This PDF file contains the front matter associated with SPIE Proceedings Volume 11820, including the Title Page, Copyright information and the Table of Contents

A call for ideas for Voyage 2050 was issued in March 2019, generating close to 100 diverse and ambitious ideas, which were subsequently distilled into a number of science themes. Topical teams, comprising many early career through senior scientists, from a broad range of space science expertise areas, carried out an initial assessment of the themes and reported their findings to a senior science committee. This committee was tasked by the Director to recommend not only science themes for the next three large-class missions following the Jupiter Icy Moons Explorer, Athena and LISA, but also to identify potential themes for future medium-class missions, and recommend areas for long-term technology development beyond the scope of Voyage 2050. The science themes were selected by ESA's Science Programme Committee at a meeting on 10 June 2021. The talk will provide an overview of the process and describe the selected themes covering the Moons of the giant planets, from temperate exoplanets to the Milky Way and new physical probes of the early universe.

Efficient ultraviolet (UV) mirrors are essential components in space observatories for UV astronomy. Aluminum mirrors with fluoride-based protective layers are commonly the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their acceptable optical performance, the single-bounce reflectance values are still too low for use in optical systems in which several reflections are required. Recently, a novel passivation procedure based on the self-fluorination of bare Al has been presented [1, 2]. This research is framed in a collaboration between the Goddard Space Flight Center (GSFC) and the Naval Research Laboratory (NRL), and the experiments are carried out in the Large Area Plasma Processing System (LAPPS) at NRL using bare aluminum samples coated at GSFC coating facilities. The passivation of the oxidized Al is accomplished by using an electron-beam generated plasma produced in a fluorine-containing background to simultaneously remove the native oxide layer while promoting the formation of an AlF₃ passivation layer with tunable thickness. Importantly, this new treatment uses benign precursors (SF₆) and does not require high substrate temperatures. This novel procedure has demonstrated improved Al mirrors with enhanced FUV reflectivity. Examples of mirrors tuned at several key FUV wavelengths are provided. The LAPPS has been recently upgraded to include a new spectroscopic ellipsometer for real-time, in situ measurements of film thickness and optical constants of the fluoride layer during the plasma treatment. Since this new capability requires precise knowledge of the complex refractive index (n,k) of AlF₃, we present optical constants in the 90-2500 nm range obtained from Al mirrors previously prepared using the LAPPS process. The derived optical properties from the AlF₃ passivation layer show similar optical properties in the FUV when compared with PVD- and ALD- hot-deposited AlF₃.

The Raman Spectrometer for MMX (RAX) as part of the JAXAs Martian Moons eXploration (MMX) mission, to be launched in 2024, is designed for in-situ science on the Martian moon Phobos. It is installed on the MMX rover to investigate the Phobos surface mineralogy complementary to the anticipated sample return mission of MMX reaching earth in 2029 [1]. To ensure high Raman signals with the RAX instrument we utilize a volume phase holographic (VPH) grating as diffracting element. The VPH grating diffracts light by refractive index modulations within a thin layer of transmissive gelatin sandwiched between two glass substrates. Optimized VPH grating parameters combined with a small spectral bandwidth lead to peak efficiencies approaching up to 100 % [2]. Due to the rather small Raman scattering efficiency they are particular suitable for space instrumentation, where initial laser intensity is relatively limited [3]. We have designed an optical setup for the characterization of 1st order diffraction efficiency and wave front aberration evaluation. A laser source similar in emission characteristics to the RAX laser (Nd:YAG at 532 nm) is widened to 14.2 mm beam diameter before illuminating the VPH grating. The VPH grating is installed axis-centered on a rotation platform within a second outer rotational platform mounting a camera for optical verification or a laser power meter for the diffraction efficiency measurement. The VPH gratings reach diffraction efficiencies up to 87 % within their specified spectral range with diffraction limited patterns nearly identical to the undisturbed reference beam and dispersed only due to the laser band width.

