5 THz. Arrays with 100+ pixels are required to build on the discoveries of Herschel and exploit the sub-millimeter/far-IR region for astronomy. Should include optics, LO beam demultipexers, and accompanying system components;
• Mixers with QL sensitivity are required across the entire far-IR range;
• For mixers, IF bandwidths of 8 GHz at shorter wavelengths (l < 100 microns) are essential to analyze entire galactic spectrum in one observation;
• Sensitive mixers not requiring cooling to 4 K (e.g., based on high critical temperature superconductors) will be essential for application on space platforms, especially with the benefit of increased IF bandwidth;
• Above 2 THz, LO sources operating at 100+ K are highly desirable;
• Far-IR/THz low-noise amplifiers are sought to mitigate the challenge of achieving QL mixers and also to boost the LO power;
• LO sources with output power levels ≥ 10 mW at frequencies > 2 THz are needed in order to pump an entire mixer array with a singe LO;
• For digital spectrometers, 8 GHz bandwidth with > 8000 spectral channels, and < 1W power per pixel will be necessary for large arrays used in space missions;
• Cryogenic IF amplifiers with reduced power dissipation also needed, e.g. 0.5 mW for 8 GHz bandwidth.
Ability to observe and map spectral lines (such as CII at 1.90 THz or OI at 4.75 THz) to study star formation and galactic chemical evolution. Observations of transitions of water are necessary to probe the early phases of planet formation, and to determine the origin of the Earth’s oceans. Development of such systems and associated technology will make imaging observations over 10´ faster. They will also significantly benefit laboratory spectroscopy and biomedical imaging. Increasing the pixel count by 1-2 orders of magnitude over existing instruments will dramatically increase imaging efficiency for high spectral resolution observations. Increased instantaneous bandwidth will enable simultaneous observation of multiple lines, further improving efficiency and relative calibration accuracy. Far-IR flagship and far-IR Probe
The technologies discussed here are directly applicable to heterodyne instrumentation on a future NASA Far-IR Flagship or a Far-IR Probe. A Far-IR Probe has been identified by Astro2020 as one of two priorities for the first Probe-class mission competition. Astro2020 has identified Far-IR as “an area where advances in technology and focused objectives can yield transformative science on a moderate-sized platform.”
Years to estimated launch or other schedule driver: 10 for the Far-IR probe, 15 for Far-IR Flagship.
Level of complexity (single tech, system of techs, or system of tech systems): System of techs
Level of difficulty (straightforward, stretch, or major stretch): Extreme Stretch
High-Performance Spectral Dispersion Component/ Device Past NASA imaging missions (e.g. GALEX, Swift-UVOT) have demonstrated science enhancement by adding relatively inexpensive transmission spectroscopy modes. Such spectrograph types are also efficient for low spectral resolution surverys. Future missions require enhancements on the current state-of-the-art with higher diffraction efficiency (> 90%) that is uniform over a large range of incident ray angles, enables near-diffraction limited resolution, and features integrated bandpass selecting or order sorting capability to avoid spectral and source confusion. The current SOTA, such as the NISP slitless spectrometer on EUCLID, achieves 90% efficiency in the ceter of the FOV, falling to ~70% at the edges where the blaze function deviates from optimum. A corrector is required to achieve the imaging and spectral resolution requirements, adding to the instrument complexity.
For the UV, the GALEX and Swift UVOT grisms are both limited to the near UV ( > 200 nm).
3 The key objectives to enhance the next-generation of slitless spectrometers are:
- Compact enough to be installed in the spectral-filter wheel as one filter element;
- Accommodate a large physical size for wife field coverage on large (> 0.5m diameter) telescopes;
- Diffraction limited (or nearly so) in the wavelength range;
- Relatively high spectral resolution (R > 500 );
- Operational bandpass between 600 to 2400 nm with the specific bandpass set by the science requirements of the future mission;
- High diffraction efficiency (>95% at peak wavelength) with small efficiency losses (< 5% from peak) at the edges of the FOV;
- Compatible with order-sorting and bandpass selecting technologies, such as filter coatings.
The science return for future missions includes dramatically reduced ghost-spectra intensity, increased throughput (transmittance) and sensitivity, and higher efficiency surveys with lower calibration systematics.
Development of high efficiency, single optic slitless spectrometers will greatly simplify future instruments, requiring less mass and volume that the current state-of-the-art.
Wide-field or all-sky spectroscopic surveys in the VIS/IR, including Explorer and Probe-class missions.
VIS/IR imaging missions where a spectroscopic mode would be science enhancing.
Compact, single-element transmission gratings with diffraction limited performance, or aberration correcting freeform profiles could have significant imact on a CubeSat or SmallSat where volumeand mass are critical.
TRL 6 demonstration by early/mid 2020’s. .
High Reflectivity Broadband FUV-to-NIR Mirror Coatings General astrophysics and exoplanet science require high-throughput observations between 100 nm and 2.5 mm. Coating should achieve >50% reflectivity at 105 nm while not compromising performance at wavelengths > 200 nm compared to existing state-of-the-art. Coating process must be scalable to meter-class segments and repeatable to ensure uniform performance across an aperture comprised of >100 segments. Al + enhanced LiF (“eLiF”) coatings have been deposited on the SISTINE secondary mirror to fly on a sounding rocket demonstration in mid-2019 (TRL 6).
Protected eLiF coatings (using MgF2 and AlF3 overcoats) have been demonstrated on samples and show no appreciable change in the base reflectivity (TRL 3). Additional development is necessary to demonstrate repeatability of the optimal coating prescription, and scalability to meter-class optics. Reference C. Updates to Ultraviolet Technology Gaps Relevant to LUVOIR (K. France, U of Colorado): submitted to Program Offices in response to current call for input. May include newly identified gaps.
3 >50% Reflectivity (100-115 nm)
>80% Reflectivity (115-200 nm)
>88% Reflectivity (200-850 nm)
>96% Reflectivity (>850 nm)
<1% Reflectance non-uniformity (over entire primary mirror) over band between 200 – 2000 nm.
Develop and optimize the coating process and implement on sub-scale mirror samples. Evaluate and verify coating performance. Repeat coating deposition multiple time (minimum of 3) to verify repeatability. Begin age-testing the sub-scale samples by storing in a controlled environment and subjecting to routine measurements.
Follow the sub-scale sample demonstration, coat a full-scale (1-meter-class) mirror and verify coating performance and uniformity. Complete optical, radiation, and environmental qualification testing to bring the Far-UV Broadband Coating technology component to TRL 6.
(see LUVOIR final report for more detailed plan, schedule, and cost)
Improved coating reflectivity can have cross-cutting benefits for any missions with broadband UVOIR science.
Stable coatings that maintain performance in ambient environments can have programmatic benefits by simplifying I&T and handling procedures.
IR/O/UV Flagship
This technology would also support the next generation of UV missions, including Explorers, Probes, and large (> 4-m apertures) future UV/Optical/IR telescopes, and is key for a LUVOIR Surveyor.
All future missions with optics, particularly missions with an important FUV or UV component, will benefit from improved coatings.
Benefits will also accrue to planetary, heliospheric, and Earth missions utilizing the UV band.
Per LUVOIR Final Report, TRL 6 prior to Phase A start in 2025 is required. See LUVOIR Final Report for detailed development plan recommendations, schedule, and cost estimate.
High-Throughput Bandpass Selection for UV/VIS - Single Filter and Detector Integrated High-throughput bandpass limiting technologiy with high out-of-band rejection does not exist or exists only in a limited form for UV instruments.
Future UV-sensitive NASA missions would greatly benefit from the development of transmissive or reflective UV filters, detector coatings, or other new technologies that would enable high-throughput observations of select, mission specific bandpasses, while limiting background contamination.
Narrow band imaging filters with Dl/l~2-3 % would also represent an enabling technology for the entire UV/VIS/IR.
Long-wavelength “red” rejection technologies are required for some detectors for applications in the UV.
Commercial optical filters are mature and widely available for visible and NUV wavelengths (> 250 nm). Current state of the art UV-transmission filters has efficiencies of < 10-20% below 180 nm and < 50% for 180-280 nm. Robust commercial high-efficiency UV-transmitting filter solutions are not frequently available. Red-blocking “Woods filters” with low efficiency (< 10-15%) and lifetime issues have been employed for solar-blind imaging.

Photocathode technologies for photoemissive detectors have demonstrated broad band sensitivity and various long wavelength cutoff profiles. Newer photocathode implementations (GaN, blue Bialkali) have steep cutoffs (0.1%) at ~350nm but are not yet fully developed.

Dichroic filters have been designed for the UV, for example the FUV/NUV split in the GALEX instrument, but efficiency in each channel (50/80%) and effective bandwidth in each channel (< 50 nm) is limited at current SOTA.

Similar designs have been implemented in using Atomic Layer Deposition directly on delta-doped Si to provide integrated red rejection capabilities with increased throughput (> 50% below 300 nm). (Hennesy et al. 2021)
3 Key Objectives Single Filters:
- Red-blocking transmittance: > 50-75% (UV), < 0.0001%-0.01% Vis-NIR;
- Dichroic: Mid-UV Split: R (FUV) > 0.8, T (NUV) > 0.9;
- Improved dielectric designs (dichroics, edge filters); dichroic: UV/Vis Split: R (UV) > 0.9,
- T (Vis) > 0.9; and

Key Objectives Detector-Integrated Filters:
-red rejection, narrowband or broadband anti-reflection coatings on Si delta-doped detectors, or bandpass tuning and red rejection photocathodes in large device formats.
High efficiency and low noise resulting from out-of-band rejection, and multi-channel instruments using dichroics allow deep astronomical surveys to be conducted with space-based telescopes on much shorter, feasible timescales, and enable instrument designs that exploit the aperture size (geometrical area) and full working FOV of the telescope.
The integration of UV-filtering elements directly on a sensor system has the potential to improve in-band sensitivity while providing out-of-band rejection. Eliminating discrete optical component can reduce instrument complexity.
Relevant experience with UV, Vis, and/or NIR filters and dichroics that achieve the technical goals set forth above is essential for maximizing the return of future space observatories. This technology is crosscutting, with applications across astrophysics, planetary, and space sciences. Commercial applications similarly benefit from high throughput and band-rejection.
Techniques capable of producing multilayer (metal-dielectrics) ultrathin (10’s nm), uniform (~ 1%), low polarization ((~1%), pinhole free layers with sharp well-defined interfaces—such as produced by atomic layer deposition (ALD)—are crucial for producing high throughput band-selecting filters.
IR/O/UV Flagship High Resolution Imager and MOS far-UV imager
COR science requires deep multi-wavelength measurements of galaxies and AGN to study their evolution from the formation of the first stars and black holes to structures observed on all scales in the present-day universe.
High priority for observations of unseen phenomena such as the cosmic web of intergalactic and circumgalactic gas and resolved light from stellar populations in a representative sample of the universe.
Other relevant missions include a Explorer missions that can conduct wide-field surveys, and small-sat experiments that can advance technology while addressing one or more COR science objectives.
Exosolar planet characterization
Time-domain UV observation of transiet events such as GW EM counterparts, stellar flaring, and life cycle of stars,
Need to demonstrate TRL 6 by mission PDR.

Level of complexity: varies; single tech in case of single fliters; system of techs in case of detector integrated filters

Level of difficulty: straightforward for single filters; extreme stretch for detector integrated filters
Advanced Cryocoolers Cryocoolers are required for achieving very low temperatures (e.g., ~4 K) for optics and as pre-coolers for sub-Kelvin detector coolers for COR missions.
Eliminating the need for expendable materials (cryogens) will increase achievable lifetime and reduce system mass and volume. Improvements are needed in terms of performance, especially low power consumption and low vibration levels.
For several-Kelvin temperature designs, the current SOTA includes pulse-tube, Stirling, and Joule-Thomson coolers which are at high TRL but are expensive, and do not yet have good enough performance. The SHI cooler used on Hitomi had the necessary cooling power at 4.5 K (Fujimoto, 2018), but the expected lifetime was 5 years rather than the 10 years required for Origins. Changes in the working fluid have been shown to produce temperatures in the 4.5 K range as required for Origins. Low vibration levels have been achieved by the miniature turbo-Brayton cryocooler of Creare down to 8 K. A currently funded SBIR Phase II will extend this operation to 4 K. DoD has some interest in these 4 K cryocoolers as well. 4 Several mission concepts require sustaining temperatures of a few Kelvin, with continuous heat-lift levels of a few dozen to ~200 mW at temperatures ranging from 4 to 18K.
Other concepts could benefit from greater heat lifts at somewhat higher temperatures. All this needs to be accomplished with < 9000 W/W input power/cooling power at 4.5 K. Such coolers need to be compact, and impose only low levels of vibration on the spacecraft.
In some applications, a sub-Kelvin cooler will be implemented, and an advanced few- Kelvin cryo-cooler able to maintain the sub-Kelvin cooler’s hot zone at a steady (e.g.) 4.5 K will be very beneficial.
Cryocoolers able to operate near 4 K, cooling detectors and optics directly, as well as serving as a backing stage for ultra-low-temperature (sub-Kelvin) coolers will enable large FIR telescopes, as well as ultra-low-noise operation of cryogenic detectors for other bands.
Increased heat lift, lower mass, lower volume, increased operational lifetime, and reduced cost will enable such missions to fly extended durations without wasting critical resources on cooling.
Large-capacity cryocoolers are required to achieve astrophysical photon-background-limited sensitivity in the FIR and meet sensitivity requirements to achieve the science goals for future FIR telescopes or interferometers.
Far-IR Flagship
Far-IR Probe
This technology is a key enabling technology for any future FIR mission, including OST.
It is also applicable to balloons, airborne platforms, Explorers, and Probes. For example, it would improve performance of STO2 long/ultra-long duration balloon and SOFIA instruments. Explorer missions would also be very positively impacted. Low vibration cooling would open up other wavelength missions, such as LUVOIR, to use of sensitive cryogenic detectors.
Need to demonstrate TRL 6 by mission PDR.
System of techs
Stretch
High-Performance, Sub-Kelvin Coolers Optics and detectors for FIR, sub-millimeter, and certain X-ray missions require very low temperatures of operation, typically in the tens of milli-Kelvins.
Compact, low-power, lightweight coolers suitable for space-flight are needed to provide this cooling.
Both evolutionary improvements in conventional cooling technologies (adiabatic demagnetization and dilution refrigerators) with higher cooling power, and novel cooling architectures are desirable.
Novel cooling approaches include optical, microwave, and solid-state techniques.
Existing adiabatic demagnetization refrigerators with low cooling power (0.4 µW) at 50 mK are at TRL 7-9 (Hitomi/SXS) (Shirron, 2015)but high-cooling-power versions (6 µW) are at TRL 4. (Shirron, 2000)
Low cooling power (<1µW) dilution refrigerators (Collaudin, 1999) and ultra low (<0.1 µW) solid-state cooling approach based on quantum tunneling through normal-insulator-superconductor (NIS) junctions are both at TRL 3.
Currently funded technology development is expected to result in a TRL 5 or 6 ADR for use over the temperature range of 10 to 0.050K with high efficiency and cooling power of 6 micro-W at 0.05 K. (Tuttle, 2017) Further development, extending the low temperature to 35 mK while maintaining the 6µW cooling capability would be enhancing.
4 A sub-Kelvin cooler operating from a base temperature of ~4 K and cooling to 50 mK with a continuous heat lift of 6 μW at 50 mK is required. To enhance detector sensitivity, cooling to 35 mK with 6 µW of cooling power is enhancing.
Features such as compactness, low input power, low vibration, intermediate cooling, and other impact-reducing design aspects are desired.
Sub-Kelvin cryocoolers are required to achieve astrophysical photon-background-limited sensitivity in the FIR and high-resolution sensitive X-ray microcalorimetry.
Techniques to lower cooling costs and improve reliability will aid the emergence of powerful scientific missions in the FIR and X-ray.
Far-IR Flagship
Far-IR Probe
This technology is a key enabling technology for any future FIR mission, including OST.
Sensors operating near 100 mK are envisioned for future missions for X-ray astrophysics, measurements of the CMB, and FIR imaging and spectroscopy.
Applicable to missions of all classes (balloons, Explorers, Probes, and flagship observatories).
High synergy with X-ray missions using microcalorimeters.
Need to demonstrate TRL 6 by mission PDR.
Large Cryogenic Optics for the Mid IR to Far IR Large telescopes (of order 10 m in diameter) provide both light-gathering power to see the faintest targets, and spatial resolution to see the most detail and reduce source confusion. To achieve the ultimate sensitivity, their emission must be minimized, which requires these telescopes to be operated at temperatures (depending on the application) as low as 4 K.
Sufficient thermal conductivity internal to the telescope segments (> 2W/m K) is required to isothermalize the primary.
Key technologies to enable such a mirror include new and improved:
- Mirror substrate materials and/or architectural designs
- Processes to rapidly fabricate and test cryogenic mirrors
- Mirror support structures that are stable at the desired scale
- Mirror support structures with low-mass that can survive launch at the desired scale
Also needed is ability to fully characterize surface errors and predict optical performance via integrated STOP (structure thermal optical performance) modeling.
These telescopes will probably need in-orbit adjustability and should be designed for low-cost optical-performance verification before launch. Some material properties, such as damping for telescope structures, are also needed.
JWST Be mirror segments meets Origins requirements now, so TRL 5 with a heavy (~ 68 kg/m2 including support structure) technology; TRL 3 exists for other materials like SiC.
Cryogenic low-dissipation actuators exist at TRL 3-5.
The MMSD program has produced 1.35-m JWST class SiC actuated hybrid mirror segments with < 14 nm rms surface figure error, < 10 Å micro-roughness (projected), and < 25 kg/m2 total. This technology is at TRL 5. Large 2-m class aluminum mirrors for balloon missions have flown successfully. EBEX flew a 1.5-m telescope 400 miles around Antarctica in 2012-13, but operated in the microwave band. The largest aluminum mirror with mid-IR precision flown-to-date was for the BETTII balloon mission in 2017 (0.5-m diameter aperture with 30 um diffraction limit). Given that the state-of-the-art for a 2-m mid-IR mirror is a 0.5-m subscale (i.e., low-fidelity system), the team assesses large-aperture aluminum mirrors for mid-IR missions to be TRL 4. Current factors limiting upscaling to 2 m are optical fabrication errors, cryogenic deformation and gravity-sag. To support balloon mission, this technology would need to be at TRL 5 by manufacturing a full-scale, 2-m class aluminum mirror with a flight traceable design and characterizing its performance in a relevant environment.
The 3.5-meter SiC Herschel Primary Mirror meets all requirements except diffraction limited performance. This mirror is diffraction limited at 80 micrometers. Herschel SiC mirror technology is TRL 9.
The balloon BLAST program is developing a 2.5-m Carbon Fiber Resin Epoxy (CFRP) or Graphite Composite mirror. Apertures of up to ~10 m are undergoing ground-based tests, including the phase 1 study for the Large Balloon Reflector.
4 Develop a feasible, affordable, and low mass (target 35 kg/m2 including support structure) approach to producing a 2-6-m-class telescope with sufficiently high specific stiffness, strength, and low areal density to be launched; while maintaining compatibility with cryogenic cooling and FIR surface quality/figure of ~1μm rms. Use of materials and techniques that do not require cryo-null figuring is required. Material property measurements at cryogenic temperatures for structures and optics such as damping, emissivity, thermal conductivity, etc.
Development is required to fabricate components and systems to achieve the following:
- Monolithic: 1 to 4 meters
- Segmented: > 4 meters
- Surface Figure: < 150 nm RMS AT TEMPERATURE
- Cryo-Deformation: < 100 nm RMS (surface)
- First Mode Frequency: >200 Hz per segment
- Areal Cost: < 2M$/m2
- Areal Density: < 35 kg/m2
A 2-m class aluminum mirror for the mid-infrared needs to have an areal density of < 30 kg/m2 and a surface figure error (SFE) of < 1 um rms with mid-spatial frequencies (removal of first 36 Zernike terms) < 700 nm. This would support a telescope having a 20 um diffraction limit. The mirror would need to maintain its SFE under G-release for space mission and over the operating elevation changes for balloon missions.
Low-cost, lightweight cryogenic optics at reasonable cost (~2M$/m2) are required to enhance development of large-aperture FIR telescopes in the 2020s.
Large apertures are required to provide the spatial resolution and sensitivity needed to follow up on discoveries from the current generation of space telescopes.
A high altitude, long duration observing platform with a mirror factors of 2-5 larger than on either SOFIA or Herschel has the potential to increase sensitivity in the far-infrared by factors of several in particular wavelength regimes over either facility, at relatively low cost. This enables scientific breakthroughs in the far-infrared including (1) detecting the dusty progenitors to `ordinary' local galaxies at redshifts up to z=3, and (2) high sensitivity mapping of debris disks around young stars. This technology is also relevant to e.g. Earth observation. Large 2-m class mirrors made from aluminum benefit from having lower material and fabrication costs compared to other mirror materials such as beryllium and silicon carbide, especially for limited production. They also have shorter lead times to produce which benefits schedule.
Far-IR Flagship
Far-IR Probe
This is an enhancing technology for any future single-aperture FIR telescope, and an enhancing technology for a FIR interferometer.
This technology development will enable (current) MIDEX-class science with a SMEX, Astrophysics Probe-class science with a MIDEX, and more ambitions larger-mission science with an Astrophysics Probe-class implementation.
Development of inexpensive 1-3-m monolithic lightweight stiff mirrors will benefit suborbital, Explorer, and Astrophysics Probe missions.
This technology is of particular relevance to ultra-long duration balloon projects, which are maturing as a viable and attractive platform for multiple astrophysics missions.
Need to demonstrate TRL 6 by mission PDR.
System of techs
Stretch
Photon Counting Large-format UV Detectors Low-noise detectors for general astrophysics as short as 100 nm QE 44% 0.115-0.18 µm with alkalai photocathode, 20% with GaN; dark current ≤0.1–1 counts/cm2/s with ALD borosilicate plates
The individual component technologies for photocathode material, microcapillary array, and readout convertor have been demonstrated. To achieve TRL 5, one device using all 3 desired components must be fabricated and demonstrate performance. HabEx would mature the MCP at the required 100 mm x 100 mm format size
Delta-doped EMCCDs: Same noise performance as visible with addition of high UV QE ~60–80% in 0.1–0.3 µm, dark current of 3×10-5 e-/pix/s beginning of life. 4k×4k EMCCD fabricated with reduced performance. Dark current <0.001 e-/pix/s, in a space radiation environment over mission lifetime, ≥4k×4k format fabricated, updated design for cosmic ray tolerance is under test, high QE for 100–350 nm wavelengths
RST maturation on EMCCDs has increased radiation tolerance and demonstrated adequate read noise; however delta doping is not planned for RST.
4 - Dark current <0.001 e-/pix/s (173.6 counts/cm2/s), in a space radiation environment over mission lifetime
- 4k × 4k format for starshade UV spectrometer (Enabling for HabEx)
- ≥4k × 8k format for UV spectrograph astrophysics camera (Enhancing for HabEx)
- QE >~60% for 0.1–0.3 µm wavelengths
Enables astrophysics UV spectrograph field of view and sensitivity requirements.
Improves dark current at all wavelengths, particularly in the UV.
For the starshade UV spectrometer, the 4k x 4k format size is enabling for the science spectral bandwidth.
For the 4k x 8k format, the delta doped EMCCDs are an alternate to MCPs
IR/O/UV Flagship A funding start in 2022 could result in TRL 5 by mid 2023.

