This PDF file contains the front matter associated with SPIE Proceedings Volume 11819, including the Title Page, Copyright information and Table of Contents

Aspera is an extreme-UV (EUV) Astrophysics small satellite telescope designed to map the warm-hot phase coronal gas around nearby galaxy halos. Theory suggests that this gas is a significant fraction of a galaxy’s halo mass and plays a critical role in its evolution, but its exact role is poorly understood. Aspera observes this warm-hot phase gas via Ovi emission at 1032 °A using four parallel Rowland-Circle-like spectrograph channels in a single payload. Aspera’s robust-and-simple design is inspired by the FUSE spectrograph, but with smaller, four 6.2 cm × 3.7 cm, off-axis parabolic primary mirrors. Aspera is expected to achieve a sensitivity of 4.3×10⁻¹⁹ erg/s/cm²/arcsec² for diffuse Ovi line emission. This superb sensitivity is enabled by technological advancements over the last decade in UV coatings, gratings, and detectors. Here we present the overall payload design of the Aspera telescope and its expected performance. Aspera is funded by the inaugural 2020 NASA Astrophysics Pioneers program, with a projected launch in late 2024.

The design of a CubeSat telescope for academic research purposes must balance complicated optical and structural designs with cost to maximize performance in extreme environments. Increasing the CubeSat size (eg. 6U to 12U) will increase the potential optical performance, but the cost will increase in kind. Recent developments in diamond-turning have increased the accessibility of aspheric aluminum mirrors, enabling a cost-effective regime of well-corrected nanosatellite telescopes. We present an all-aluminum versatile CubeSat telescope (VCT) platform that optimizes performance, cost, and schedule at a relatively large 95 mm aperture and 0.4 degree diffraction limited full field of view stablized by MEMS fine-steering modules. This study features a new design tool that permits easy characterization of performance degradation as a function of spacecraft thermal and structural disturbances. We will present details including the trade between on- and off-axis implementations of the VCT, thermal stability requirements and finite-element analysis, and launch survival considerations. The VCT is suitable for a range of CubeSat borne applications, which provides an affordable platform for astronomy, Earth-imaging, and optical communications.

In this paper, we establish the mission operation concept for the Orbiting Configurable Artificial Star mission, a hybrid space-ground observatory, which aims to enable ground observations of near-diffraction limited resolution and exquisite sensitivity. We present the mission requirements, introduce a potential orbit solution that can meet them, detail the concrete operational steps to be taken to enable such observations, and develop a mission planning tool which generates a mission schedule that meets all mission requirements and can be altered in real time in the case of disruptions to the mission. Finally, we show the the mission could enable 300 adaptive optics and 1500 flux calibration observations throughout its lifetime.

The Photo-z InfraRed Telescope (PIRT) is an instrument on the Gamow Explorer, currently proposed for a NASA Astrophysics Medium Explorer. PIRT works in tandem with a companion wide-field instrument, the Lobster Eye X-ray Telescope (LEXT), that will identify x-ray transients likely to be associated with high redshift gamma-ray bursts (GRBs) or electromagnetic counterparts to gravitational wave (GW) events. PIRT will gather the necessary data in order to identify GRB sources with redshift z >6, with an expected source localization better than 1 arcsec. A near real-time link to the ground will allow timely follow-up as a target of opportunity for large ground-based telescopes or the James Webb Space Telescope (JWST). PIRT will also allow localization and characterization of GW event counterparts. We discuss the instrument design, the on-board data processing approach, and the expected performance of the system.

Measurements on astronomical survey data are the link between scientific questions and scientific findings that help to answer these questions. In acknowledgement of their importance, NASA requires proposers to specify what measurements their space mission concept can make and what physical properties or processes can be derived from those measurements. NASA is now requiring a plan for actually making these measurements and how they will be made available in useable form to astronomical community. We will explore the benefits and issues involved in having a NASA mission take responsibility for making and distributing astronomical measurements. We use as a case study the NASA Probe mission concept, CETUS (Cosmic Evolution Through UV Surveys) posted at arXiv:1909.10437.