The dominant selection characteristic between candidate mirror materials with acceptable strength and “polishability” is dimensional stability. 250K to 290K represents the temperature of many spaceborne telescopes. Furthermore, pointing and orbital motion, changing solar view factors create thermal transients. Traditionally, stability has been passively managed either with high thermal diffusivity materials (e.g. Al, Be, SiC) or low thermal expansion materials (e.g. ZERODUR®, ULE, ClearCeram and Carbon Fibers). Recently Kyocera introduced a high thermal diffusivity material, Cordierite CO720, with its Coefficient of Thermal Expansion (CTE) passing from negative to positive near 23C, postulating this would provide simultaneous low expansion and high diffusivity. We examine this postulate, noting both CO720’s CTE(T)’s high slope and Zero CTE at a warmer temperature than typical for space telescopes. Our conclusion from FEM simulations is that CO720 does not change the trade space

Modern observatories including ultra-stable spectrographs, optical telescopes and gravitational wave observatories rely on ultra-stable structures to meet their science objectives. These structures must exhibit pm to nm level length stability over a few seconds to a few hours and m-level length stability over mission duration of several years in some cases. The use of ultra-low CTE glass substrates provide the required stability while being highly fragile, having limited adaptability while driving turnaround times longer. We characterized structures made using materials that can provide the required stability while improving on the adaptability, turnaround times, structural mass and strength. These include a compound structure made using ALLVAR Alloy, a metal with a negative CTE, a second structure made of HB-Cesic, a full-ULE structure and a metal-ULE hybrid structure. In this work, we present a comparative analysis of the measured length noise and the long-term length stability for these structures.

The extremely large telescope (ELT) will be the largest telescope in the world with a primary mirror of 39 m aperture. In January 2019 SCHOTT has already successfully finished and delivered the 4.25 m ZERODUR® mirror blank for the secondary mirror (M2) of the extremely large telescope (ELT) of ESO, followed by 4 m ZERODUR® mirror blank for the tertiary mirror (M3) in January 2020. The ELT primary mirror M1 consists of 798 hexagonal mirror segments, each with slightly different aspherical shape to cope with the aspherical surface. Until 2024 SCHOTT will produce 949 cylindrical ZERODUR® mirror segment blanks (including spares) for the ELT M1 that will be polished and hexagonal shaped by SAFRAN REOSC in France. The production started in 2019 with the manufacturing of 18 verification blanks. The serial production started in December 2019 and will continue with roughly one blank per day until 2024. The ELT M1 specification requires tight tolerances on the scattering of the absolute CTE, high CTE homogeneity, excellent internal quality at the mirror surface and bonding interface with tight geometrical tolerances of the functional surface and back surface after acid etching. Generating high demands on the reproducibility of serial production in all relevant steps from melting, ceramization to CNC grinding and acid etching. This paper summarizes the status of the ELT M1 segment blank production at SCHOTT.

The Predictive Thermal Control (PTC) project is a multiyear effort initiated in Fiscal Year 2017 to mature the Technology Readiness Level (TRL) of technologies required to enable ultra-thermally-stable ultraviolet/optical/infrared (UVOIR) space telescope primary-mirror assemblies for ultra-high-contrast observations of exoplanets. PTC has three objectives: validate thermal optical performance models, derive thermal system stability specifications, and demonstrate predictive thermal control. PTC has successfully accomplished all of its objectives and matured the technology to at least TRL-5 via demonstration in a relevant thermal/vacuum environment with the 1.5-m ULE© AMTD-2 mirror.