Single tech
Stretch
UV/Opt/NIR Tunable Narrow-Band Imaging Capability High throughput UV, Optical and Near Infra-red narrow-band filters (Dl/l~2-3 %) with continuously selectable central wavelength capable of covering an un-vignetted and un-aberrated FOV of several arcmin (linear size) or better. A Tunable Filter in the NIR (Fabry-Pérot etalon with piezo electric actuation) was developed for JWST. The design advanced capabilities beyond existing ground based operational systems to provide low-order gap, very wide waveband, and operation in a cryovacuum environment. However, difficulties occurred in providing a stable and predictable gap separation after exposure to vibration, shock, and cryogenic cycling and was not successfully qualified for flight.
The JWST development did not investigate performance down into the lower UV-Vis wavelengths.
Space environment challenges with the tunable Fabry-Pérot etalon for the JWST fine guidance sensor. Available from: https://www.researchgate.net/publication/258718456_Space_environment_challenges_with_the_tunable_Fabry-Perot_etalon_for_the_JWST_fine_guidance_sensor [accessed Jul 4, 2017].
3 High throughput UV, Optical and Near Infra-red narrow-band filters (Dl/l~2-3 %) with continuously selectable central wavelength capable of covering an un-vignetted and un-aberrated FOV of several arcmin (linear size) or better. The availability of variable narrow band filters at UV, Optical and Near Infrared wavelengths will enable systematic studies of a broad range of astrophysical problems in galaxy evolution that are core to the Cosmic Origins program that currently can either not be done or are done very sub-optimally (with grism/slitless spectroscopy, which partially destroys spatial information) or can only be done in few, very lucky cases when the targeted emission line is fortuitously redshifted to the wavelength of available onboard fixed filters. Tunable narrow band filters will allow us to study the formation and evolution of the proto-cluster environment, the proto-galaxy environment and star-formation in the cosmic web. The device will enable high-angular resolution, large-scale spatial tomography of line-emission processes from galaxies, globular clusters and gaseous nebulae in general. This capability is key, and currently unavailable to address a broad range of problems, from spatial reconstruction of the cosmic web, to satellite star-formation and quenching in massive halos, to the formation of globular clusters, to gas accretion and expulsion in galaxies. These physical processes are key to the Cosmic Origins program and currently cannot be addressed in a systematic way.  
Warm Readout Electronics for Large-Format Far-IR Detectors Readout schemes compatible with cryogenic multiplexing and room temperature ADCs and RF electronics for these arrays need to be developed. The cryogenic electronics are covered in a separate gap. Room temperature readout of cryogenic amplifier outputs currently use FPGAs and ADCs for bandwidths up to 4 GHz having power dissipation of about 50W per channel. Dedicated ASICs would potentially lower the input power requirement by a factor of 10. Origins has baselined using a rad-hard version of a commercial RFSoC using about 50 W per channel, currently at TRL 4.(enabling for Origins) Raising to TRL 6 requires using rad-hard parts. (enhancing for Origins) 4 Using current technology with rad-hard parts is enabling. Recovering the signal to noise with lower-power room-temperature electronics needs to lower the input power by up to a factor of 10. ASIC development takes time and money. Sensitivity reduces observing times from many hours to a few minutes (≈ 100× faster), while array format increases areal coverage by ×10-100. Overall mapping speed can increase by factors of thousands.
Sensitivity enables measurement of low-surface-brightness debris disks and protogalaxies with an interferometer. This is enabling technology.
Suborbital and ground-based platforms can be used to validate technologies and advance TRL of new detectors.
Far-IR Flagship
Far-IR Probe
FIR detector technology is an enabling aspect of all future FIR mission concepts, and is essential for future progress.
This technology can improve science capability at a fixed cost much more rapidly than larger telescope sizes.
This development serves Astrophysics almost exclusively (with some impact on planetary and Earth studies).
Need to demonstrate TRL 6 by mission PDR.
Compact, integrated spectrometers for 100 to 1000 µm Compact, integrated spectrometers operating in the 100 mm to 1 mm band which can provide a wide (e.g., 1:1.6) instantaneous bandwidth at resolving power (R = λ/Δλ = n/Δn) ~500 with high efficiency in a compact (~10 cm) package that could be arrayed in a focal plane to provide integral-field mapping or multi-object spectroscopy capability.
Si immersion technology can provide increased spectrometric capability (R~1x105) with smaller size (factor of 3) over standard Echelle gratings.
Multiple compact spectrometers are under development: including compact silicon gratings and grating analogs, as well as superconducting filter banks. These systems are promising, and in some cases are approaching photon-noise limited performance suitable for ground-based observations, but have not yet been demonstrated in a scientific application. 3 An integrated spectrometer + detector array system would demonstrate 1:1.7 bandwidth (or greater), high efficiency (>50%, including detector absorption), resolving power > 400, and a coupling scheme compatible with a telescope beam e.g., an f/4 Gaussian beam. To enable the observatories with hundreds of spectrometers, a single spectrometer + detector array would be a packaged on a silicon wafer on order tens of square cm in size (i.e., less than one 4” wafer). Large-format spectrometers in the far-IR through millimeter enable 3-D spatial-spectral surveys over large areas. The combination of large spatial coverage (many to tens of square degrees) and spectral bandwidth (giving redshift, or line-of-sight distance) will simultaneously find galaxies and measure their redshifts in large numbers (e.g., on order millions with Origins). This measurement addresses key questions in galaxy evolution and the reionization epoch. This is enabling technology. Far-IR Flagship
Far-IR Probe
Compact spectrometers would greatly reduce the system mass for future far-IR flagship such as Origins, but they also enable interim opportunities such as involvement with SPICA, balloon-borne far-IR experiments which are being proposed, and SOFIA.
Need to demonstrateTRL 6 by mission PDR.
Short-Wave UV Coatings Allows astrophysics imaging as low as 0.1 µm For a 0.1 µm cutoff, Al + LiF + AlF3 has been demonstrated at the lab proof-of-concept level with test coupons achieving reflectivities of 80%+ for >0.2 µm and 60% at 0.1 µm and 3-year lab environment stability 3 - Reflectivity from 0.3–1.8 µm: >90%
- Reflectivity from 0.115–0.3 µm: >80%
- Reflectivity below 0.115 µm: >50%
- Operational life: >10 years
Development performance on cm size blanks needs to scale to meter class. Durability is critical with accelerated lifecycle tests taking 1-3 years.
Improves astrophysics science output for UVS IR/O/UV Flagship TRL 6 by mission PDR.
Far-IR Spatio-Spectral Interferometry Wide field-of-view spatio-spectral interferometry with cold telescopes in the Far-IR provides sensitive integral field spectroscopy with sub-arcsecond angular resolution and R ~ 3000 spectral resolving power. This technique will give the Far-IR Surveyor the measurement capabilities envisaged in the Astrophysics Roadmap. With those capabilities the community will learn how habitable conditions develop in nascent planetary systems and will overcome source confusion to measure the spectra of individual high-z galaxies, complementing JWST to understand their formation and evolution. The angular resolution achievable with a structurally connected interferometer will vastly exceed that of any practical single-aperture telescope, and the resolution, coupled with spectroscopy, is essential to mapping the distribution of water vapor and ice in protoplanetary disks. Experiments conducted with an existing laboratory testbed interferometer and parallel algorithm development are needed to advance spatio-spectral interferometry to flight-ready status for the Far-IR Surveyor.
The gap can be closed in 2 to 3 years of concerted effort, and depends entirely on funding. Nearly all of the required hardware exists in an established testbed, as does the optical system model. The testbed is currently housed in a world-class facility, which offers the stability of a quiescent space environment. The graduate student and postdoc have moved on to new positions, so new qualified experimentalists will have to be hired.
Current SOTA in detector technology is the only other pacing item for a far-IR interferometer, and current investments may yield detectors – TES bolometers or KIDs – that satisfy mission requirements. (The detector requirements and performance goals are relaxed relative to those for OST.)
A structurally connected interferometer could enter development in the early 2020s and fly by the end of the decade. Interferometry is perceived to be complex, but the engineering challenge is greatly relaxed at long (far-IR) wavelengths. Wavefront sensing and control for JWST, a mature technology, is a harder engineering problem than far-IR spatio-spectral interferometry.
Wide field-of-view spatio-spectral interferometry has been demonstrated in the lab (GSFC) at visible wavelengths with a testbed that is functionally and operationally nearly equivalent to a space-based far-IR interferometer, and testbed experiments have been conducted with an astronomically realistic hyperspectral test scene. The most important error terms are well understood. Further, single-pixel spatio-spectral interferometry has been demonstrated in the lab (University College London) at THz frequencies, demonstrating the desired broadband far-IR wavelength response of the beam combiner.
This work was the subject of two successfully-defended PhD theses, one by Dr. Roser Juanola-Parramon, and another by Dr. Alexander Iacchetta. During Dr. Juanola-Parramon’s tenure as a NASA Postdoctoral Fellow, she adapted her “Far-Infrared Spectro-Spatial Space Interferometer” computational simulator to model the optical testbed.
4 Additional effort is required to fully characterize the hyperspectral scene projector used in the testbed, as well as minor testbed optical aberrations, and then to close the loop by demonstrating reconstruction of a hyperspectral astronomically realistic scene that matches an independent measure of the “truth” scene to high fidelity, with residual differences explained. All necessary equipment is in place to characterize the scene projector. Testbed optical aberrations will be understood with the aid of an existing, thoroughly tested computational optical system model of the testbed. Finally, experimentation with “single dish” (standard FTS) mode and simulation and experimentation with data acquired with a rotating interferometer, will close the gap and provide enabling technology for a space-based far-IR interferometer. Interferometric baselines in the tens of meters, up to ~100 m, are required to provide the spatial resolution needed to follow up on discoveries made with the Spitzer and Herschel space telescopes, and to provide information complementary to that attainable with ALMA and JWST. The capability to definitively map the distributions of gas, dust, and ice in protoplanetary disks, to find structures (gaps or holes) indicating the presence of young planets, and to learn how the conditions for habitability arise during the planet formation process, is particularly strong motivation for the Far-IR Surveyor. Only a space-based far-IR interferometer will have these capabilities.
Programmatically, as explained in the Astrophysics Roadmap, Enduring Quests, Daring Visions, the far-infrared is the best “training ground” for space-based interferometry, but eventually interferometers will be needed across the entire electromagnetic spectrum.
Far-IR Flagship
Far-IR Probe
Wide-field spatio-spectral interferometry is the critical path technology for a Far-IR astrophysics mission consistently given high priority by the Far-IR astrophysics community since the 2000 Decadal Survey, and most recently in the NASA Astrophysics Roadmap, Enduring Quests, Daring Visions. The first interferometer will be structurally connected and might resemble the SPIRIT mission concept recommended as a Probe-class mission to the 2010 Decadal Survey (https://asd.gsfc.nasa.gov/cosmology/spirit/). Later interferometers would rely on the same technology, but could use formation flight to provide long interferometric baselines and correspondingly improved angular resolution (Harwit et al. 2006, see http://adsabs.harvard.edu/abs/2006NewAR..50..228H). The Astrophysics Roadmap explains the need for space-based interferometers across the electromagnetic spectrum, from far-infrared to X-rays.
Potential applications also exist in NASA’s Planetary and Earth science programs.
This technology is the pacing item for a space-based far-IR interferometer.
The future envisaged in the Astrophysics Roadmap will be delayed until NASA invests to close the gap on wide-field spatio-spectral interferometry, so the urgency is great.
High-QE, Solar-blind, broad-band NUV detector Single photon-counting, high-QE (>50%), megapixel, solar-blind, broad-band NUV detector (180-360 nm) MCP detectors employing solar-blind photocathodes (CsTe or bi-alkali) have low QE (10-17%) in the NUV.
The Fireball-2 balloon experiment (Hamden+2020) employs an EM-CCD detector made UV-sensitive over a narrow band (FWHM~15 nm) by delta-doping and AR coatings (Nikzad+2017
The SPARCS CubeSat payload planned for flight in late 2021 (Scowen+2020) has delta-doped and AR-coated CCD’s with detector-integrated out-of-band rejection and red-blocking reflection filters (Nikzad+2017), but it is purely an imaging mission and doesn’t cover the full NUV (180-360 nm).
TRL details: Broadband NUV CCD’s incorporating >103 out-of-band rejection (Nikzad et al. 2017)
3-6 QE>50% over the full NUV wavelength range (180-400 nm) but extremely insensitive (QE<10-3) to out-of-band light (i.e. solar-blind) Scientific benefit: Accurate measurements of the NUV continuum spectral energy distribution (SED) and important diagnostic spectral features in the NUV spectra of red stars (e.g., a dwarf M star hosting a potentially habitable planet) , 2nd-generation stars in the universe, and in elliptical galaxies, etc. without serious effects of red-light leak. COR – LUVOIR, UV Probe missions (Phase A start in FY23)
ExEP – HabEx
PCOS – future high-energy Probe-class missions with a NUV component like that in the Gehrels Swift observatory (UVOT)
Years to estimated launch or other schedule driver: 8 years; 2 years to Phase A for Probe missions
Level of complexity (single tech, system of techs, or system of tech systems): detector and grating technology
Level of difficulty (straightforward, stretch, or major stretch):
Our-of-band rejection of optical/red light – straightforward
Low-scatter NUV gratings – straightforward-stretch
FUV imaging bandpass filters FUV imaging with a far UV spectrograph+imager (e.g., the LUVOIR LUMOS concept) relies on a set of medium- and wide-band filters, extending the imaging capability of IR/O/UV Flagship below the optical/NUV bandpass. This instrumentation capability was specifically called out, per the above words from the 2020 decadal survey.
1) develop the technical approach, e.g., multi-layer deposition or other technique, for an optimized a FUV filter set that meets the science requirements identified by the LUVOIR STDT's science program: medium (20nm) and wide (40nm) bandpass filters covering 100-200nm. Demonstration should be made on shaped optics with diameter >= 5” to be representative of the needs of a IR/O/UV Flagship-like instrument and to identify non-uniformities in the deposition process when working with shaped optics,
2) calibrate these filters in the laboratory with representative environment aging and reflectance testing: goals should include > 80% peak reflectance within the central 15nm for medium band filters, > 60% peak reflectance within the central 30nm for wide band filters, < 1% transmission at 121.6nm, and < 1E-4 transmission at lambda > 300nm when combined with typical solar-blind detector system, and
3) demonstration in an vacuum-ultraviolet imaging system operating from 100 - 200 nm to demonstrate their use as an optical system in a representative environment. Pre-phase-A development should include demonstration of the full FUV filter set in the laboratory, including integration-like environmental testing and degradation/lifetime testing (peak reflectivity decline < 10% relative, out of band rejection stable to 20% relative with exposure to RH <= 50% for 6 months), and in-band performance demonstration for comparison with existing FUV photometric calibrations (e.g., GALEX FUV).
Current state of the art FUV-transmission filters have efficiencies of < 10-20% below 180 nm, and either no red rejection (i.e., the long-pass filters on HST), or large out-of-band response owing to large resonant wings from the dielectric materials, leading to unacceptably high Lyman-alpha contamination and inter-band cross-talk.
Dielectric multi-layer mirror coatings have been developed to TRL 3, with a reflective filter designed to select Lyman alpha have flown on a heliophysics sounding rocket. Narrow band filters operating at Lyman-alpha are operating on the HST ACS/SBC (TRL 6+), but these filters have very small (< 10%) peak throughput and are narrower than the IR/O/UV Flagship requirements.
Other multi-layer dielectic have been demonstrated in the lab on 2” wafers (Honrado-Benítez et al. 2018; Rodríguez-De Marcos et al. 2018), demonstrating TRL 3.
3 Medium (20 nm) and wide (40 nm) filter set covering 100-200 nm
Work with shaped optics
>80% peak reflectance within central 15 nm for medium band filters
>60% peak reflectance within central 30 nm for wide band filters
<1% transmission at 121.6 nm and <1E-4 transmission at >300 nm when combined with solar blind detector systems
EOL peak reflectivity decline <10% relative and EOL out-of-band rejection stable to 20% relative with exposure to RH<=50% for 6 months