The Polstar mission will provide for a space-borne 60cm telescope operating at UV wavelengths with spectropolarimetric capability capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter massive stars' evolution, and finding the consequences for the stellar remnant population and the stirring and enrichment of the interstellar medium, by addressing four key science objectives. In addition, Polstar will determine drivers for the alignment of the smallest interstellar grains, and probe the dust, magnetic fields, and environments in the hot diffuse interstellar medium, including for the first time a direct measurement of the polarized and energized properties of intergalactic dust. Polstar will also characterize processes that lead to the assembly of exoplanetary systems and that affect exoplanetary atmospheres and habitability. Science driven design requirements include: access to ultraviolet bands: where hot massive stars are brightest and circumstellar opacity is highest; high spectral resolution: accessing diagnostics of circumstellar gas flows and stellar composition in the far-UV at 122-200nm, including the NV, SiIV, and CIV resonance doublets and other transitions such as NIV, AlIII, HeII, and CIII; polarimetry: accessing diagnostics of circumstellar magnetic field shape and strength when combined with high FUV spectral resolution and diagnostics of stellar rotation and distribution of circumstellar gas when combined with low near-UV spectral resolution; sufficient signal-to-noise ratios: ~103 for spectropolarimetric precisions of 0.1% per exposure; ~102 for detailed spectroscopic studies; ~10 for exploring dimmer sources; and cadence: ranging from 1-10 minutes for most wind variability studies, to hours for sampling rotational phase, to days or weeks for sampling orbital phase. The ISM and exoplanet science program will be enabled by these capabilities driven by the massive star science.

Astronomy has always been a technology driven science. This drive to ever greater sensitivity and performance is placing great pressure on the development community to meet this ongoing need. Extrapolating demand indicates a fundamental problem of affordability and timely development for both space and ground based systems. We note, that this trend is not unique to UV/VIS band. Other systems such as very long baseline interferometry in space leads to the need of large aperture radio telescopes, gravitational wave experiments like the Laser Interferometer Space Antenna (LISA) and occulter and starshade missions for coronography demand for high performance at acceptable cost. This begs the question, what can be done about it? In this initial paper, we plan to explore the roles of standardization, specialization and trans-national partnerships to realize future system design and implementation. We conclude our discussion with examples of areas where standardization in hardware and engineering approaches will improve productivity to help realize the next generation of cutting edge systems.

Starting with a conceptual thermal model of LUVIOR-A, provided by NASA/Goddard Space Flight Center, alternate concepts of operations are explored. A candidate technology was identified to use solar power for rough first stage heating. That innovation reimagines what functions a solar array can have in a large telescope architecture. By combining electricity generation, stray-light control, and thermal control into one agile deployable structure, the heater power requirements could be improved by reducing the inefficiency of solar power conversion.

Two observatories under consideration in the 2020 decadal survey include coronagraphs for characterizing the atmospheric composition of exoplanets. The telescope in those observatories must provide a wavefront that is stable to the picometer level for the coronagraph to provide enough starlight rejection to capture light reflected off of an Earth-like exoplanet. Therefore, picometer dimensional stability and control are required for these missions. Analysis indicates the thermal environment around the primary mirror must be stable to milliKelvin for the primary mirror’s thermally induced wavefront error to be stable to picometers. Marshall Space Flight Center (MSFC) partnered with L3-Harris Corporation to design and build a 1.5m thermal control system capable of milliKelvin stability. MSFC tested this thermal control system in the X-Ray and Cryogenic Facility’s large chamber to characterize how well the system controlled a 1.5m ULE® mirror.

Starship Cargo Vessel’s hold is 8 meters in diameter, nine meters tall to shoulder, then conical shape to 2 meters diameter at 16 meters tall. Lifting capacity to Low Earth Orbit is rated at 150 metric tons per vessel. Shock loads remain well within industry standards, also wellknown are figures on vibration issues and G-force records. This launch vessel’s volume of the cargo hold matches well the proposed size and support structure for 8.4-meter borosilicate glass mirrors in the seven-mirror pattern utilized by GMTO. Lift capacity also compares well against the weight of the mirrors, and infrastructure. Fleet vessel numbers and variety are under development (SpaceX, 2020).