Covering the FUV bandpass is important for measurements of stellar populations, circumgalactic structures, exoplantary aurorae, and other cosmic origins science. However, no standard set of FUV filters that meet the LUMOS reference design have been built, tested, and calibrated (as noted above, the long-pass filter set on HST suffer from red leaks and rely on uncertain filter-differencing techniques to quantify in-band fluxes and do not meet the LUVOIR science requirements). IIR/O/UV Flagship, as well as any probe, explorer, pioneer, cubesat, or sounding rocket focused on FUV imaging science. We will note that these filter systems would also be beneficial for FUV imaging on NASA’s planetary science missions. but their primary focus is astrophysics missions at all scales. Years to estimated launch or other schedule driver: 4 years, approximate Phase A start for IR/O/UV Flagship
Level of complexity (single tech, system of techs, or system of tech systems): single tech, but demonstration requires a vacuum demonstration system to be developed
Level of difficulty (straightforward, stretch, or major stretch): Extreme stretch
Improving the Calibration of Far-Infrared
Heterodyne Measurements
Relative instrument flux calibration (flux calibration of one spectral line relative to another line in the same spectrum) of about 1% is required for measurements of molecular isotopic line ratios in the THz spectral range (~0.5 – 1.3 THz). The HIFI instrument on Herschel represents the current SOTA for FIR space-based heterodyne spectrometers. The relative calibration accuracy was 5-7%, depending on the frequency band, limited by the sideband ratio and optical standing waves. Other factors, such as the overall beam efficiency, influence the absolute flux calibration, but not the relative flux calibration. The full solution consists of developing an instrument, in which multiple lines are downconverted to the same intermediate frequency band, so that they can be observed simultaneously. 1 Increase the RF and IF bandwidth of the current FIR spectrometers to allow simultaneous observations of multiple spectral lines. Improve the characterization and stability of the sideband ratios, instrument standing waves, as well as the thermal environment to eliminate gain variations between different signal paths. Isotopic ratios have been shown to be critically important tracers of the origin and thermal history of interstellar and solar system materials. Such measurements, e.g., those of oxygen isotopic ratios in water, require ~1% relative flux calibration of the instrument, a factor of 5–10 better than what has been demonstrated in past space-based applications. Closing the technology gap discussed here will enable new types of investigations that have not been possible with past space-based and airborne assets, such as Herschel, SOFIA, or Rosetta. The technology discussed here is directly applicable to heterodyne instrumentation on a future NASA FIR Flagship or a FIR Probe. A FIR Probe has been identified by Astro2020 as one of two priorities for the first Probe-class mission competition. Astro220 have identified FIR as “an area where advances in technology and focused objectives can yield transformative science on a moderate-sized platform.” Years to estimated launch or other schedule driver: 10 for the FIR probe, 15 for FIR flagship
Level of complexity (single tech, system of techs, or system of tech systems): System of techs
Level of difficulty (straightforward, stretch, or major stretch): Stretch
Improving the photometric and spectro-photometric precision of time-domain and time-series measurements Higher precision observations are needed to probe some of the most interesting time-variable astrophysical phenomena. Systematic errors caused by detector intrapixel response variations in the presence of pointing jitter and other noise sources should be reduced to allow near photon-limited performance for future missions. Kepler, HST, and TESS all reduced but did not eliminate such noice by defocusing their telescopes or cameras,. This imperfect solution also led to other problems, particularly contamination by nearby objects that had to be resolved by ground-based follow-up observations.
New technologies are needed to improve the photometric and spectrophotometric precisions of small-amplitude time-domain measurements. Ideally these solutions would work over many different wavelength ranges and with different existing detector technologies.
Ideally the problem could be addressed with optical technologies that allow existing detectors to be used to achieve high photometric and spectrophotometric precision in time-series measurements in the presence of pointing jitter and perhaps some thermal variations. I believe that the best current candidate technology is the Densified Pupil Spectrograph (DPS), as first described by Matsuo et al. (2016 ApJ, 823, 139) and subsequent works. I am aware of a single prototype DPS that has been fabricated, and it has been tested cryogenically (for IR detectors; should also work fine with visible) but not with injected pointing jitter or other disturbances. The DPS concept uses reflective optics for its slicer and re-imaging components, so it should function over the UV to mid-IR wavelength range with suitably stable detectors and with appropriate design parameters optimized for the desired wavelength interval within this range.
TRL details:
I believe that the DPS is currently entering TRL given that its concept has been developed (and that a prototype has been built and tested statically from the NPR 7123.1C Appendix E TRL definitions.
3 The goal is to produce an end-to-end system capable of spectroscopic or photometric precision <= 10 ppm for time-series observations spanning ~10 hours. This period is relevant for observing exoplanet transits, small-amplitude stellar oscillations, and perhaps small accretion events in circumstellar environments. Of course this 10 ppm value can only be achieved with detectors that have instrinsic stability bellow (better than) this value. Ideally this technology would operate over the UV to mid-IR wavelength range (l ~0.1 – 20 mm) with sufficiently stable detectors. This would provide adequate precision for measuring a number of different astrophysical processes with missions in the new Time Domain Astrophysics Program, the highest priority Sustaining Activity for Space in the Astro2020 “Pathways…” Decadal Survey Report (p.S-8, 1-17, 7-18, 7-19). Filling this technology gap would improve our knowledge of stellar physics (oscillation), accretion of black holes and young stars, and also exoplanets (e.g., via high-precision transmission or emission spectra). Pursuing an optical solution could have substantial engineering and programmatic benefits if existing detectors could be used (saving cost and development time). An optical solution like the DPS could be implemented with conventional optical design practices, materials, and tolerances. Time Domain Astrophysics Program, the highest priority Sustaining Activity for Space in the Astro2020 “Pathways…” Decadal Survey Report (p.S-8, 1-17, 7-18, 7-19).
This gap cuts across at least COR and ExEP. Developing technologies to bridge this gap would improve our knowledge of stellar physics (oscillation), accretion of black holes and young stars, and also exoplanets (e.g., via high-precision transmission or emission spectra). Missions in the new Time Domain Astrophysics Program, the highest priority Sustaining Activity for Space in the Astro2020 “Pathways…” Decadal Survey Report (p.S-8, 1-17, 7-18, 7-19) would benefit most from this effort.
Years to estimated launch or other schedule driver: 5-7. It will be most useful to develop a solution to TRL ~5 by completion of concept studies for the first missions in the the new Time Domain Astrophysics Program.
Level of complexity (single tech, system of techs, or system of tech systems): Single tech for gap, but system of technologies to increase system TRL. Increasing TRL will require developing a high spectrophotometric precision testbed with a stable (or calibratable) light source, the new optical technology solution (e.g., a prototype densified pupil spectrograph), a laboratory thermal control system (to < 10 mK, within commercial laboratory solutions), a detector with known time-series stability, and some sort of telescope simulator with injectable disturbances characteristic of what is expected on orbit.
Level of difficulty (straightforward, stretch, or major stretch): Straightforward, but it would need to be started relatively soon with a stable funding source to be successful.
Large aperture deployable antennas for FIR/THz/sub-mm astronomy for frequencies above 100 GHz Large aperture deployable antennas are needed to advance FIR-submillimeter radio astronomy. Balloon-borne (e.g. BLAST) and spaceborne (e.g. Herschel Space Observatory) observatories have made key advances in multiple areas of Cosmic Origins science. However, small antenna aperture (e.g. 3.5-m Herschel) has limited performance. Antennas as large as 30-m have been deployed in space since the 1960's by NASA and other agencies, primarily operating below 50 GHz and not developed for astrophysics applications. Inflatable membrane antennas have been studied for the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) mission concept. Perimeter-truss mesh antenna (8-meter) operatin up to 100 GHz has been studied for geostationary atmospheric sounding. Composite reflector operating >500 GHz is used on Microwave Limb Sounder and up to 200 GHz on numerous Earth-orbiting radiometers. Extending operating frequency and aperture size will have broad application in astrophysics as well as planetary and Earth science. There are 3 main architurectures possible: arrayed composite reflectors; deployable mesh surface; inflatable membrane surface. Antennas as large as 30-m have been deployed in space since the 1960's by NASA and other agencies, primarily operating below 50 GHz and not developed for astrophysics applications. Inflatable membrane antennas have been studied for the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) mission concept. Perimeter-truss mesh antenna (8-meter) operating up to 100 GHz has been studied for geostationary atmospheric sounding. Mesh materials for operation >200 GHz are in development under SBIR program. Composite reflector operating >500 GHz is used on Microwave Limb Sounder and up to 200 GHz on numerous Earth-orbiting radiometers. Expanding aperture size should be possible by paneling multiple composite reflectors to create an effective single reflecting surface. 3 aperture size >20m-meters; surface roughness ~30um; deployed volume appropriate for Falcon or Atlas faring. Greater sensitivity and more accurate spatial resolution than is currently achievable. For membrane/mesh solutions in particular, benefits included potentally lower cost (lower mass, stowed volume compared to solid composite) Space-based submillimeter VLBI; single dish FIR-submillimeter missions; FIR flagship and probe missions, as recommended by Astro2020 Years to estimated launch or other schedule driver: 5 – 8 years
Level of complexity (single tech, system of techs, or system of tech systems): System of techs
Level of difficulty (straightforward, stretch, or major stretch): Stretch
Precision timing for space-based astrophysics Precision timing and frequency standards are the beating heart of several types of space missions, including distributed interferometric arrays, probes of fundamental physics, and measurements of exoplanets. These target fundamental science questions in the Cosmic Origins and Physics of the Cosmos programs. Separate missions place priorities on different oscillator characteristics, and ground based projects often merge separate technologies in order to achieve good stability over a range of integration times. To optimize size, weight and power considerations, space missions require compact systems that are the subject of ongoing development. Of considerable interest is the range from 1-1000s where interferometry in particular requires RF standards that have Allen Standard Deviations ("sigma-tau") in the range of 1e-14 and better. This enables submm wavelngth signals received at widely separated space platforms to be coherently combined to produce angular resolutions approaching sub-micro arc second. Several types of systems can deliver state of the art stability over 1-1000s times scales. These include Hydrogen Masers, which achieve sigma-taus of (1s: 7e-14, 10s: 2e-14, 100s: 4e-15, 1000s: 1.5e-15). These have been, or are scheduled to be, flown in space (e.g., ACES, Gravity Probe A). Cryogenic sapphire oscillators (CSO) can deliver an order of magnitude better sigma-tau from 1-100s, but become equivalent to Hydrogen Masers around 1000s; CSOs have not operated in space. Optical comb systems (by e.g., MenloSystems) deliver sigma-tau in the RF on par with the CSO on 1-10 second scales, but then decrease in stability on longer time scales. The comb systems have flown on sounding rockets, but not with the required sigma-tau achieved on the ground. 4 To optimize size, weight and power considerations, space missions require compact systems that are the subject of ongoing development. For submm wavelength interferometric applications, target requirements of 2e-14 @1s, 5e-15 @10s, 5e-16 @ 100s. This technology is enabling for submillimeter wavelength space-based VLBI, which to date has only been achieved on the ground and in space at radio wavelengths. This technology will also enable new investigations into the physics of gravity, dark matter and tests for variations of fundamental constants. Space-based submillimeter VLBI; single dish FIR-submillimeter missions; FIR flagship and probe missions, as recommended by Astro2020 Years to estimated launch or other schedule driver: 5 – 8 years
Level of complexity (single tech, system of techs, or system of tech systems): Single tech
Level of difficulty (straightforward, stretch, or major stretch): Stretch
Far-IR imaging interferometer for high-resolution spectroscopy A technology enabling far-infrared (FIR: 50 microns to 500 microns) imaging at a high angular resolution (~0.1” arcsecond) comparable to ALMA and optical systems with AO is needed. High sensitivity at such angular resolution should be achieved at a sub-Kelvin level in brightness temperature or 1-σ noise < 10-23 W/m2 in 1-hour. A spectral resolution of (λ/Δλ > 100,000) is also needed for identifying line emission and probe gas dynamics in protoplanetary disks and star-forming regions. The Herschel Space Observatory is the current SOTA for the FIR spaceborne telescope that has the capability of imaging astronomical objects. With its 3.5-meter primary reflector, the best angular resolutions are 5” at 70 microns and 37 microns at 500 microns. Another SOTA is BETTII, which is the twin telescope in FIR on a high-altitude balloon. The angular resolution of BETTII is as small as 0.5”, but, practically, it does not obtain images but measures visibilities. Also, the spectral resolution is λ/Δλ ~ 100. A full-solution consists of developing a high-speed data communication, high-precision telemetry, and formation flying system that can combine multiple Herschel-like telescopes into a FIR interferometer. 2 Develop a FIR heterodyne interferometric system with multiple interferometric terminals. The critical subsystems to be developed or improved include the following: expand the high-accuracy telemetry system (e.g., GRACE-FO) to be applied to multiple interferometric terminals (number of terminals >4). Expand the high-speed optical communication links from 1-to-1 links to a 1-to-N link for collecting large data from multiple interferometric terminals to either a data-processing satellite or a ground facility. Develop most efficient orbits for formation flying, maintaining the baseline and the uv-coverage required to science in the Astro2020 decadal report. Measuring the spatial distribution and the dynamics of gas and water in protoplanetary disks has been highlighted as critical science in the Astro2020 decadal report and NASA Strategic Plan 2018. A direct probe of the gas distribution and the snowline in disks requires higher than 0.1” resolution with high spectral resolution in the crucial tracers for the gas and the snowline, singly deuterated hydrogen molecule (HD at 2.7 & 5.3 THz), and water (H2O at 0.5, 1.1, & 1.6 THz), which are not accessible from the ground. Galaxy evolution and black hole science are also key science topics in Astro2020 and require observations of atomic fine-structure lines at 1 – 2 THz at sub-arcsecond resolution. Closing the technology gap described here will bring a major step forward to understand key science by matching the angular resolution of FIR observations to those of optical and radio observations. Far-IR Flagship
Far-IR Probe
The technology discussed here is directly applicable to the mission technology of a future NASA FIR Flagship or FIR Probe. A FIR Probe has been identified by Astro2020 as one of two priorities for the first Probe-class mission competition. Astro220 has identified FIR as “an area where advances in technology and focused objectives can yield transformative science on a moderate-sized platform.”
Years to estimated launch or other schedule driver: 10 for the FIR probe, 15 for FIR flagship
Level of complexity (single tech, system of techs, or system of tech systems): System of tech systems
Level of difficulty (straightforward, stretch, or major stretch): major stretch
High Efficiency, Low Scatter, High and Low Ruling Density, High and Low Blazed Angle, UV Gratings The diverse science cases prioritized by the LUVOIR and HabEx observatories requires a flexible suite of gratings that will push current fabrication techniques. Past major UV spectrographs (HST-COS and HST-STIS) used multi-channel grating wheels to address the different observing requirements. The UV instrument on a “IR/O/UV Flagship” mission would need to follow suit, as demonstrated in the design of LUVOIR-LUMOS. We consider three distinct grating categories that would populate these various channels, each individually contributing to its own relevant science cases:
1) High resolution echelles, with scatter and efficiency constraints not achievable by current mechanical ruling techniques
2) Gratings with holographic solutions that may not be feasible using current recording assemblies
3) Ultra-low blaze angles (< 2°), for high efficiency, low resolution spectroscopy of faint objects.
In order for a LUMOS-type instrument to address all of its stated goals, progress in each of these three categories is required.

Additionally, low-scatter, low-dispersion (R~1000) NUV grating needed to obtain greatly improved low-resolution NUV spectra.
Modern echelle gratings are mechanically ruled (TRL 9). They can suffer from decreased efficiency and prohibitive scatter. KOH etched echelle gratings show improved efficiency and scatter, but still fall short of predictions (TRL 4; Kruczek et al. 2021) and have never been coated with a broadband UV mirror coating.
Low scatter, high efficiency holographically ruled gratings have a rich flight history (TRL 9). Obtaining a blazed facet requires post-processing. The possible grating solutions are limited by the recording wavelengths and assemblies, placing constraints on instrument designs. Adapting existing free-form ruling methods (mechanical or lithographic) could expand the parameter space for designing variable spaced curved grooves gratings that provide uniform point spread functions over the extended fields of view required by a multiobject spectrograph.
Both mechanically ruled and KOH etched gratings can achieve blaze angles below 2° (TRL 3; Underwood et al. 1997 & France et al. 2019). Etched single crystal silicon (Si) gratings should have superior scatter performance but likely need more process development. For both methods, complications arising from coatings filling the grooves requires additional study.

Hubble’s NUV medium-resolution spectrographs have utilized holographic gratings having very low scatter, but Hubble’s low-dispersion spectrographs have used ruled gratings, which suffer from in-band grating scatter of radiation from red sources.
3 The technical goal should include the development and in-vacuum demonstration of several classes of diffraction gratings:
1) Low ruling density, high blaze angle echelle gratings capable of achieving R ≥ 50,000 with ≥ 80% peak order groove efficiency and I/I0 < 10-3 at Δλ = 1 nm post-coating, with supporting simulations predicting the observed performance. This performance should extend through the far ultraviolet (FUV; 90 – 180 nm) bandpass.
2) Blazed, free-form gratings with holographic solutions on curved substrates demonstrating groove efficiencies ≥ 50% post-coating.
3) Ultra-low blaze angle (≤ 2°) gratings demonstrating ≥ 50% groove efficiency, R ≥ 500 post-coating.
4) Demonstrated compatibility with state-of-the-art FUV coating techniques.

Another need is for NUV low-resolution gratings with grating scatter <<few %.
Diffraction gratings that meet these specifications have broad scientific benefits to a future IR/O/UV Flagship instrument both by enabling investigations that have already been identified and providing flexibility to address yet to be identified guest observing goals. High efficiency and low scatter gratings facilitate a faster cadence of observations and provide a more robust signal for muti-object observing. At the highest resolutions, they can provide an unprecedented view of important biosignature tracers. For high sensitivity observations of ionizing radiation escape, CGM emission, and the UV background, low resolution blazed gratings offer ~2x the sensitivity of sinusoidal gratings. These developments are required to realize the promise of MOS for answering the science questions outline in Table I.1 of the Decadal 2020 report Pathways to Discovery – under the Themes of Cosmic Ecosystems, and Worlds & Suns in Context.
An improved understanding of the grating design and fabrication process and how it relates to the final measured efficiency will further provide programmatic benefits in the form of streamlined testing and calibration schedules. This is includes the ability to develop targeted grating solutions without the need to test multiple potential samples.
UV/O/IR Flagship
Any UV spectrograph, from sounding rockets and CubeSat’s to IR/O/UV Flagship. This technology is relevant to COR, PCOS and ExEP. It is also relevant to non-strategic missions, development of which can be brought to bear in support of their maturation for IOU-ST. The blazed Si grating technology could be employed in LYNX and OST type strategic missions.
Years to estimated launch or other schedule driver: Phase A for IR/O/UV Flagship (4-5 years)

Level of complexity: single tech

Level of difficulty: stretch
Key Multi-Object Spectrograph Components: High Efficiency Far-UV Mirror One of the 4 key components required to enable UV-MOS system on a 6-meter telescope is a high reflectance mirror coatings that extend from 3200 Å to 1000 Å with stretch goal of extending to 912 Å and is immune to degradation in the face of high humidity. A number solutions are in the works to enhance the reflectance and improve the stability of LiF/Al mirror coatings in the face of high relative humidity. They involve various forms of enhanced protection in the form of thin layers of MgF2 or AlF3 over coating a thin layer of LiF on freshly deposited (unoxidized) Al. The means of coating can either be direct sputtering, atomic layer deposition (ALD) or reactive Physical Vapor Deposition (rPVD) using XeF2. These coatings have been successfully deposited on small mirrors ~ 50 – 100 mm but have yet to successfully be demonstrated on large optics ~ 0.5 – 1 m.
The XeF2 rPVD process has promise for extending the wavelength sensitivity down to 912 Å and to provide stability in the face of high relative humidity.
Thin ALD of MgF2 (~20 Å) on fresh Al could in principal provide nearly unattenuated Al reflectance(~90% 3200-912Å).
SiC offers 40% reflectance below 1000Å, but its reflectance longward 1000 Å is flat at about 40%.
TRL details:
MgF2/LiF/Al are TRL ~ 5
AlF3/LiF/AlF3/Al are TRL ~ 4.
ALD thin MgF2/Al are TRL ~ 3
SiC is not competitive
3-5 Far-UV coatings – R >=40 % between 912 – 1000 Å, 80 – 90 % between 1000 – 1150 Å, 90% longward of 1150 Å This technology is required to realize the promise of MOS for answering the science questions outline in Table I.1 of the Decadal 2020 report Pathways to Discovery – under the Themes of Cosmic Ecosystems, and Worlds & Suns in Context.

Extending LiF/Al mirror reflectance to below 1000 Å is important to allow use of critical diagnostics for hydrogen, deuterium, C III l977, S VI ll933,944 and molecular nitrogen (0-0) band of c′41Σu+ - X1Σg+ at 958 Å, to name a few.
IR/O/UV Flagship
These technologies are relevant to COR, PCOS and ExEP. They are also relevant to non-strategic missions, which can be brought to bear in support of their maturation for IOU-ST.
Years to estimated launch or other schedule driver: Elimination of the shortwavelength gap is essential to the Great Observatories Mission and Technical Maturation Program.
5 years of support through a combination of a directed testbed and entrepreneurial proof of concept (in small flight programs) will push most of these techs to TRL = 6 or higher.
Level of complexity (single tech, system of techs, or system of tech systems): single tech
Level of difficulty (straightforward, stretch, or major stretch): Individually straightforward given enough support.
High-resolution Direct-Detection Spectrometers for Far-IR Wavelengths Spectrometers in the Far-IR with resolution of <1km/s to resolve emission lines like [OI], HD, [OIII], etc. These spectrometers are in particular important in the wavelength range where direct detection is more sensitive than heterodyne spectroscopy (IR wavelengths shorter than approximately 200 microns). In addition, these spectrometers need to have many spatial pixels as well. Spectrometers using VIPAs and other technology are currently proposed or under development. However, partially the limiting devices are detectors. However, appropriate detectors are listed under item 10. 2 Achieve resolution of <1km/s or R>1E5 with <100 spectral elements (goal of 1000). Being able to simultaneously observe in 100 or more spatial positions. Sensitivity: tbd. Closing this gap would greatly (possibly a factor of 100 to 100) increase the mapping speed of extended Galactic sources or of extra-galactic surveys.    
Mirror Technologies for High Angular Resolution (UV/Vis/Near IR) The capability to resolve the habitable zones of nearby star systems in the UV/Vis/NIR band, with a large space telescope. Monolith: 3.5-m sintered SiC with < 3 µm SFE (Herschel); 2.4-m ULE with ~10 nm SFE (HST); Waterjet cutting is TRL 9 to 14" depth, but TRL 3 to >18" depth. Fused core is TRL 3; slumped fused core is TRL 3 (AMTD); 4-m class Zerodur mirrors from single boules are TRL 4.
Segmented: (no flight SOA): 6.5 m Be with 25 nm SFE (JWST); Non-NASA: 6 DOF, 1-m class SiC and ULE, < 20 nm SFE, and < 5 nm wavefront stability over 4 hr with thermal control
4 Large (4–16 m) monolith and multi-segmented mirrors for space that meet SFE < 10 nm rms (wavelength coverage 400–2500 nm); Wavefront stability better than 10 pm rms per wavefront control time step; CTE uniformity characterized at the ppb level for a large monolith; Segmented apertures leverage 6 DOF or higher control authority meter-class segments for wavefront control.

Sub-gaps that could partially or fully close this gap:
- Mirror Substrate and Structure
- Mirror Positioning Actuators
- Gravity Sag Ofloader
- Coefficient of Thermal Expansion Characterization
- Mirror Polishing
- UV Coatings: Wavefront Effects
This gap is likely to be closed by development of large monolithic or segmented telescopes. Aside from angular resolution, large primary mirrors enhance planet sensitivity due to reduction in science integration time with greater collecting areas and throughput enabling probing of a larger number of more distant stars’ habitable zones and improved spectral resolution. IR/O/UV Great Observatory Demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the mid-to-late 2020’s.
Coronagraph Contrast and Efficiency The capability to suppress starlight and receive planet light with a coronagraph to the level needed to detect and spectrally characterize Earth-like exoplanets in the habitable zones of Sun-like stars. unobscured pupil: 4×10-10 raw contrast at 10% bandwidth, angles of 3-15 λ/D (Lyot coronagraph demo in HCIT); obscured pupil: 1.6×10-9 raw contrast at 10% bandwidth across angles of 3-9 λ/D (Roman CGI Lab Demos); segmented/unobscured pupil:2.5×10-8 raw contrast in monochromatic light across 6-10 λ/D (Lyot coronagraph demo in HiCAT) 3 Maximized science yield in imaging and spectroscopy for a direct imaging telescope/mission. ≤ 10-10 raw contrast, >10% throughput, inner working angle ≤ 3 λ/D, outer working angle >= 45 λ/D [TBD], 20% bandwidth; obscured/segmented pupil

For the two distinct cases of monolith and segmented primary mirrors, Sub-gaps that could partially or fully close this gap:
- Coronagraph Architecture
- Deformable Mirrors
- Computational Throughput on Space-rated processors
- High bandwidth optical communcation between space and ground
- Coronagraph Efficiency
- Autonomous onboard WFSC architectures
This gap is likely to be closed by improvements in coronagraph masks and optics, wavefront sensing and control (deformable mirrors), data post-processing, and integrated models. IR/O/UV Great Observatory;, or any other coronagraph-based exoplanet direct-imaging mission. Demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the mid-to-late 2020’s.
Coronagraph Stability The capability to maintain the deep starlight suppression provided by a coronagraph for a time period long enough to detect light from an exo-Earth. RST CGI demonstrated ~10-8 contrast in a simulated dynamic environment using LOWFS (which obtained 12 pm focus sensitivity)
SIM and non-NASA work has demonstrated nm accuracy and stability with laser metrology
Capacitive gap sensors demonstrated at 10 pm
80 dB vibration isolation demonstrated
Gaia cold gas microthrusters and LISA pathfinder colloidal microthrusters can reduce vibrations
3 Contrast stability on time scales needed for spectral measurements (possibly as long as days). Achieving this stability requires an integrated approach to the coronagraph and telescope, possibly including wavefront sense/control, metrology and correction of mirror segment phasing, vibration isolation/reduction
This stability is likely to require wavefront error stability at the level of 10-100 pm per control step (of order 10 minutes).

Sub-gaps that could partially or fully close this gap:
- Ultra-stable Telescope
- Integrated Modeling of Telescope/Coronagraph system
- Disturbance Reduction and Observatory Stability
- Wavefront Sensing (low-order and out-of band)
- Laser Gauges for Metrology
- Segment Relative Pose Sensing and Control
- Thermal Sensing and Control
- Wavefront Sensing and Control Algorithms
- Observatory Pointing Control
This gap is likely to be closed by a combination of many factors in a coronagraph/observatory system, including active wavefront control at the coronagraph level, thermal control, active and passive ultra-stable structures, and disturbance isolation/ reduction. Integrated modeling for tracability to flight environments is likely to be a key capability to close this gap. IR/O/UV Great Observatory; or any other coronagraph-based exoplanet direct-imaging mission. Demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the mid-to-late 2020’s.
Vis/NIR Detection Sensitivity The capability to detect single photons in the Vis and NIR to enable imaging and spectroscopy of Earth-like exoplanets.
Vis: 1k×1k silicon EMCCD detectors provide dark current of 7×10-4 e-/px/sec; CIC of 0.01 e-/px/frame; zero effective read noise (in photon counting mode) after irradiation when cooled to 165.15 K (Roman); 4k×4k EMCCD fabricated but still under development

NIR: HgCdTe photodiode arrays have read noise ≾ 2 e- rms with multiple nondestructive reads; 2k×2k format; dark current < 0.001 e-/s/pix; very radiation tolerant (JWST), high QE down to 750nm;HgCdTe APDs demonstrated dark current ~10–20 e-/s/pix, RN << 1 e- rms and 1k×1k format

Cryogenic superconducting photon-counting, energy-resolving detectors (MKID,TES): 0 read noise/dark current; space radiation tolerance not systematically studied; <1k×1k format
4 Near IR (900 nm to 2.5 μm) and visible-band (400-900nm) extremely low noise detectors for exo-Earth spectral characterization with spectrographs or intrinsic energy resolution. NIR Read noise << 1 e- rms, dark current noise < 0.001 e-/pix/s, Vis band read noise < 0.1 e- rms; CIC < 3×10-3 e-/px/frame; dark current < 10-4 e-/px/sec , functioning in a space radiation environment over mission lifetime (5-10 years); may need large ≥ 2k×2k format

Sub-gaps that could partially or fully close this gap:
- NIR Low-noise Detector
- UV/VIS Low-noise Detector
- Rad-Hard, High-QE, Energy Resolving, Noiseless Single Photon Detector Arrays for the NIR, VIS, and UV
Single-photon counting detectors in the Vis/NIR bands allow characterization of very faint objects, including exo-Earths. IR/O/UV Great Observatory Demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the late 2020’s.
Detection Stability in Mid-IR The capability to detect mid-infrared light with ultrastable detectors to carry out transit spectroscopy of terrestrial exoplanets in the Habitable Zone of M-dwarf stars. JWST/MIRI is expected to achieve 10-100 nm transit stability.
Spitzer IRAC Si:As detector data have demonstrated about 60 ppm precision in transit observations of several hours
  Ultrastable detectors (< 10 ppm over 5 hours) for the mid-infrared band (7 - 20 microns) enabling transit spectroscopy of rocky exoplanets in the Habitable Zone of M-dwarfs.   A mid-IR transit spectroscopy mission, potentially the Far-IR flagship or probe, explorer concept. Need to demonstrate TRL 6 by mission PDR.
Starshade Deployment and Shape Stability The capability to deploy on-orbit a starshade that is stowed in a launch vehicle fairing to a precise shape, and to maintain that shape precision during all operational environments. Manufacturing tolerance (≤ 100 µm) verified with low fidelity 6 m prototype. Petal deployment tests conducted on prototype petals to demonstrate rib actuation.

Petal deployment tolerance (≤ 1 mm) verified with low fidelity 12 m prototype; limited environmental testing.
4 A system that will deploy the petals from a launch-stowed configuration to the needed shape (to better than ≤ 1 mm (in-plane envelope) and maintain petal edges to ≤ 100 micron (in-plane tolerance profile for a 7 m petal on the 34 m-diameter starshade; tolerances scale roughly linearly with starshade diameter), and be optically opaque.

Performance goals are under re-evaluation for the IROUV Great Obsevatory. Overall starshade diameter likely to be > 50m.
The capability to stow, survive launch, and deploy the petals and opaque inner disk of the starshade to within the deployment tolerances budgeted to meet the shape, and thus the contrast requirements. The shape must be maintained within a stability envelope to enable imaging and spectroscopy of Earth-like exoplanets. IR/O/UV Great Observatory; other future starshade missions TRL 6 needed by mission PDR.
Starshade Starlight Suppression and Model Validation The capability of a starshade to suppress diffracted on-axis starlight and scattered off-axis Sunlight to levels needed to characterize Earth-like exoplanets. The capability to experimentally validate model of the starshade’s optical performance at subscale. 1e-10 contrast at inner working angle demonstrated over 10% bandpass using 24 mm starshade in Princeton testbed with F = 13. Validated optical model with demonstrated 1e-6 suppression at white light, 58 cm mask, and F = 210. Optical model validated to within a factor 2 at 1e-8 contrast at F=13.

Etched amorphous metal edges with anti-reflection coating meet scatter specs with margin; integrated in-plane shape tolerance is to be demonstrated.
4 Experimentally validate at flight-like Fresnel numbers (F) the equations that predict starshade starlight contrast: total starlight contrast <=1e-10 in a scaled flight-like geometry, F between 5 and 40, across a broad UV/optical/IR bandpass. Contrast model accuracy validated to better than 25%.

Limit edge-scattered sunlight and diffracted starlight with optical petal edges that simultaneously meet scatter requirements and in-plane shape tolerances. Limit solar scatter lobe brightness to better than visual magnitude (V) ~26.

Performance goals are under re-evaluation for the IROUV Great Obsevatory.
Limit edge-scattered sunlight and diffracted starlight with optical petal edges that simultaneously meet scatter requirements and in-plane shape tolerances. Limit solar scatter lobe brightness to better than visual magnitude (V) ~26
The starshade must create contrast levels to better than 10-10 in the image plane with shape tolerances specified by an error budget. The precise positioning and manufacture of the starshade edges minimize the diffraction of on-axis starlight and scatter/diffraction of off-axis Sunlight detected at the science focal plane. The starlight suppression capabilities of the starshade must be demonstrated on the ground to validate optical models and the error budget, which are used to predict performance in a space environment. IR/O/UV Great Observatory; other future starshade missions TRL 6 needed by mission PDR.
Stellar Reflex Motion Sensitivity: Extreme Precision Radial Velocity Capability to measure exoplanet masses down to Earth-mass. Ground-based RV: state-of-the-art demonstrated stability is currently 28 cm/s over 7 hours (VLT/ESPRESSO).
Laser frequency combs demonstrated on ground-based observatories with correct mode spacing, non-NASA work is advancing miniaturization. Fiber laser-based optical frequency combs demonstrated on sounding rocket though with closer line spacing than useful for RV.
3 Capability to measure exoplanet masses down to Earth-mass. The radial velocity semi-amplitude of a Solar-mass star due to an orbiting Earth-mass planet at 1 AU is 9 cm/s.
Technology to make radial velocity mass measurements may include using a space-based instrument to avoid atmospheric telluric lines and simultaneous measurements of stellar lines across a broad band (both Vis and NIR). Stability of the instrument and its absolute calibration must be maintained on long time scales in order to enable the measurement.
Theoretical understanding of astrophysical noise sources (stellar jitter) and how to mitigate them.