The Nancy Grace Roman Space Telescope Coronagraph Instrument will be the first large scale coronagraph mission with active wavefront control to be operated in space and will demonstrate technologies essential to future missions to image Earth-like planets. Consisting of multiple coronagraph modes, the coronagraph is expected to characterize and image exoplanets at 1E-8 or better contrast levels. An object-oriented physical optics modeling tool called POPPY provides flexible and efficient simulations of high-contrast point spread functions (PSFs). As such, three coronagraph modes have been modeled in POPPY. In this paper, we present the recent testing results of the models and provide quantitative comparisons between results from POPPY and existing tools such as PROPER/FALCO. These comparisons include the computation times required for PSF calculations. In addition, we discuss the future implementation of the POPPY models for the POPPY front-end package WebbPSF, a widely used simulation tool for JWST PSFs.

The Ultraviolet Transient Astronomical Satellite (ULTRASAT) is a scientific space mission carrying an astronomical telescope. The mission is led by the Weizmann Institute of Science (WIS) in Israel and the Israel Space Agency (ISA), while the camera in the focal plane is designed and built by Deutsches Elektronen Synchrotron (DESY) in Germany. Two key science goals of the mission are the detection of counterparts to gravitational wave sources and supernovae.¹ The launch to geostationary orbit is planned for 2024. The telescope with a field-of-view of ≈ 200 deg², is optimized to work in the near-ultraviolet (NUV) band between 220 and 280 nm. The focal plane array is composed of four 22:4-megapixel, backside-illuminated (BSI) CMOS sensors with a total active area of 90 x 90mm^2.2 Prior to sensor production, smaller test sensors have been tested to support critical design decisions for the final flight sensor. These test sensors share the design of epitaxial layer and antireflective coatings with the flight sensors. Here, we present a characterization of these test sensors. Dark current and read noise are characterized as a function of the device temperature. A temperature-independent noise level is attributed to on-die infrared emission and the read-out electronics' self-heating. We utilize a high-precision photometric calibration setup³ to obtain the test sensors' quantum efficiency relative to PTB/NIST-calibrated transfer standards (220-1100 nm), the quantum yield for λ >300 nm, the non-linearity of the system, and the conversion gain. The uncertainties are discussed in the context of the newest results on the setup's performance parameters. From the three ARC options Tstd, T1 and T2, the last assists the out-of-band rejection and peaks in the mid of the ULTRASAT operational waveband. We recommend ARC option T2 for the final ULTRASAT UV sensor.

Space telescopes for studying astrophysical phenomena from the far ultraviolet (FUV) to the near infrared (NIR) require durable mirror coatings with high and uniform reflectance over a very broad spectral region. While coatings for the optical and NIR region are well developed with proven performance, the FUV band presents significant challenges, particularly below 115 nm. Recent developments in physical vapor deposition (PVD) coating processes of aluminum mirrors that are protected with a metal-fluoride overcoat to prevent oxidation (such as LiF, MgF₂, or AlF₃) have improved reflectance in the FUV. While the emphasis in these studies has been placed on improving the deposition conditions of the metal-fluoride overcoats, less attention has been devoted to how deposition parameters (such as vacuum conditions or deposition rates) may affect the quality of the aluminum mirrors. This paper presents characterization of Al+MgF₂ coupons made by ash evaporation of aluminum followed by resistive evaporation of MgF₂. Samples were manufactured under a variety of processing conditions and the relationship between processing variables and mirror FUV re ectivity is analyzed. Performance characterization was based on the measured near-normal reflectance in the FUV (90-180 nm), and normal-incidence transmittance in the visible was done to analyze the possible presence of pinholes in the mirror. We demonstrated pinhole-free Al/MgF₂ mirrors deposited at room temperature with a reflectivity of 0.91 at 122 nm wavelength. This reflectivity enhancement was achieved solely through parameter optimization.