Sub-gaps that could partially or fully close this gap:
- Detectors for high-resolution, cross-dispersed spectrographs
- High-Precision, High-Throughput, High-Spectral Resolution Dispersive Optics
- Advanced Photonics for extreme-precision radial velocity spectroscopy
- Visible-light Adaptive Optics
- Precision calibration for extreme-precision radial velocity spectroscopy
Interpretation of spectra of Earth-like exoplanets in reflected light requires the measurement of mass. IR/O/UV Great Observatory If a need for space-based RV is identified, then need TRL 6 in the late 2020s. If ground-based supporting role is identified, then capability is needed well ahead of launch in early 2030s.
Stellar Reflex Motion Sensitivity: Astrometry Capability to measure exoplanet masses down to Earth-mass. GAIA preliminarily achieved 34 micro arcsecond error but ultimately could achieve 10 microacrseconds on bright targets after all systematics are calibrated
Demonstration (Bendek) of diffractive pupil showed 5.75×10-5 l/D or 1.4 microarcsecond on a 4m telescope (limited by detector calibration)
Preliminary study of 1-m space telescope and instrument with in-situ detector calibration can achieve 0.8 micro arcsecond in 1 hr
3 Astrometric detection of an exo-Earth at 10pc requires 0.1 microarcsecond uncertainty.
Technology with the stability neeed to make astrometric measurements to this level, possibly requiring detector metrology and/or diffractive pupils
Theoretical understanding of astrophysical noise sources (star spots) and prospects for mitigating them.
Interpretation of spectra of Earth-like exoplanets in reflected light requires the measurement of mass. IR/O/UV Great Observatory If a need for a precision astrometry mode in the IR/O/UV Great Observatory is identified, then demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the late 2020’s.
UV Detection Sensitivity The sensitivity to perform imaging spectroscopy of exoplanets in the ultraviolet. Lab: Micro-channel Plates (MCP): 0 read noise, 90 – 300 nm, spurious count rate 0.05 - 0.5 counts/cm2/s; QE 20-45%; resolution element size 20 mm. EMCCD: 0 read noise, dark current > 0.005 e-/res/hr; QE 30-50%; resol. el. size 20 mm
Flight: HST HRC: In relevant UV band (250 nm): QE 33%, read noise 4.7 e-, dark current 5.8×10-3, 1024×1024 format
3-4 Low-noise ultraviolet (200-400 nm) detectors to characterize exoplanets with an imaging spectrograph.
Read Noise: 0 e-; Dark Current: 0 e- /resolution/s; Spurious Count Rate: < 0.05 counts/cm2/s; QE: 75% ; Resolution size ≤ 10 mm; Tolerant to space radiation environment over mission lifetime.
Enables UV coronagraphy and/or UV spectroscopy with a starshade. IR/O/UV Great Observatory Demonstration of feability and as much risk reduction as possible prior to mission formulation. TRL 6 in the late 2020’s.
Fast, Low-noise, Megapixel X-ray Imaging Arrays with Moderate Spectral Resolution Probe and Great Observatory both require X-ray imaging arrays covering wide fields of view (≥ 60 x 60 mm) with excellent spatial resolution (i.e. <16 μm pixels, or equivalent X-ray position resolution), and moderate spectral resolution (comparable to modern scientific CCDs).
These detectors must have good detection efficiency across the soft X- ray band pass, 0.2-12 keV, and excellent detection in the low-energy (0.2 – 1 keV) end of this band pass is essential. Therefore, optical blocking filters with minimal attenuation of soft X-ray will also be required.
These strategic X-ray missions feature large collecting area (10x to 30x Chandra) , so fast frame rates (i.e. > 20 -100 frame/s) are required to minimize pile-up, reduce non-X-ray background, and maximize time resolution.
The Lynx High-Definition Imager Technology Roadmap (https://www.lynxobservatory.com/blog/roadmaps) identifies three candidate technologies, all judged to be TRL 3 for Lynx. Despite technical progess since completion of that roadmap, no sensor technology has yet achieved TRL 4. Current CCDs provide excellent noise performance but frame rates need to be increased and application-specific readout circuits must be demonstrated. Hybrid and monolithic CMOS silicon active pixel sensors (APS) currently provide high frame rates but hybrid sensors need lower noise and monolithic sensors need thicker depleition layers. Further work is needed on these technologies to meet all requirements simultaneously. 3 Probe and Great Observatory requirements are nearly identical. The only differences are that the Probe may require somewhat lower frame rates (20 vs 100 frames/s) for full field of view, while the Great Observatory requires higher frames rates in a subfield on axis (10,000 vs 100 fr/s in a sub-field on axis,) . Common requirements include:
• Large format X-ray detectors with sufficient spatial resolution so as not to compromise the imaging performance of the optics (notionally with 0.5” half power diameter, HPD, requiring ≤ 16 μm pixels for both AXIS-like and Lynx-like missions);
• Multi-chip abuttablility to build detector surface approximating best focal surface for the mirrors;
• Roughly Fano-limited spectral resolution in the 0.2-12 keV energy band, e.g. 70 eV FWHM at 0.3 keV for Lynx-like mission;
• Sensor-specific fast, low-noise, low-power readout circuitry;
• Optical-blocking filters with minimal X-ray absorption above 0.2 keV;
• Radiation hardness supporting 5-20 years of science operations in their respective orbirts (e.g., low-inclincation LEO for Probe, L2 for Great Observatory).
Enables an X-ray a strategic mission meeting Astro-2020 objectives ( complementing ATHENA, with high-spatial and/or high spectral resolution) to be launched in this decade. Raises technical readiness, increases scientific capability , and potentially reduces risk, instrument power and mass requirements and/or cost, for a range of smaller missions including Medium and Small Explorers and Small Satellites, as described below. X-Ray Probe
X-Ray Flagship
In addition to Probe- and Great Observatory-class missions, wide-field time-domain X-ray monitoring missions as recommended by Astro 2020, (p 197) for Missions of Opportunity/SmallSat, Small or Medium Explorers or missions in a dedicated Time-Domain Astrophysics Program, e.g. Joint Astrophysics Nascent Universe Satellite (JANUS) / X-ray Time Domain Explorer (XTiDE) -like, or any other focused X-ray optics, or coded-aperture wide-field X-ray-monitoring, or X-ray-grating mission (larger than an Explorer but less than than half the cost of a Probe). Any future mission with X-ray optics for imaging and/or gratings for high-resolution spectroscopy.
Years to estimated launch or other schedule driver: ≲8 years to launch of probe-class X-ray mission
Level of complexity (single tech, system of techs, or system of tech systems): System of technologies;
Level of difficulty (straightforward, stretch, or major stretch): Straightforward/Stretch; TRL 4-to-5 Advancement Degree of Difficulty 5 per Lynx HDXI technology roadmap.
High-Efficiency X-ray Grating Arrays for High-Resolution Spectroscopy Light-weight, high-efficiency (> 45%), large-format X-ray grating arrays enable high spectral resolving power R ~ 7500 in the soft X-ray band (~ 0.2 - 2 keV) for absorption- and emission-line spectroscopy using large-area X-ray telescopes. These would provide the resolving power needed to address key science goals in the soft X-ray band, such as studying the physical state of baryons in galactic halos and in the Cosmic Web, detailing matter and energy feedback from supermassive black holes (SMBH), and characterizing stellar lifecycles from birth to death. Proven technologies (grating spectrometers on Chandra and X-ray Multi-mirror Mission-Newton, XMM-Newton) fall short in efficiency, collecting area, and resolving power, by factors of 5-10. There are two technology candidates with potential to meet strategic spectrometer performance: Off-plane reflection gratings (OPG) and Critical-Angle Transmission (CAT) gratings. High-efficiency blazed (sawtooth groove profile) OPGs have been demonstrated that place > 50% of the incident soft X-ray light into the diffracted orders. Separately, individual laminar groove profile OPGs have demonstrated R > 2200-4500. Recently, a pair of aligned CAT gratings has achieved R ~ 12,000. Over 40% diffraction efficiency has been demonstrated with CAT gratings, but only over a narrow band. Both technologies have been vetted at TRL 4 in 2016, but TRL 4 has been questioned for Lynx performance. Currently no gratings exist that perform at a strategic level, simultaneously providing reasonably large form factor, and the required high diffraction efficiency over the whole grating area and high enough resolving power.
References:
“Manufacture and Performance of Blazed Soft X-ray Transmission Gratings for Arcus and Lynx,” R. K. Heilmann et al., Proc. SPIE 11822, 1182215 (2021). --- “Toward Volume Manufacturing of High-Performance Soft X-ray Critical-Angle Transmission Gratings,” R. K. Heilmann et al; Proc. SPIE 11444, 114441H (2020).
“Demonstration of Resolving Power λ/Δλ > 10,000 for a Space-Based X-ray Transmission Grating
Spectrometer,” R. K. Heilmann et al., Appl. Opt. 58, 1223 (2019). --- “Large-format X-Ray Reflection Grating Operated in an Echelle-like Mounting,” C. T. deRoo et al., Ap. J. 897, 92 (2020). --- “ Performance Testing of a Large-Format X-ray Reflection Grating Prototype for a Suborbital Rocket Payload,” B. D. Donovan et al., J. Astron. Instrum. 9(3), 2050017 (2020). --- Lynx Concept Study Report: https://wwwastro.msfc.nasa.gov/lynx/docs/LynxConceptStudy.pdf. --- “Fabrication and Diffraction Efficiency of a Large-format, Replicated X-ray Reflection Grating,” D. M. Miles et al., ApJ, 869, 95 (2018). --- “Reflection grating concept for the Lynx X-ray Grating Spectrograph,” R. L. McEntaffer, JATIS, 5(2), 021002 (2019).
3-4 Demonstrate CAT gratings and OPGs with high efficiency (>45%) and resolving power (> 7500) with size > 50 x 50 mm2. CAT grating open area (illuminated by x rays and not blocked by support structures) > 70%. This requires a scalable (many hundreds of gratings), large-area fabrication process for ~ 6 mm deep, ~ 40 nm wide grating bars and narrow support structures. OPGs require fabrication processes for modestly blazed grooves (~30-50°) in radial groove patterns that can be replicated onto thin substrates. Alignment into large arrays needs to be demonstrated. Large, high-efficiency x-ray grating arrays enable sensitive, high-resolution soft x-ray absorption and emission line spectroscopy that cannot be achieved with other technologies. Priority science goals for soft X-ray spectroscopy include studying the physical state of baryons in galactic halos and in the Cosmic Web, detailing matter and energy feedback from SMBH, and characterizing stellar lifecycles from birth to death. CAT gratings operate at near-normal incidence and are thin, light weight, and alignment insensitive. OPGs are highly efficient and can be mass produced through replication. High efficiency reduces required mirror aperture and grating array size. Larger gratings lead to reduced fabrication, testing, and assembly cost. The technology is mission enabling for an x-ray probe complementary to ATHENA, and mission enhancing for a high spatial and spectral resolution X-ray strategic mission. X-ray Flagship
X-ray Probe to complement the ATHENA mission and a high spatial and spectral resolution X-ray strategic mission with some of the capabilities of the proposed Lynx. Soft x-ray grating Explorers such as Arcus. Broadband soft x-ray polarimetry such as REDSoX or GOSoX. Laboratory astrophysics (improved laboratory data on transition energies, electron impact ionization collision strengths, photoexcitation, and ionization).
Years to estimated launch or other schedule driver: 7-15
Level of complexity (single tech, system of techs, or system of tech systems): system of technologies
Level of difficulty (straightforward, stretch, or major stretch): straightforward/stretch
High-resolution, Lightweight X-ray Optics Needed is a technology capable of manufacturing an X-ray mirror assembly meeting two requirements simultaneously: (1) angular resolution better than 1 arcsec half-power diameter (HPD) for X rays in the 10-200 keV band and better than 0.5 arcsec for 0.1-10 keV X rays; and (2) mass per unit mirror surface area less than 3 kg/m2. for an X-ray Flagship, but possibly up to 10 kg/m2 for an X-ray Probe. In other words, compared to Chandra’s technology, this one must achieve the same 0.5” angular resolution or close to it, while reducing the mass by up to an order of magnitude. In addition, this technology must be scalable to at least 2 m2 effective area, and amenable to mass production such that the production cost per unit mirror area must also be reduced commenserately from that of Chandra so that future missions like an X-ray Flagship, which requires up to 20 times more mirror surface area, will also be financially feasible. The development path should include empirical demonstrations, building progressively more substantial sub-assemblies to show the feasibility of a complete process for building a mirror assembly required by missions like an X-ray Flagship or Probe. High Resolution Chandra optics:
- Angular resolution <~ 0.5 arcsec;
- Effective area 750 cm2 at 1 keV; and
- Mirror mass 951 kg.
The Lynx Technology Roadmaps (https://www.lynxobservatory.com/blog/roadmaps) identifies three candidate mirror technologies. As of December 2021, of the three candidate technologies, the silicon meta-shell technology has shown that it is possible to achieve sub-arcsecond image quality with lightweight mirrors (areal density less than 2 kg/m2). (W.W. Zhang, PCOS Annual Technology Report, 2020). The associated mirror alignment and bonding techniques need further development to meet both precision and structural strength requirements.
The low TRL is mainly due to a lack of empirical demonstration of “substantial” sub-assemblies. The advancement of TRL is characterized by two parameters: angular resolution or PSF, and the “size” of the sub-assembly relative to the final assembly required for Lynx or AXIS.
3-4 The technical objectives are to meet the following performance requirements:
1. Better than 1” HPD angular resolution for 10-200 keV and better than 0.5" HPD for 0.1-10 keV, as measured with full X-ray illumination
2. Less than 3 kg/m2 mass per unit mirror surface area (though up to 10 kg/m2 may be acceptable for some applications)
3. Scalable up to at least 2 m2 effective area
This technology will enable next generation X-ray astronomical missions in the 2020s, 2030s, 2040s, and 2050s. In particular it will enale missions like Lynx and AXIS which will scientifically and observationally complement JWST, Roman, ATHENA, and many other missions. It will retire the most important technical and programmatic risks of those missions, insuring credible and reliable cost estimates for implementing those missons. X-Ray Probe
X-Ray Flagship
A scalable X-ray mirror technology would be applicable to missions of all sizes, from flagship missions like Lynx, to Probe missions like AXIS, TAP, and HEX-P, to Explorer missions like STAR-X and Arcus, and to sounding rocket experiements like OGRE.
Years to estimated launch or other schedule driver: < 8 years to support a Probe mission as receommended by Astro-2020. This means that the technology development must be ramped up as soon as possible to enable NASA to start the implementation of a Probe mission in less than 3 years.
Level of complexity (single tech, system of techs, or system of tech systems): single technologies that can be developed individually and in parallel; then they must be implemented together to make the mirror assembly to form a system of technologies.
Level of difficulty (straightforward, stretch, or major stretch): Straightforward to stretch.
Large-format, high-spectral-resolution, small-pixel X-ray focal plane arrays The X-ray Great Observatory will require X-ray imaging arrays covering wide fields of view (≥ 15 x 15 mm) with excellent spatial resolution (i.e. £ 25 μm pixels, matching future X-ray optics (<~ 1 arcsec) and spectral resolution (< 2 eV for energies < 7 keV).
Different regions of the array should be optimized for different types of measurements in a hybrid configuration. The innermost region should have the smallest pixels with an energy range extended to > 7 keV. Another region should be optimized for energies less than 1 keV to provide the greatest possible spectral resolution (R>~ 2,000) at these energies.
These detectors must have good detection efficiency across the soft X- ray band pass, from 0.2-12 keV, and excellent efficiency in the low-energy (0.2 – 1 keV) end of this band pass is essential. Therefore, optical blocking filters with minimal attenuation of soft X-ray will also be required.
The Lynx X-ray Microcalorimeter Technology Roadmap (https://www.lynxobservatory.com/blog/roadmaps) identifies two candidate detector technologies. Transition-edge sensors (TES) (detectors only) are judged to be at TRL-4 for Lynx, and Magnetic Micro-calorimeters (MMCs) are judged to be TRL 3 for Lynx. Further work is needed on these technologies to meet all TRL-5 requirements simultaneously.
1. S. J. Smith et al., “Toward 100,000-Pixel Microcalorimeter Arrays Using Multi-absorber Transition-Edge Sensors,” J Low Temp Phys, vol. 199, no. 1–2, pp. 330–338, Apr. 2020, doi: 10.1007/s10909-020-02362-0.
2. S.J. Smith et al., “Multi-absorber transition-edge sensors for X-ray astronomy”, Journal of Astronomical Telescopes, Instruments, and Systems 5(2), 021008 (Apr–Jun 2019), special ed. on The Lynx X-ray Observatory, 2019.
3. F. T. Jaeckel et al., “Calibration and Testing of Small High-Resolution Transition Edge Sensor Microcalorimeters With Optical Photons”, IEEE Trans. On Appl. Sup. 31(5), 2100305 (2021).
4. A.M. Devasia et al., “Large-scale metallic magnetic calorimeter arrays for the Lynx X-ray Microcalorimeter”, accepted for publication in the Journal of Low Temperature Physics, (2021).
Readout using microwave resonators, such as with microwave SQUIDs are needed to read out these detectors. The desired multiplexing factor is ~ 400 to 1000:1 for TESs and MMCs, with the multiplexing factor limited by the slew rates of the various different sub-array designs.
4 P Parameter | R Requirement |
T Technical Requirement


    Main Array:
P Energy Range
R 0.2-7 keV for 3 eV

P Field of View
R ≥ 5 arcmin´ 5 arcmin
T ≥ 300´ 300 pixels ~100k pixels

P Pixel size
R ≤ 1 arcsec´ 1 arcsec
T ≤ 50 µm´ 50 µm

P Energy Resolution
R 3 eV (FWHM)

    Enhanced Main Array:
P Energy Range
R 0.2-7 keV for 3 eV

P Field of View
R ≥1 arcmin´ 1 arcmin
T ≥ 120´ 120 pixels ~14.4k pixels

P Pixel size
R ≤ 0.5 arcsec´ 0.5 arcsec
T ≤ 25 µm´ 25 µm

P Energy Resolution
R ≤ 2 eV (FWHM)

    Ultra-Hi-Resolution Array:
P Energy Range
R 0.2-0.75 keV

P Field of View
R ≥1 arcmin´ 1 arcmin
T ≥ 60´ 60 pixels ~3.6k pixels

P Pixel size
R ≤ 1 arcsec´ 1 arcsec
T ≤ 50 µm´ 50 µm

P Energy Resolution
R 0.3 eV (FWHM)
Enables an X-ray GO strategic mission meeting Astro-2020 objectives by raising technical readiness, increases scientific capability, and potentially reduces risk, instrument power and mass requirements and/or cost. It also enables a range of smaller missions including a Medium Explorers, as described below. In addition the X-ray Great Observatory mission, X-ray microcalorimeters could potentially be used on a variety of different future X-ray missions. e.g. The Light Element Mapper LEM (A. Ogorzalek. Probing physics of galaxy formation with a wide-field X-ray microcalorimeter, to be presented at the Nineteenth Divisional Meeting of the HEAD in Pittsburg, PA, 2022). LEM is a potential Probe or MIDEX concept. Opportunities might also exist for their use in the Time-Domain Astrophysics Program (larger than an Explorer but less than than half the cost of a Probe). Critical technologies need to be demonstrated in order to have high degree of confidence in schedule and estimated cost of the X-ray Great Observatory prior to its build-up scheduled for the latter half of this decade. It is imperitive to have high-TRL demonstrations in order for cost estimates to be similar to current estimated grass-roots cost estimates, and be within forseen cost-cap of the X-ray Great Observatory. This has been demonstrated very clearly through the review process of the two most recent Decadal Surveys. For these demonstrations, a full cryogenic detector system is needed comprising of a system of technologies that includes a detector, cold readout electronics, and warm readout electronics. This is prior to the program of development needed in the latter part of the decade where a flight-like prototype will be needed integrating a flight-like focal plane assembly, cooling system (cryocoolers and ADRs), and cryostat.
Advanced Millimeter-Wave Focal-Plane Arrays for CMB Polarimetry This is an update to the 2019 gap "Advanced Millimeter-Wave Focal-Plane Arrays for CMB Polarimetry"
A CMB Probe requires arrays of detectors with system noise temperatures near those of the sky (CMB + foregrounds), dual-polarization detection capability, and control of systematic errors at multiple frequencies between ~10 and ~1000 GHz for foreground removal (see attachment). Architectures must be scalable to large arrays for the requisite sensitivity. Simultaneous multiband operation, high multiplexing factors, and efficient detector and readout focal-plane packaging are necessary design characteristics. Detector systems must be compatible with the space environment. This includes low dielectric exposure to low-energy electrons and robust performance in the presence of cosmic rays. Continued deployment in ground-based and balloon-borne platforms will likely benefit development efforts; however, the performance requirements (see Technical Goals) on detectors for space are much more stringent than those on detectors for ground and balloon experiments, and additional development is mandatory.
Current ground-based experiments hope to reach upper limits on the tensor to scalar ratio of about 0.01. The Astro2020-recommended ground-based CMB-S4 experiment hopes to constrain r £ 0.001 (95%), with frequency bands from 20 to 270 GHz. The JAXA space mission LiteBIRD also aims to constrain r £ 0.001 (95%), with frequency bands from 40 to 400 GHz (although simulations done by CMB-S4 show that 40 GHz is not low enough to deal with synchrotron foregrounds effectively). The CMB Probe will aim to constrain r £ 0.0001 (95%), requiring total errors in the maps, including systematics and foreground residuals, at a level of about 1 nanoK per pixel. No detector system existing or being built has demonstrated or simulated this level of performance. 3 Frequencies 10-1000 GHz, in large arrays at most frequencies
Total system noise levels <2 x sky level (CMB = 2.7 K; foregrounds vary with frequency)
Sum of all systematic errors <10% of total signal
Insensitive to cosmic rays and other radiation
Excellent gain/noise stability
Reduced integration complexity, and specifically reduced number of wire interconnects in arrays
A further explanatory note was attached, regarding the necessary wavelengths
Detection of B-mode polarization in the CMB and hence a gravitational wave background would reveal the energy scale of inflation. Stringent upper limits on B-modes would strongly reduce the allowed variety of models of inflation. Excellent measurements of B-modes due to lensing would afford unique determination of neutrino masses. Probe #2 (CMB probe) is the principal target, but improved detectors from 10-1000 GHz could be used for Explorers aimed at other science areas. Years to estimated launch or other schedule driver: Proposals for Probe #2 will be solicited around 2030, if NASA follows the Astro2020 recommendation of one Probe mission per decade.
Level of complexity (single tech, system of techs, or system of tech systems): System of techs, as over this wide a frequency range there are multiple aspects to the detector systems that have to work together.
Level of difficulty (straightforward, stretch, or major stretch): Stringent requirements on systematic effects will require extensive and realistic simulations, and will probably be harder to meet than raw sensitivity requirements. As a result, this development is not straightforward, but rather stretch.
Optical Blocking Filters for X-ray Instruments Future EUV and X-ray missions will have large effective area optics and silicon-based detectors on the focal plane. These large X-ray optics will focus optical photons as well as the photons of interest, causing the silicon-based detectors to be swamped by an optical photon background, deteriorating their signal-to-noise performance. A new generation of filters is needed to block these undesirable photons. A stack of these filters, located at various temperatures, needs to be able to block optical, UV, and infrared photons from being absorbed by the detector array. They need to be optimized as thin as possible to allow for maximum transmission at low energies, down to 0.2 keV. The method used to attenuate the undesirable photons needs to offer high transmission of the target photons, good optical photon attenuation, high throughput, and relative immunity to contamination issues. Current state-of-the-art optical blocking filters (OBFs) fall into two categories: Directly deposited and free standing. Directly deposited filters are deposited onto the surface of the detector. They can be thin and don't require a support structure, but are as cold as the detectors. Contaminants used in the payload are attracted to the coldest surface, so a cold filter will not prevent detector contamination buildup. Directly deposited filters have been used on RGS on XMM-Newton (TRL 9) and examples suitable for future EUV and X-ray missions are currently under development (TRL 5). The full solution technology is based around lithographic support meshes that have a high transparency (> 90%) and thin Al films (50 nm). They are a new technology that has had minimal testing and so has a TRL of 3. Free standing filters of the general type have successfully flown on Hitomi (TRL 9). For future EUV and X-ray missions, higher transmission is desired for soft X rays requiring thinner aluminum and polyimide, thinner than has so far been developed or demonstrated. The filters also need to be significantly larger (> 6 cm). Thus for the full solution, the TRL is 3, with the principle having been demonstrated experimentally. The waveguide cut-off filter option has been demonstrated for 15 micron holes, but not at small enough holes for this application (~1-2 microns).
- Ryu, K. K. et al. Development of CCDs for REXIS on OSIRIS-REx. Proc. SPIE 9144, 91444O (2014).
- Bautz, M., et al. Directly-deposited blocking filters for high-performance silicon X-ray detectors. Proc. SPIE 9905, 99054C (2016).
- Brinkman, A. C. et al. The Refection Grating Spectrometer on board XMM. in SPIE EUV, X-ray and Gamma Ray Instrumentation for Astronomy 2808, 463–480 (1996).
- den Herder, J. et al. The reflection grating spectrometer on board XMM-Newton. Astron. Astrophys. 365, L7–L17 (2001).
- Pollock, A. M. T. Status of the RGS Calibration. XMM-Newton Users Group, European Space Astronomy Centre, Villanueva de la Canada, Madrid, Spain (2008). doi:10.1002/hed.21900
- Chandra X-ray Observatory. HRC Calibration Information. HEX IPI Team, CXC Calibration Team, cfa Harvard (2014). At http://cxc.harvard.edu/cal/Hrc/detailed_info.html#uvis_trans
Free-standing filters are held in-front of the detectors. This removes them from the detector so they can be warm (reducing contamination issues). However, they require a support structure, often have a polyimide structure to support the blocking material, and are thicker than directly deposited filters (>100 nm Al) which all affect instrument effective area. Free-standing filters have been used on Chandra, XMM-Newton, and Hitomi (TRL 9), but filters on highly transmissive frames are at TRL 4 or 5.
- McCammon, D. et al. The X-ray quantum calorimeter sounding rocket experiment: Improvements for the next flight. J. Low Temp. Phys. 151, 715–720 (2008).
- Takahashi, T. et al. The ASTRO-H Mission. in SPIE Space Telescopes and Instrumentation 7732, 18 (2010).
- Koyama, K. et al. X-Ray Imaging Spectrometers (XIS) on Board Suzaku. Publ. Astron. Soc. Japan 59, 23–33 (2007).
- Chandra X-ray Center. The Chandra Proposers’ Observatory Guide. Chandra Project Science, MSFC Chandra IPI Teams, Version 14.0 (2011). http://cxc.harvard.edu/proposer/POG/html/chap6.html
- Gastaldello, F. et al. Status of the EPIC thin and medium filters on-board XMM-Newton after more than 10 years of operation : II - analysis of in-flight data. in Proceedings of the SPIE 8859, 885914 (2013).
2-3 Optical blocking filter technology needs to be advanced along both categories of filters. Directly deposited OBFs have, to date, consisted entirely of a thin film of Al deposited on the back surface of a Si sensor. There are complex trade-offs between optical blocking and X-ray transmission for other filter materials, but these have not been fully explored because of the chemical and electrical effects that metal deposition has on the back-surface of pixellated Si sensors. Investigation of direct deposition of thin films of Ti, W, and other materials onto Si sensors needs to be undertaken to truly optimize soft X-ray sensitivity and minimize optical contamination.
Alternatively, free-standing filters that can be thin enough to compete with directly deposited filters (50 nm or thinner), with a structure that supports the filter film that is better than 90% transparent also need to be explored. This structure, with large area films (> 6 cm) supported by fine meshes with as thin as 10 nm of aluminum and 20 nm of polyimide, would have to be strong enough to survive launch vibrational loads and the thermal cycling environment that would be expected in a space mission. Another approach is the use of waveguide cutoff filters that have no films betweena support mesh, with extremely tiny holes (1-2 microns across).The filters would have to have a high X-ray transmission over the 150 eV to 2 keV energy range, while maintaining a good optical attenuation performance. The requirement would be >40 % X-ray transmission above 200 eV and >10% X-ray transmission below 200 eV. The optical attenuation at a thickness of 50 nm should be 10-3. It would also be advantageous to have the ability to control the temperature of the filters so that any contamination that did build-up on them could be removed through heating.
The potential benefit of advanced filter technology is significantly greater effective area of the instrument, below 2 keV through high X-ray transmission. Having more than an order of magnitude transmission available for the softest X-ray makes more of the collecting area of the X-ray optic available for science. The X-ray optic would have a corresponding order of magnitude more area for some scientific measurements. Measurements include observations of highly redshifted sources such AGN and baryons from distant groups of galaxy clusters.
The signal-to-noise performance of the instrument can be optimized by limiting the amount of optical photons that would be able to reach the detectors without affecting target photon energy throughput.
Contamination build-up can be controlled by free-standing filters as they can be thermally isolated from the detectors and so can act as a warm barrier between the cold detectors and the hydrocarbon contamination within the payload.
Free standing filters allow multiple filter foil thicknesses to be included in an instrument on a filter wheel, optimizing filter characteristics for a particular observation.
X-Ray Flagship
X-Ray Probe
Advances in OBF technology will be applicable to any missions that use detectors on the focal plane that are sensitive to optical photons but that target photons in the EUV to soft X-ray bandpass (50 eV to 2 keV). This would include missions falling under both the X-ray Flagship and Probe designs that have a large effective area optic and an imaging camera on the focal plane that would be silicon based. Effective optical blocking filters will allow the signal-to-noise of the detected photons to be maximized by attenuating optical photons, while maximizing effective area with highly X-ray transparent filters. The technology is also synergistic with an enabling technology for the US contribution to ATHENA.
Filter technologies will have to be identified in the early 2020s and need to reach TRL 6 by mission PDR.. Filters will also be required in any future X-ray spectrograph, either for grating readouts, imaging readouts, or calorimeter filters. Many future mission concepts (observatory class and probes) will emphasize high-redshift science where maximizing the instrument response below 0.5 keV will optimize the sensitivity.
Low Stress, High Stability X-ray Reflective Coatings Great Observatory and Probes require a light-weight X-ray telescope with large collecting area and a couple of arcsec angular resolution (0.1-10 keV: < 1 arcsec, 10-150 keV: < 5 arcsec). The X-ray mirrors used to construct the telescopes will comprise thin, curved substrates (either segmented or full-shell) coated with X-ray reflective thin films. The thin-film coatings must provide high reflectance over the target energy band, and must maintain or reduce the high-frequency surface roughness of the substrate (to minimize losses due to X-ray scattering). Additionally, and crucially, coating stress must be controlled and stabilized, and remaining stress must be compensated in order to mitigate stress-driven substrate deformations that degrade angular resolution. Iridium single-layer, iridium/boron-carbide (Ir/B4C) bilayer films, and similar combinations (Ir/SiC) have been demonstrated by several groups to have high X-ray reflectance over the soft energy band (0.1-10keV) [Massahi+ 2020, Appl. Optics 59, 10901]. Additionally, preliminary work on more complex multilayer coatings has demonstrated even better X-ray performance [Yang+ 2020, JATIS 6, 044001-1]. At hard X-ray energies (10 - 150 keV), even more complex multilayers of the Pt-, W-, and Ni- families as demonstrated by NuSTAR and Hitomi are required [Madsen+,2018, SPIE, 10699]. Coatings typically exhibit high stress when deposited under sputtering conditions that provide high film density and low surface roughness, and thus maximal X-ray reflectance. Ir-based coatings having low net stress (as measured on flat test substrates) have been demonstrated through various methods [Broadway+ 2015, SPIE 9510, 95100E]. However, with these low stress coatings, preservation of figure error on curved, thin mirror segments has not yet been demonstrated at the arcsecond-level. Recently-developed 2D film stress patterning methods (e.g., using ion implant, oxide etch, or ultra-fast laser patterning) have successfully demonstrated high-precision correction of substrate distortion caused by coating stress [Yao+ 2019, JATIS 5, 021011]. While these methods have successfully corrected coating distortion on Wolter flight prototype mirrors, no method has been fully incorporated into a mirror production process, and X-ray tests have not yet been performed. Limited studies of potential low-stress coatings have been performed on mirror coupons, and stress evolution with time has been noted. However, systematic studies of methods to stabilize stress in single-layer and multilayer coatings are needed. Advancement to TRL 4 will require demonstration of temporally stable X-ray reflective coatings deposited on thin, curved substrates (segmented or full-shell) with acceptable figure preservation (after compensation), demonstration of preservation (or improvement) of high-frequency substrate surface roughness, and demonstration of acceptable X-ray reflectance, using techniques that are scalable for mass production. 3 Thin-film single-layer and multilayer coatings deposited onto figured, thin-shell substrates that (a) provide high X-ray reflectance over the target bandwidth, (b) have low high-frequency surface roughness (to minimize losses due to X-ray scattering), (c) preserve the underlying substrate figure, after coating deposition and compensation, to minimize and stabilize coating-stress-driven substrate deformations that degrade angular resolution, and (d) demonstrate performance parameters are temporally and environmentally stable over the target mission lifetime. Coating deposition and stress compensation methods must be scalable for mass production. To reduce cost, processes should first be developed on flat test substrates, moving to curved Wolter optics after successful demonstrations. High X-ray reflectance and low surface roughness are needed to achieve high telescope collecting area. However, without the development of effective methods to mitigate coating-stress induced substrate deformations, arcsecond telescope resolution (< 5-arcsec ) will not be possible using thin-shell mirror substrates. Systematic study of various coating processes, pre-coat surface preparations, and post-coat annealing and correction processes will build a body of knowledge that can be used to optimize coatings and ensure a well understood manufacturing process. Probe class X-ray mission
X-ray Great Observatory
Astro2020 enumerated numerous science objectives driving large aperture, high resolution x-ray mirrors. Small or Medium Explorer missions would also benefit from sub-arcsec imaging enabled by this technology.
Years to estimated launch or other schedule driver: 5-10 years to launch of Explorer; <10 years to launch of probe-class X-ray mission; <20 years for Great Observatory.
Level of complexity (single tech, system of techs, or system of tech systems): system of technologies.
Level of difficulty (straightforward, stretch, or major stretch): stretch
Polarization-Preserving Millimeter and Far IR Optical Elements The CMB probe will likely operate at frequency bands between 20 and 800 GHz (15 mm to 0.375 mm; see the PICO Probe report). A far IR space mission will likely have frequency coverage within the range between 25 and 600 micron. Both probes require refractive optical elements operating over broad bandwidth. The CMB probe will certainly be a polarimeter. If the far IR probe resembles the instruments proposed for the Origins Space Telescope it will include a polarimetric instrument. For both probes the optical elements operate at cryogenic temperatures. The technology capability needed is for a cryogenically-robust, broad-band, polarimetric-worthy optical elements. This capability can be thought of in terms of two separate but complementary entries: broad-bandwidth, and polarization preserving optical elements. Both require cryogenically robust solutions. Refractive optical elements include vacuum windows, absorbing filters, half-wave plates, and lenses. Materials for optical elements in the far IR and millimeter-wave include plastics such as polyethylene and teflon and ceramics primarily silicon, alumina, and sapphire. Approaches to providing broad-bandwidth depend on the anti-reflection coating (ARC) through applying multi-layers (ML), fabricating sub-wavelength structures (SWS), or synthesizing indices of refraction by other means. (Occasionally, the words ‘fabricating meta-material’ is used, but ‘meta-material’ is ambiguous at best.) Over the last decade there has been progress with using SWS on silicon between ~50 GHz and up to ~1 THz with finite fractional bandwidths. Good progress has been reported with other materials. Applying multi-layers has spotty record. Some groups report success (after much invstement), others report failures, even when using the same approach that were reported to succeed elsewhere. In aggregate: solutions with preferred materials are not yet available for the bandwidths, and wavelengths needed.
TRL details:
Some references for the assessment provided at left: Nadolski,A.W. “Broadband, millimeter-wave antireflection coatings for cryogenic sintered aluminum oxide optics”, 2020, PhD thesis, http://hdl.handle.net/2142/107910; Hill, C.A. “Sensitivity Simulations and Half-Wave Plate Polarization Modulators for Cosmic Microwave Background Observatories”, 2020, PhD thesis; https://www.proquest.com/docview/2510190971 ;Coughlin,K.P. et al. “Pushing the Limits of Broadband and High-Frequency Metamaterial Silicon Antireflection Coatings”, 2018,JLTP,http://adsabs.harvard.edu/abs/2018JLTP..tmp..141C ;Takaku,R. et al. “A Large Diameter Millimeter-Wave Low-Pass Filter Made of Alumina with Laser Ablated Anti-Reflection Coating”, https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-25-41745 Chrisopher,M.M. et al. “Planar Silicon Metamaterial Lenslet Arrays for Millimeter-wavelength Imaging”, 2020, SPIE, https://arxiv.org/abs/2012.08636
3 Develop robust multi-layer coatings for broadband applications for commonly used dielectrics (e.g., silicon, alumina, and sapphire).
Develop thermal filtering technologies suitable for large Focal-Plane Arrays (FPAs) operating at sub-Kelvin temperatures, using low index of refraction materials to enable filters with high transmission and low ripple over a broad passband.
Develop space-compatible modulators, including work on frequency-selective surfaces and mechanisms compatible with the space radiation environment. Minimizing dielectric cross-section to low-energy electrons is a priority.
Develop and compare strategies for instrument architectures with and without rapid modulators.
A secondary goal is to ensure that the technology can be implemented in a cost-effective way for large optical elements. Large in this context is up to 100 cm in diameter.
For frequencies 20 GHz – 20 THz GHz, a ratio of 1:10 bandwidth with reflection <=1%, cryogenically robust, polarization preserving optical elements. The words ‘polarization preserving’ mean that they induce instrumental polarization at a level of <=0.1%. The ratio 1:10 bandwidth quantifies the ratio of the low frequency to the high frequency edges of the pass-band. For example, 20 to 200 GHz; or 100 to 1000 GHz.