Improving the spectral multiplexing efficiency of ultraviolet (UV) instruments is one of the essential technology developments for future large missions. This is particularly hard in the far UV (FUV), where high reflectivity coatings and high material transmission are difficult to achieve. We present here the concept and design of the Ultraviolet Micromirror Imaging Spectrograph (UMIS), which utilizes Analog Micromirror Arrays (AMAs) as the spectral multiplexing element. These Micro-Opto-Electro-Mechanical-Systems (MOEMS)-based mirrors can be dynamically programmed to probe multiple points of interest across a wide field of view without spectral confusion limitations. We have assembled a benchtop version of UMIS, to characterize the individual arrays and to evaluate the performance of the overall system in optical and FUV wavelengths. The instrument consists of a 75 cm off-axis telescope, with two AMAs placed on either side of the focal plane, and an optical spectrograph with a resolution of about R=1000. The individual mirror orientations can be adjusted by varying their bias voltages, which are controlled using custom electronic boards designed and fabricated by Ball Aerospace and LASP. The same testbed will be reconfigured with an FUV grating and detector, to calibrate the instrument in a vacuum environment and qualify the AMAs for future flight missions.

With the upcoming extremely large telescopes (ELTs), the volume, mass, and cost of the associated spectro- graphs will scale with the telescope diameter. Astrophotonics offers a unique solution to this problem in the form of single-mode fiber-fed diffraction-limited spectrographs on a chip. These highly miniaturized chips offer great flexibility in terms of coherent manipulation of photons. Such photonic spectrographs are well-suited to disperse the light from directly imaged planets (post-coronagraph, collected using a single-mode fiber) to characterize exoplanet atmospheres. Here we present the results from a proof-of-concept high-resolution astrophotonic spectrograph using the arrayed waveguide gratings (AWG) architecture. This chip uses the low-loss SiN platform (SiN core, SiO₂ cladding) with square waveguides (800 nm ~ 800 nm). The AWG has a measured resolving power (=) of ~ 12,000 and a free spectral range (FSR) of 2.8 nm. While the FSR is small, the chip operates over a broad band (1200 - 1700 nm). The peak on-chip throughput (excluding the coupling efficiency) is ~40% (- 4 dB) and the overall throughput (including the coupling loss) is ~ 11% (- 9.6 dB) in the TE mode. Thanks to the high-confinement waveguide geometry, the chip is highly miniaturized with a size of only 7.4 mm x 2 mm. This demonstration highlights the utility of SiN platform for astrophotonics, particularly, the capability of commercial SiN foundries to fabricate ultra-small, high-resolution, high-throughput AWG spectrographs on a chip suitable for astronomical applications.

During a NASA Phase II SBIR project the Goodman Technologies (GT) team developed and competed two advanced processes for producing large silicon carbide (SiC) mirror substrates and structures, both processes which could ultimately be performed in the microgravity environment of space. The process of scale-up from Phase I mirror substrates was anything but easy; over 250 unsuccessful trials were performed before achieving a printable substrate. A new Z-process allowed for conversion of moldable (but not printable) nanopastes into Pathfinder mirror components. The team also overcame delays associated with shutdowns due to COVID-19. The work did ultimately result in the demonstration of the world’s large 3D/AM SiC mirror substrates (25-cm scale). The first process, the Robocasting or Direct Ink Writing (DIW) printing process, was employed to 3D print engineered nanopastes consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. A large computer numerically controlled (CNC) platform with a 1.2-meter by 1.2-meter build bed was modified to become a large prototype “robocaster”. Modifications included incorporation of a large build plate (an optical bench top), a fluid supply system and syringes, a 750 W infrared (IR) heater, and programming. More than 200 experiments were conducted on the prototype robocaster, unsuccessfully, over a span of 16-months. Every print job would crack, and/or warp, and/or delaminate either during printing, or during low temperature curing, or during low-temperature polymer infiltration pyrolysis (PIP), or during high-temperature (PIP). Through massive effort, we finally overcame the engineering issues. Using a production robocaster, the ability to 3D print, join and then cure individual off-axis parabolic (OAP) mirror substrate segments (4 of them) to make a 25-cm monolithic Pathfinder mirror substrate for subsequent densification and pyrolysis was ultimately demonstrated. Two 25- cm monolithic substrates were printed and cured successfully. All of the robocast parts ultimately warped or bowed and/or cracked during low-temperature pyrolysis or high-temperature PIP steps. Through internally funded efforts performed by GT it was confirmed that as cured robocast material can be silicon melt infiltrated to form a very low silicon content of reaction bonded silicon carbide. GT also found that robocast material densified via polymer infiltration pyrolysis results in a polishable material. The second process GT developed and employed with UHM is the Z-process. The Z-process starts with a moldable (but not printable) nanopaste consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. The moldable nanopaste is compacted in a custom designed metallic and graphitic tooling set which also contains a precision mandrel. Compaction serves to squeeze out “extra” liquid phase material from the nanopaste while conforming the nanopaste to the shape of the mandrel prior to curing. Once cured the part is almost theoretical density and requires only a few steps of PIP to fully densify the part. PIP is accomplished at temperatures much lower than silicon melt infiltration (>160°C), chemical vapor deposition (>1450°C), or conventional sintering (>2200 °C), providing enormous energy savings. The Z-process tooling can be used multiple times providing economy of numbers. The Z-process was successfully used to join 4-OAP segments and a backside lattice supporting structure. This paper shall discuss the results of progress made towards the manufacturing of Large SiC Space Optics traceable to meter-class segments for a far-infrared surveyor (the Origins Space Telescope, OST). The technology also is intended to fill priority technology gaps for the LUVOIR Surveyor, and Habitable Exoplanet Observatory (HabEx).