Detection of B-mode polarization in the CMB or setting a stringent upper limit. Robust rejection of foregrounds. Enabling broadband and polarimetric measurements in the far IR. Far IR Probe; CMB Probe; Balloon-borne payloads that are science and technology pathfinders for both probes. The far IR probe is scheduled for this decade. The CMB probe is scheduled for 2030s.
Level of complexity (single tech, system of techs, or system of tech systems): between single technology to system of technologies. There are several candidate technologies, each may be suitable for a specific region in parameter space.
Level of difficulty (straightforward, stretch, or major stretch): between ‘straightforward’ to ‘stretch’. Progress done over the last decade points to solutions that may fully mature this decade.
Rapid readout electronics for X-ray detectors Future NASA X-ray missions in both the long term (Lynx, which is endorsed by Enduring Quests Daring Visions Report and currently under STDT study) and nearer term (Probe and Explorer-class opportunities), as well as the ATHENA mission, led by the European Space Agency (ESA), will have focal-plane instruments that have many pixels (> 10 Mpix) and very fast frame rates. A key technology that will need further development to support these focal planes will be electronics boards that can read out and process the events from these detectors at very rapid rates. The concept for a board that would accomplish the requirements for both JANUS and ATHENA was initially developed when JANUS was in a phase-A study and ATHENA had a potential opportunity for US contribution to these electronics.
The need still exists for such a board for other future missions such as Lynx, Arcus, or other upcoming Explorer and/or Probe-class missions.
A design was developed using a Xilinx Field Programmable Gate Array (FPGA) and parallel event processing that enables the required speeds.
A prototype board was built, with firmware being finalized. Current TRL is assessed by the team as 4, and planned to be at TRL 5 in summer 2017.
4-5 Development of digital electronics that can accept data and detect and characterize X-ray events at rates required by Lynx (> ~ 100 Mpix/s) Enable rapid detector readout and event characterization for a variety of possible X-ray missions, large and small: flagship, Probes, Small Explorer (SMEX), Medium-class Explorer (MIDEX), and CubeSats.
In particular, this will enable missions such as Arcus, which is being proposed, and/or Lynx, which is endorsed by Enduring Quests Daring Visions and under study by an STDT in preparation for the 2020 Decadal Survey.
It would also be useful for other possible missions such as probe-class missions like TAP or AXIS; SMEX and Explorer like Arcus; and even CubeSat opportunities. These developments could also contribute to efforts by our European colleagues on technology needed for the ATHENA mission.
X-Ray Flagship
X-Ray Probe
Lynx (currently under study by a NASA STDT as a large strategic mission concept) is one example of a potential mission requiring rapid readout electronics with these characteristics (Kouveliotou et al. 2014, “Enduring Quests, Daring Visions;” & Vikhlinin et al. 2012 gives a possible strawperson example of how this mission might look). The large-FOV instrument on this mission specifically required large-format APS. This instrument would provide a large FOV with excellent spatial and temporal resolution and moderate spectral resolution. The heart of the instrument is a > 16 Mpix focal plane with a readout requirement of >100 frames/second and is comprised of an array of X-ray photon-counting APS. Similar requirements would apply to any other similar mission that couples large effective area with a need for large-format/many-pixel detectors, or missions that require rapid readout of many pixels for other purposes such as timing resolution or background suppression.
Examples: Lynx, Arcus, ATHENA, Probe-class X-ray missions with more targeted capabilities, JANUS-like missions, XTiDE-like missions.
Need to achieve TRL 6 by mission PDR.
Advancement of X-ray Polarimeter Sensitivity Standard photoelectric X-ray polarimeter designs are both quantum-efficiency (QE) -limited and challenging to calibrate due to diffusion of electron signal as it drifts through the gas. A new generation of polarimeter is needed to enable larger detector areas that can be at the focus of larger diameter mirrors and single reflection concentrators. To enable this, the diffusion of the electron signal must be decoupled from the sensitivity. Doing so will enable a large improvement in sensitivity without driving cost and with only moderate increase to mass and power of the detector and/or instrument. Furthermore, the energy band will be tunable to maximize science return.
Several photoelectric polarimeter concepts such as Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS, previously Gravity and Extreme Magnetism Small Explorer, GEMS), Imaging X-ray Polarimeter Explorer (IXPE), and Polarimetry of Energetic Transients (POET) etc. were proposed in 2014 to provide the next substantial step exploiting X-ray polarization to answer key scientific questions for some of the brightest sources in the sky.
However, proposed measurements remain photon-limited and the need for higher-sensitivity polarimeters for both faint persistent sources such as Active Galactic Nuclei (AGN) and bright transient sources such as Soft-Gamma Repeaters (SGRs) by way of Explorer missions and probe-class missions in the next decade remains critical.
The goal of this development is to make practical the technology that will provide an order-of-magnitude improvement in polarization sensitivity over current-generation instruments.
  Development of gas electron multipliers optimized for negative ion gas.
Development of finer-pitch strip readouts to improve the sensitivity at lower energies and higher pressures.
Optimization of gas mixtures to maximize sensitivity (QE vs. track length).
Demonstrate lifetime of gas and detector materials is commensurate with mission requirements.
These developments will allow a factor-of-10 improvement in sensitivity without decreasing the sensitivity per unit mass and without increasing the relative cost of an instrument. Flagship and Probe-class X-ray missions.
Explorer-class X-ray missions.
Sounding rocket experiments.
Named missions:
Development needed for 2020 Decadal: No
Other drivers: Explorers, Probes, and Missions of Opportunity (MOs).
Very-Wide-Field Focusing instrument for Time-Domain X-Ray Astronomy There exists considerable support in the astronomical community for a Probe-type mission dedicated to Time-Domain X-ray Astronomy. Given the large number of X-ray sources across the sky that are variable, or transient, the key instrument would be a focusing telescope with an extremely large field of view, i.e. several steradians. The type of optics with the large field of view is known as a “lobster-eye.” There are two types. One is based on an array of square pores slumped onto a spherical surface. Small units have been constructed and, in fact, such an optic is scheduled to be aboard the ESA mission to Mercury. The other type of lobster-eye is a hybrid consisting of an equally spaced array of flat mirrors that lie along the radii of a cylinder. Both faces of the mirrors are coated with a heavy metal with good X-ray reflectivity. It provides position information in one dimension. A circumferential cylindrical coded mask provides positions in the other dimension. Both types require similar detector systems, which would consist of an array of CCD, CMOS or other type of pixelated detectors. While the detector chips currently exist, there has not been an array of X-ray devices that covers efficiently a very large area (up to a square meter). Compared to the 2D channel-plate optic, the hybrid has an order of magnitude more effective area and much broader bandwidth but more background. For very short-lived transients, such as gravitational waves (GWs) and short gamma-ray bursts (GRBs), where little background accumulates, the larger area and broader bandwidth is desirable. Small prototype telescopes of both types have been constructed but are not close to the optics size required for the Transient X-ray Astrophysics Probe. Leicester University in the UK has been leading the thermally slumped channel-plate effort. So far, the angular resolution of small units has been ~5 arcmin, far short of the 20-arcsec theoretical value. Also, the coatings on the walls of the channel plate have not been optimal. Small versions of the cylindrical-geometry 1D focusing lobster-eye telescopes have been constructed at SAO and in the Czech Republic. However, the problem of a low-mass structure for the mirrors has not been addressed and no effort has been made to develop a circumferential cylindrical coded mask to go with the cylindrical 1D lobster-eye. While optical astronomers have successfully made large arrays of optical/infrared CCDs, there has not been the need so far for a comparable X-ray detector.
Technologies for the channel plate version are ~TRL 4. Technologies for the hybrid exist in concept but insufficient support delays design of a prototype optic integrated with a coded mask. Optics are at ~TRL 2 and the 1D cylindrical coded mask at TRL 3 thanks to the success of Swift and XMM coded masks. The UK is supporting most channel-plate telescope development efforts. Support for the hybrid would have to be provided by NASA. The detector array for both 2D and 1D focusing systems is at TRL 3. Comparable-size arrays have been constructed for optical telescopes but with less need for continuous focal surface coverage. Kepler is an example of a large focal plane detector array in space.
4 With a 1-m focal length and 120° azimuth coverage, the lobster-eye focal plane would be a closely packed cylindrical array of pixelated X-ray detectors over a focal surface with ~1-m2 total area. Ideally, the detector active region would occupy a very large fraction of that area, efficient to at least 10 keV, with 50-µm pixels sufficient.
Telescope technical goals are several arcmin or better angular resolution and sub-arcmin position determination (by finding image centroid) with a FOV ≥ 2 ster.
Sensitivity for GRB detection at least ×10 better than Swift, capable of detecting and positioning all kinds of variable (and static) sources at least ×10 better than the scanning Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor and Japan’s Monitor of All-sky X-ray Image (MAXI) on the International Space Station (ISS).
Many sources are nearly constantly in the FOV (except during Earth occultation in Low Earth Orbit, LEO) for both types of lobster-eye telescopes. The hybrid’s larger exposure time and area ensure superior sensitivity to the scanning collimated monitors, with background reduction of focusing adding another level of superiority.
Both types of lobster-eye telescopes are new types of X-ray optics that have not been in orbit. The channel-plate type will be very low mass. The FOV of both types is several ster whereas current and previous focusing telescopes, e.g. Chandra and XMM, have fields that are a fraction of a square degree. The difference in sky coverage is a factor of 104. Large non-focusing detectors with similar large FOVs, e.g. Swift, do not cover the same bandwidth, and have much less sensitivity and angular resolution. The lobster-eye telescopes enable an entire new class of measurements, detecting and positioning short-lived transient sources such as distant GRBs and likely transients with sub-second duration transients associated with GWs. They have much more sensitivity than the non-focusing scanning all-sky monitors for all types of temporal variations. X-Ray Probe
The very-wide-field lobster-eye X-ray telescope enables the development of the Transient Astrophysics Probe, a candidate PCOS mission that carries out studies of multiple sources varying over a large range of time scales simultaneously from the same pointing position. By detecting and positioning very distant GRBs, Cosmic Origins (COR) objectives will be fulfilled because the distant GRBs’ host galaxies will be the youngest galaxies in the universe. Lobster-eye telescopes can map X-ray emission from planets and asteroids in our solar system. Depending on the viewing distance and the size of the object, X rays from a large part or the entire front-facing surface of the planet can be mapped much more quickly with the lobster-eye telescope’s large FOV.
TRL 6 by mission PDR.
Gravitational Reference Sensor (GRS) Gravitational wave missions rely on measuring picometer changes in the separation of test masses on spacecraft millions of kilometers apart. The test masses are housed in drag-free subsystems, called Gravitational Reference Sensors, which give rise to a substantial part of the disturbance noise budget. The capability needed is a reduction of disturbances arising within the GRS itself. LISA Pathfinder demonstrated a level of performance adequate for LISA. Future missions, like ALIA, require 10X better performance at higher (>1 mHz) frequencies. Leading contributors to the LPF acceleration noise are shown in the Astro2020 APC White Paper “Space based gravitational wave astronomy beyond LISA” Current SOTA is LISA Pathfinder and LISA. LPF demonstrated a GRS in-flight at a level necessary for LISA. The LISA GRS incorporates UV-LED charge control developed by University of Florida/NASA (now at TRL 5). Improvements like interferometric sensing of the test mass and larger gaps to reduce stiffness noise are TRL 4. 4 The technical goal is to reduce the total acceleration noise budget from all sources (cf. Fig. 4 in White Paper), both external to and arising within the GRS by an order of magnitude above 1 mHz. Obviously, this goal is intertwined with acceleration noise from the GRS and specific mission design choices for spacecraft and payload through a total noise budgeting process. This is an intrinsic property of gravitational wave missions. The detection of gravitational waves is repeatedly cited by Astro2020 as one of the major breakthroughs in astronomy and physics in this century. LISA is arguably the most promising currently planned project for furthering gravitational wave astronomy. NASA needs to prepare the technology foundation in the 2020s and early 2030s for the next generation gravitational wave mission. Selection of a conceptual mission design is strongly dependent on the interplay between detailed design choices and technology development progress in the Gravitational Reference Sensor and other areas. The primary benefit of technology development in this area is to enable mission concept selection in the 2030s for the development of the next generation gravitational wave mission in the 2040s. Next Generation Gravitational Wave Mission – LISA is scheduled to launch in 2034 and operate into the mid 2040s. A next-generation strategic gravitational wave mission concept would optimally be selected in the 2030 Decadal for launch in the late 2040s. Technology development during the 2020s would establish the achievable performance. Astro2020 strongly endorses past and future gravitational wave observations (p. S-1, 2-1, Section 2.2). They are a crucial part of the New Messengers and New Physics theme (pp. S-1, 1-6, 1-9), the Cosmic Ecosystem theme (pp. 2-37, 2-37, 2-48), the New Windows on the Dynamic Universe priority area (pp. 1-6, 2-33), the Time Domain Astrophysics Program (pp. 1-17, 2.2.5), and Program of Record (p. 7-2, Table 7-2, Section 7.7.3, pp. 7-36 – 7-37).
High or low frequency next generation gravitational-wave mission concepts (see APC White Paper link above). An additional mission benefitting from a GRS for drag-free operation is GRACE 2.0/Next Generation Gravity Mission (NGGM, Earth Sciences).
Years to estimated launch or other schedule driver: 10 yrs to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): collection of technologies
Level of difficulty (straightforward, stretch, or major stretch): stretch
High-power high-stability Laser Gravitational wave missions use a stabilized laser for interferometrically measuring changes in the separation between inertial test masses in widely separated spacecraft. Shot noise is the lead contributor to the measurement sensitivity at the high frequency end of the detector sensitivity. Higher power, and thus better measurement sensitivity, delivered over tens of thousands to hundreds of millions of kilometers can be achieved by a combination of higher transmitted laser power and larger diameter transmitting and receiving telescopes. The laser must be frequency and power stabilized to a very high degree, and must be reliable and redundant. Representative Next Generation Gravitational Wave Missions and their laser requirements are outlined in the Astro2020 APC White Paper “Space based gravitational wave astronomy beyond LISA” (https://baas.aas.org/pub/2020n7i243/release/1). The low-frequency mission requires 1.5X higher power than LISA; the high frequency mission requires 15X higher power. The current SOTA is the LISA laser being developed at NASA GSFC (now at TRL 5). A comparable laser is being developed by ESA in Europe. Much higher power with exquisite stabilization has been demonstrated by LIGO, but not in an appropriate system or environment. Reliability and redundancy strategies are always challenging in laser systems for spaceflight. 4 The technical goal is to augment the laser power without compromising frequency or power stability, reliability and other key performance parameters (mass, power, volume, environmental robustness). System architectural solutions to improve overall system reliability and availability should be explored in addition to component development. This may include, for example, cross-strapping of life-time limiting components in a master-oscillator power amplifier architecture. The detection of gravitational waves is repeatedly cited by Astro2020 as one of the major breakthroughs in astronomy and physics in this century. LISA is arguably the most promising currently planned project for furthering gravitational wave astronomy. NASA needs to prepare the technology foundation in the 2020s and early 2030s for the next generation gravitational wave mission. Selection of a conceptual mission design is strongly dependent on the interplay between detailed design choices and technology development progress in interferometric metrology. The primary benefit of technology development in this area is to enable mission concept selection in the 2030s for the development of the next generation gravitational wave mission in the 2040s. Next Generation Gravitational Wave Mission – LISA is scheduled to launch in 2034 and operate into the mid 2040s. A next-generation strategic gravitational wave mission concept would optimally be selected in the 2030 Decadal for launch in the late 2040s. Technology development during the 2020s would establish the achievable performance. Astro2020 strongly endorses past and future gravitational wave observations (p. S-1, 2-1, Section 2.2). They are a crucial part of the New Messengers and New Physics theme (pp. S-1, 1-6, 1-9), the Cosmic Ecosystem theme (pp. 2-37, 2-37, 2-48), the New Windows on the Dynamic Universe priority area (pp. 1-6, 2-33), the Time Domain Astrophysics Program (pp. 1-17, 2.2.5), and Program of Record (p. 7-2, Table 7-2, Section 7.7.3, pp. 7-36 – 7-37).
High or low frequency next generation gravitational-wave mission concepts (see APC White Paper link above). Development would also profit other missions relying on powerful frequency-controlled lasers for laser metrology (e.g. adaptive optics, starlink positioning) and optical communications.
Years to estimated launch or other schedule driver: 10 yrs to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): single technology
Level of difficulty (straightforward, stretch, or major stretch): straightforward
MicroNewton Thrusters for GW missions Gravitational wave missions use micronewton thrusters to achieve drag-free flight. The spacecraft propulsion system is commanded to follow the inertial trajectory of a free-falling test mass contained within the payload. The thrusters have to provide thrust levels from below 10 µN to ~150 µN, depending on spacecraft mass with noise levels approaching 0.1 µN/√Hz in the 0.1 – 100 mHz band. The specific impulse must be high enough so that the propellant for 10 yrs of operation does not require an unreasonable amount of mass. Representative Next Generation Gravitational Wave Missions are outlined in the Astro2020 APC White Paper “Space based gravitational wave astronomy beyond LISA” (https://baas.aas.org/pub/2020n7i243/release/1). The current SOTA are the cold gas (nitrogen) microthrusters demonstrated in-flight by LISA Pathfinder (LPF). These are the baseline for LISA, but have very low specific impulse and require hundreds of kilograms of propellant and tankage for LISA’s 10 yr design lifetime. This enormous mass has dramatic consequences for system structure, total spacecraft mass and, ultimately launch mass. Further, the LPF thrusters had an undesirable noise source of unknown origins at higher frequencies. JPL developed an alternative technology with high specific impulse based on colloidal droplets for LISA, which also flew successfully on LPF, but ESA did not select them for LISA. Colloid micronewton thrusters approached the thrust and lifetime requirements, but the development was aborted before TRL 6. The thrust required for LISA or a subsequent GW mission may need higher maximum thrust than was demonstrated. Other technologies for microthrusters emerge frequently, and should be investigated, if promising. 5 The technical goal is to engineer a micronewton propulsion system with high specific impulse so that the total flight system is not burdened by the mass of the propulsion system and propellant. While the requirements are somewhat mission dependent, a total mass of 100-150 kg would be a suitable target. The detection of gravitational waves is repeatedly cited by Astro2020 as one of the major breakthroughs in astronomy and physics in this century. LISA is arguably the most promising currently planned project for furthering gravitational wave astronomy. NASA needs to prepare the technology foundation in the 2020s and early 2030s for the next generation gravitational wave mission. Selection of a conceptual mission design is strongly dependent on the interplay between detailed design choices and technology development progress in interferometric metrology. The primary benefit of technology development in this area is to enable mission concept selection in the 2030s for the development of the next generation gravitational wave mission in the 2040s. Next Generation Gravitational Wave Mission – LISA is scheduled to launch in 2034 and operate into the mid 2040s. A next-generation strategic gravitational wave mission concept would optimally be selected in the 2030 Decadal for launch in the late 2040s. Technology development during the 2020s would establish the achievable performance. Astro2020 strongly endorses past and future gravitational wave observations (p. S-1, 2-1, Section 2.2). They are a crucial part of the New Messengers and New Physics theme (pp. S-1, 1-6, 1-9), the Cosmic Ecosystem theme (pp. 2-37, 2-37, 2-48), the New Windows on the Dynamic Universe priority area (pp. 1-6, 2-33), the Time Domain Astrophysics Program (pp. 1-17, 2.2.5), and Program of Record (p. 7-2, Table 7-2, Section 7.7.3, pp. 7-36 – 7-37).
As the current LISA design shows, a more mass-efficient propulsion system that meets current performance standards would have significant positive impact on spacecraft mass, spacecraft structure, system accommodation, launch mass and project cost. Micro- and milli-newton propulsion systems have wide ranging applications for precision stationkeeping, e.g., star shade spacecraft.
Years to estimated launch or other schedule driver: 10 yrs to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): single technology
Level of difficulty (straightforward, stretch, or major stretch): straightforward
Laser phase measurement chain for a decihertz GW mission LISA-like Decihertz missions such as ALIA use laser heterodyne interferometry to measure distance changes between free-falling test masses on widely separated spacecraft. The gravitational wave signal is embedded in the phase evolution of a laser beat signal in the 10 MHz range. This signal is measured with a phase measurement chain composed of the photoreceivers, cables, ADCs and a FPGA-based phasemeter. ALIA’s proposed characteristic strain sensitivity of a ~3e-23 at 30mHz requires a phase measurement system capable of measuring below 10fm/√Hz; a 200-fold improvement over LISA capabilities. Current SOTA is the LISA phase measurement chain composed of photo receivers, cables, analog RF-components, ADCs and an FPGA based phasemeter. The timing jitter of the ADCs, dispersion in analog components, and temperature-driven phase noise in cables all scale with the laser beat frequencies which in turn depend on the orbits. This is described as chain noise in Figure 5 in the Astro2020 APC White Paper. “Space based gravitational wave astronomy beyond LISA”. It also shows that while relative intensity noise of the local oscillator laser (which typically increases towards lower laser beat frequencies) is not yet a problem for LISA, it would be a limiting factor for ALIA. ALIA’s required 200-fold improvement will likely be a compromise between smaller beat frequencies (enabled by the shorter arms), reduced timing jitter and better analog components.
TRL details:
The estimated TRL for the full solution is at least 3 but certainly not yet 4. LISA’s phase measurement chain is TRL 5 but it has to be adapted to meet ALIA’s requirements.
There is also some experience in hand with the phase measurement chains of LISA Pathfinder and the Laser Ranging Instrument of Grace Follow-on (both TRL 10) which include some technologies that can be useful for ALIA.
3-4 The first objective is to design a phase measurement chain (PMC) concept that will meet ALIA’s requirements and evolve a suitable requirement breakdown for all components. The second objective is to demonstrate that all components meet their requirements at the TRL 4/5 level (breadboard validation in laboratory/ relevant environment).
The third objective is to demonstrate that these components can then be combined to meet ALIA’s requirements and measure phase changes equivalent to 10fm/√Hz above 20mHz.
The desired phase measurement chain (PMC) is the central sensing system for ALIA, a new gravitational wave mission, with vastly improved sensitivity above ~5mHz compared to LISA. Gravitational wave observatories are limited by acceleration noise at low frequencies and phase measurement noise at high frequencies. While ALIA requires improvements in both frequency ranges, the higher frequencies targeted by ALIA require more improvements in the PMC. Identifying limits from the PMC then defines the required received laser power, which in turn informs the mission design in terms of spacecraft distance, telescope diameter, transmitted laser power and acceleration noise. The overwhelming endorsement for multi-messenger astronomy in ASTRO2020 indicates the need for continuing space-based GW observational capability after LISA. An early development of the PMC will be crucial to meet this need, by enabling a mature ALIA design in time for a launch when LISA’s lifetime ends. ALIA (or the next gravitational wave mission beyond LISA). While this is beyond the scope of ASTRO 2020, the Decadal report stressed that it is important to enable the development of technologies that prepare for future large strategic missions in space.
ASTRO2020 strongly emphasized the importance of gravitational waves and that LISA will cover three frequency decades centered around 3 mHz of the spectrum. Together with Pulsar Timing Arrays (PTAs) around a nHz and Cosmic Explorer (CE)/Einstein Telescope (ET) above a few Hz, this leaves two major frequency gaps as likely targets to be closed by the 2050s. As described in a White Paper submitted to ASTRO 2020 the Advanced Laser Interferometer Antenna (ALIA) could not only close the gap between LISA and CE/ET but also cover the LISA band, while the gap between PTAs and LISA is the target of the Folkner mission. To close these gaps on this time scale, advanced technology development support will be essential in the next decade. The phase measurement chain technology gap described here applies mostly to ALIA.
Years to estimated launch or other schedule driver: 10 years to next generation concept selection.
Level of complexity (single tech, system of techs, or system of tech systems): system of technologies.
Level of difficulty (straightforward, stretch, or major stretch): Stretch.
Complex ultra-stable structures for future GW missions The performance of gravitational wave observatories is often compromised by the stability of the used materials and structures. The path from the primary telescope mirror to the surface where the interferometer signal is formed is part of the interferometer arm. As our sensing systems start to reach their fundamental limits, noise associated with the intrinsic noise of the materials and structures used in this path will become more and more dominant. We typically distinguish between in-band noise (measured in m/√Hz) which directly increases the noise floor and slow material creep (measured in m/s) which reduces the performance of the instrument. In-band noise can be thermal noise driven by the temperature of the material or thermal expansion driven by external temperature changes. The ideal materials have very low CTEs and mechanical losses. Also athermal structures are common to reduce the effective CTE but now require low mechanical losses. Current SOTA is LISA but also large diffraction limited optical telescopes (e.g. GAIA), Xray-mirrors (Chandra, ATHENA) and ground-based gravitational wave observatories faced or face these issues. For LISA, typical materials of choice for the most critical distances and structures are ultra-low CTE materials (Zerodur, ULE, ClearCeram). To form complex structures such as optical benches and telescopes, hydroxide bonding and low-CTE epoxies are used. The Folkner mission requires sub-nm/√Hz stability in the µHz band. This is the frequency range where material creep will dominate. The ALIA mission requires a few fm/√Hz stability above a mHz. Temperature driven thermal expansions and potentially also thermal noise will become a challenge.
TRL details:
The TRL scale is difficult to apply as for example the relevant environment could include a requirement for the thermal stability which is yet undefined. The here suggested activity is rather a study of what is possible, of methods and design principles which later allow to design and build final structures.
However, gaining experience by building complex ultra-stable structures which meet the Folkner or ALIA requirements would allow to judge if passive designs might be sufficient or if actively controlled designs are needed. The knowledge base created is very essential for any future design.
3-4 The main objectives are to build relevant telescope-like mechanical structures using different materials and joints and show that they can meet the Folkner and the ALIA requirements. The desired structures and joints will help to better understand which materials and building techniques are available for a Folkner mission or an ALIA mission. Compare this with the huge strides in the last decades that were needed to improve precision measurement such as LIGO and LISA Pathfinder to where they are today. The new frequency domain of ALIA and Folkner require a similar sustained effort. ALIA and the Folkner mission as described in the white paper.. While this is beyond the scope of ASTRO 2020, the Decadal report stressed that it is important to enable the development of technologies that prepare for future large strategic missions in space.
ASTRO2020 strongly emphasized the importance of gravitational waves and that LISA will cover three frequency decades centered around 3 mHz of the spectrum. Together with PTAs around a nHz and CE/ET above a few Hz, this leaves two major frequency gaps as likely targets to be closed by the 2050s. As described in a White Paper submitted to ASTRO 2020, ALIA could not only close the gap between LISA and CE/ET but also replace LISA while the gap between PTA and LISA is the target of the Folkner mission. To close these gaps on this time scale, advanced technology development support will be essential in the next decade. The technology gap described here applies to both missions although the solutions will likely be different.
Years to estimated launch or other schedule driver: 10 years to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): single technology (does not really apply here)
Level of difficulty (straightforward, stretch, or major stretch): Stretch.
Stable telescopes Gravitational wave missions use an intersatellite laser link to interferometrically measure changes in the separation between inertial test masses in widely separated spacecraft. Shot noise is the lead contributor to the measurement sensitivity at the high frequency end of the detector sensitivity. Higher power, and thus better measurement sensitivity, delivered over large separations can be achieved by a combination of higher transmitted laser power and larger diameter transmitting and receiving telescopes. The delivered laser power to the far spacecraft scales as the transmit laser power and the fourth power of the telescope diameter. Representative Next Generation Gravitational Wave Missions and their laser and telescope diameter requirements are outlined in the Astro2020 APC White Paper “Space based gravitational wave astronomy beyond LISA” (https://baas.aas.org/pub/2020n7i243/release/1). A 1-meter diameter telescope is a factor of 120X improvement over the current 0.3-meter diameter design. The current SOTA is the LISA Engineering Development Unit Telescope being developed at NASA GSFC in partnership with L3Harris Corporation in Rochester, NY. The contract with L3Harris is currently 21 months into a 3 year contract with the goal of demonstrating TRL 6 for this design before adoption currently scheduled for early 2024. Telescopes with 1 meter primary diameters are not unusual, and a 1 meter scale size optic is targeted for other missions such as proposed large segmented mirror designs, but needs to be coupled with ultra-stable structural design techniques to meet requirements at the TRL 6 level for space-based gravitational wave missions. 4 The technical goal is to scale the telescope design from the current 0.3-meter primary diameter to the full 1.0 meter goal while maintaining the key performance parameters (mass, power, volume, environmental robustness, and resistance to cross-coupling of angular pointing jitter to length) as well as design robustness so that the units can be efficiently replicated in quantities of ~ 6-10. A 1-meter diameter telescope is a factor of 120x improvement over the current 0.3-meter diameter design in light delivery efficiency. The current design is a single low-CTE material (Zerodur) and there are several technical challenges with this approach including the surface treatment (etching) of the ceramic to maintain adequate strength and the selection or possible development of a suitable adhesive with a near room temperature cure to accommodate Zerodur and a high glass transition temperature and the corresponding joint design that may enable meeting requirements without using a single material. Development should also explore using new low-CTE materials or a mix of materials that meets requirements through clever compensation of environmental disturbances such as thermal fluctuations. Note that some developments can be accomplished with models incorporating key elements rather than a full-size telescope. The detection of gravitational waves is repeatedly cited by Astro2020 as one of the major breakthroughs in astronomy and physics in this century. LISA is arguably the most promising currently planned project for furthering gravitational wave astronomy. NASA needs to prepare the technology foundation in the 2020s and early 2030s for the next generation gravitational wave mission. Selection of a conceptual mission design is strongly dependent on the interplay between detailed design choices and technology development progress in interferometric metrology. The primary benefit of technology development in this area is to enable mission concept selection in the 2030s for the development of the next generation gravitational wave mission in the 2040s. High or low frequency next generation gravitational-wave mission concepts (see APC White Paper link above). Development would also profit other missions proposing large segmented mirrors, optical communications, including support of applications such as the EHT, and laser ranging applications Years to estimated launch or other schedule driver: 10 yrs to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): system of techs
Level of difficulty (straightforward, stretch, or major stretch): straightforward and stretch
Disturbance Reduction Gravitational wave missions rely on measuring picometer changes in the separation of test masses on spacecraft millions of kilometers apart. The test masses are housed in drag-free subsystems, called Gravitational Reference Sensors, to isolate them from disturbances originating within the spacecraft, such as mechanical noise from mechanisms, thermal noise from environmental changes, or gravitational noise from changing mass distributions. The capability needed is for strategies to model/predict disturbances and mitigate them with subsystem designs. For example, phased array comm antennas eliminate the vibrations associated with antenna repointing. Low-noise, precision mechanisms reduce mechanical disturbances from telescope re-pointing. Stabilized power dissipation from avionics and static solar illumination reduce thermal gradient changes. Mass balance and mass balance changes have to be modeled and managed from design through operations. Conventional spacecraft design and construction lacks tools and techniques for addressing these challenges which will be relevant to future cutting-edge missions. Current SOTA is LISA Pathfinder and LISA. LPF demonstrated disturbance reduction in-flight at a level necessary for LISA. LISA, now entering Phase B, addresses disturbances associated with two measurement link terminals (telescope, optical bench and GRS) in a single spacecraft with a 10 yr (extended) mission lifetime. The LISA design has a pointed comm dish, moving measurement terminals, changing illumination, and substantial propellant mass changes. Example future mission concepts are described in the Astro2020 APC White Paper. “Space based gravitational wave astronomy beyond LISA” The ALIA mission concept described there and elsewhere requires an order of magnitude improvement in disturbances (aka, acceleration noise) over LISA performance above 1 mHz.  3-4 The technical goal is to reduce the total acceleration noise budget from all sources (cf. Fig. 4 in White Paper), both external to and arising within the GRS by an order of magnitude above 1 mHz. This goal is intertwined with acceleration noise from the GRS and specific mission design choices for spacecraft and payload through a total noise budgeting process. This is an intrinsic property of gravitational wave missions. The detection of gravitational waves is repeatedly cited by Astro2020 as one of the major breakthroughs in astronomy and physics in this century. LISA will probe uncharted frequency regions in gravitational wave astronomy, but higher and lower frequency bands accessible only from space remain. NASA needs to prepare the technology foundation in the 2020s and early 2030s for the next generation gravitational wave mission. Selection of a conceptual mission design is strongly dependent on the interplay between detailed design choices and technology development progress in disturbance reduction and other areas. The primary benefit of technology development in this area is to enable mission concept selection in the 2030s for the development of the next generation gravitational wave mission in the 2040s. High or low frequency next generation gravitational-wave mission concepts (see APC White Paper link above). Other missions with precision measurement, optical stability and metrology needs, like adaptive optics, can profit from this technology development program. Years to estimated launch or other schedule driver: 10 yrs to next generation concept selection
Level of complexity (single tech, system of techs, or system of tech systems): collection of technologies
Level of difficulty (straightforward, stretch, or major stretch): straightforward (e.g., Ka band phased-array antennas, advanced thermal design/analysis/construction, mass modeling/assembly control/verification)
Broadband X-Ray Detectors The 2020 Decadal Survey on Astronomy and Astrophysics identifies “Unveiling the Drivers of Galaxy Growth” via “peering into the dusty hearts of galaxies to reveal enshrouded accreting black holes, or tracing the hottest phases of gas driven outward by this same accretion, with the spatial and spectral resolution needed to isolate critical physical quantities” [1] as one of the three key Priority Areas for the next decade and beyond. In practice this requires X-ray telescopes with broadband sensitivity, e.g., to conduct a full census of Active Galactic Nuclei (AGN) from the present to distant past. As a result, future instruments require sensitivity in the hard X-ray regime, beyond the capability of today’s silicon detectors, to detect distant sources whose soft X-ray emissions are heavily obscured by absorbing gas and dust in the young galaxy environs.
1. National Academies of Sciences, Engineering, and Medicine, "Pathways to Discovery in Astronomy and Astrophysics for the 2020s," 2021. [Online]. Available: https://doi.org/10.17226/26141.
While silicon detectors have been the workhorse for X-ray observatories, they lack sensitivity to hard X-rays. The 450 µm-thick detectors for ATHENA’s WFI [1] – approaching the maximum thickness at which silicon can be fully-depleted – have a cutoff (<50% QE) around 17 keV. Today’s SOTA is represented by the CdZnTe detectors flown on NuSTAR [2]; while the proposed detectors for HEX-P [3] represent considerable advancement of this technology, they still lag silicon in terms of format, energy resolution, and spatial resolution. The energy resolution of CZT if limited by the purity of the crystal growth of the crystals and the ability to cut the crystal from a good region of the CZT slide [4]. The low enegy triggering threshold of the CZT is also tied to the crystal purity and to the electronic noise of the readout system. Germanium is an alternative detector material but is less mature. The hard X-ray detectors flown on Hitomi combined a Si detector with CZT by having the Si on top of the CZT detector [4]. A detector that combines the capabilities of silicon detectors (in noise, format, and resolution) with broadband sensitivity would enable simultaneous spectroscopy of time-variable sources at low (<2 keV) and high (> 10 keV) energies, providing new insights into the complex physics of accretion and feedback, and could potentially eliminate the need to perform calibrations between different instruments or observatories.
1. N. Meidinger, M. Barbera, V. Emberger and others, "The Wide Field Imager instrument for ATHENA," SPIE Proceedings, vol. 10397, 2017.
2. F. Harrison, W. W. Craig, F. E. Chritensen and others, "The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-ray Mission," The Astrophysical Journal, vol. 770, p. 103, 2013.
3. K. Madsen, R. Hickox, M. Bachetti and others, "HEX-P: The High-Energy X-Ray Probe," Bulletin of the AAS, vol. 51, no. 7, 2019.
4. Kazuhiro Nakazawa, et al, "Hard x-ray imager onboard Hitomi (ASTRO-H)", JATIS, 021410 (2018).
3 A future X-ray Probe would benefit from large-format, low-noise detectors with sensitivity spanning both the soft and hard X-ray bands. Key performance parameters include:
- Sensitivity: HEX-P targets a range of 2–200 keV. Additional sensitivity to soft X-rays (< 2 keV), while maintaining good spectral resolution, would expand the range of science goals for a future observatory. Ideally a single detector for future missions would exhibit sensitivity to the entire X-ray band, from about 0.2 – 200 keV. A single detector solution (in contrast to focal plane arrays with multiple detector technologies or even multiple FPAs optimized for different energy regimes) that fulfills all requirements would substantially reduce the focal plane mass, power and system complexity.
- Format: While HEX-P targets [3] 70 kpixel, even larger formats would be preferred. Detectors with four-side abuttability and minimal gap widths will enable wide field of view mosaics of individual detectors. Four-side abuttable detectors also enable a focal plane with the detector surface approximating the best focal surface for the mirror.
- Spatial Resolution: The HEX-P goal is 5 arcsec (0.3 mm pixel pitch, 20 m focal length).
- Spectral Resolution: Approaching Fano-limited energy resolution across the entire range of sensitivity.
- Requisite radiation hardness for a multiyear mission.
Achievement of these goals will enable X-ray telescopes with wide field of view, excellent spatial resolution, and moderate spectral resolution for both the soft and hard X-ray bands, building on and complementing the capabilities of NuSTAR and ATHENA. Prospective X-ray Probe, or other mission requiring wide field of view, high-resolution detectors with good spectral resolution, and sensitivity extending into the hard X-ray band. Years to estimated launch or other schedule driver: 5-10 years (Explorer-class mission), 15 years (Probe-class mission)
Level of complexity (single tech, system of techs, or system of tech systems): Single technology
Level of difficulty (straightforward, stretch, or major stretch): Stretch

 





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