Commercially available UV fibers suffer from high absorption and solarization in the far-UV range at λ<200 nm. Recently, new hollow-core anti-resonant fiber optics (UV-HCFs) have been developed with demonstrated guidance for the first time deep into the far-ultraviolet. These fibers are fabricated at the University of Bath and tested in facilities at the University of Colorado (CU) Laboratory for Atmospheric and Space Physics (LASP). We present the optical characterization and possible applications of UV-HCFs for the 100 { 200 nm regime. Testing of the fibers involved measuring the throughput of several fiber designs and lengths, that were optimized for transmission at peak wavelengths of 160 nm and 185 nm. The transmission of the fibers is measured using a far-UV monochromator and deuterium light source, connected to a custom alignment apparatus contained within a nitrogen-purged enclosure. The throughput is detected and logged using a photomultiplier tube and supporting autonomous control and data collection software. Our measurements show more than 50 percent throughput for a 20 cm fiber at approximately 160 nm, and performance that matches model predictions at wavelengths as low as 122 nm. The performance of the fibers will allow for the extension of the energy range of fiber-fed spectrographs and Raman spectrometer/reflectometers. Our results show that UV-HCFs have promise for future scientific applications.

The Star-Planet Activity Research CubeSat (SPARCS) 1 far ultra-violet (FUV) instrument will be tested and thermally characterized in a thermal vacuum (TVAC) chamber. The development and understanding of the thermal characteristics of the TVAC system are crucial to the verification of the thermal capabilities of the SPARCS payload. A TVAC chamber for testing FUV CubeSat instruments is in development at Arizona State University (ASU). The chamber will be used to test the SPARCS payload and future CubeSat missions. A thermal model of the thermal chamber has been developed for use with the SPARCS payload to correlate the model to test data. Correlating the model to test data will provide more realistic temperature predictions and reduce risk to the mission. The chamber model will be used along with the payload thermal model to determine preliminary test procedures creating a more realistic timeline for the testing.

The cryogenic etching of the black silicon (BSI) has been demonstrated as a superior absorber in par with other ultraabsorbers such as carbon nanotubes in the visible and near-infrared spectrum. In this work, we discuss the fabrication, modeling, and characterization of the BSI targeting the 2.5-5 microns range. We investigated a series of cryogenic parameters such as temperature, pressure, oxygen flow rate, power, and etching duration and fabricated a series of uniformly etched wafers. Additionally, we established a three-dimensional mathematical model of a unit cell and manipulated the silicon needle geometry and shape. Our preliminary results show five orders of magnitude specular reflectance in the infrared region. The technique employed here could be used to scale the etching process and enhance the absorption in the far-infrared and submillimeter range.