Design To Unit Production Cost (DTUPC) is of crucial importance to the viability and implementation of constellations which require tens to hundreds of small to moderate sized spaceborne telescopes (10-cm to 50-cm aperture). Emerging technologies enable cost-effective alternatives to the traditional design and build of such telescopes. This is especially true for the structure of Optical Telescope Assemblies (OTAs) addressing environments where orbital phase modulated thermal gradients and transients dominate design. The implementation of a dimensionally stable structure is crucial to the performance of a telescope, and typically, compensation with focus mechanisms is not a cost-effective option in this size. ALLVAR Alloys, a family of emerging aerospace materials exhibiting Negative Thermal Expansion (NTE), can offer novel and cost-effective approaches to OTA metering. These Titanium-based alloys’ NTE can be used to dial in a specific thermal expansion or zero expansion profile by compensating for the natural expansion of other telescope components. For the first time, a telescope designer can passively control thermal stability of a telescope’s metering/support structures. This is true both for designs based on materials with low or high Coefficients of Thermal Expansion (CTE) and low or high thermal diffusivity (for example ULE®, ZERODUR® or ClearCeram metered with CFRP and Invar vs. all Aluminum or all SiC approaches). NTE ALLVAR Alloys offer a brand-new solution for athermalizing telescope structures and offer several benefits over Invar and CFRP including lower cost, faster lead time, and greater thermal stability control in modulated orbital thermal environments.

To achieve the ambitious science goal of performing direct imaging of earth-like exoplanets with a high contrast coronagraph, future space-based astronomical telescopes will require wavefront stability several orders of magnitude beyond state-of-the-art. The Ultra-Stable Large Telescope Research and Analysis – Technology Maturation (ULTRA-TM) program is maturing key component-level technologies for this new regime of “ultra-stable optical systems” through hardware testbeds that demonstrate component performance in the desired picometer regime and with path-to-flight properties. This paper describes the initial results from these testbeds – which address key capabilities across the ultrastable architecture and include active components like segment edge sensors, actuators and thermal sensing and control hardware, as well as passive components like low distortion mirror mounts and stable composites for structures. These promising experimental results are the first steps in our team’s technical maturation plan to credibly enable a large, ultrastable telescope in space. The resulting component, sub-system and system roadmaps are meant to support planning for technology development efforts for future NASA missions.

The Star-Planet Activity Research CubeSat (SPARCS) is positioned to revolutionize our understanding of M-dwarf star evolution, activity, variability, and the habitability of surrounding exoplanets. SPARCS will be the first mission to observe M stars for long periods of time simultaneously using a dual channel FUV (153 – 171 nm) and NUV (260 - 300 nm) imaging system. Anticipated to launch in 2023, SPARCS will provide key UV context to future observations by TESS and JWST, and the spaceflight application of advanced new detector technologies will pave the way for their implementation into future missions like LUVOIR and HabEx. To realize the scientific potential of SPARCS against the challenges of the ultraviolet spectrum, we are developing the specialized facilities, procedures, and tests necessary to assemble, integrate, and test the SPARCS science payload and spacecraft. A thorough testing campaign will verify the performance of individual payload components and obtain calibration baselines from the fully assembled science instrument that are vital to the data reduction process and in-flight contamination monitoring. SPARCS requires extensive contamination control to maintain its sensitivity in the FUV and NUV, which means all of AIT must occur in controlled and precisely monitored environments. This work will result in: (1) The delivery of the assembled and tested SPARCS spacecraft for launch in 2023. (2) A comprehensive performance validation and calibration baseline for SPARCS including a measurement of system throughput to for every wavelength across the SPARCS bandpasses, maps of NUV and FUV sensitivity across the payload field of view, and a full set of calibration products like flatfield images and dark current measurements for data reduction and comparison with calibration products acquired in orbit to monitor spacecraft conditions. (3) The establishment of a fully operational CubeSat AIT laboratory at ASU equipped to handle CubeSats up to 6U in size requiring meticulous contamination control up to the levels required for working in the FUV. This paper presents the work completed so far on the development and early operation of assembly, integration, and testing facilities for SPARCS. A custom thermal vacuum (TVAC) chamber facility was created and one of Arizona State University’s cleanroom environments was retrofitted to accommodate a 6U ultraviolet CubeSat requiring strict contamination control. We will describe the TVAC facility design and early testing, the cleanroom operation and contamination monitoring, and the development of an optical system and procedures to characterize the optical performance.

The Cherenkov Telescope Array (CTA) is the next-generation ground-based observatory for very-high-energy gamma rays. One candidate design for CTA's medium-sized telescopes consists of the Schwarzschild-Couder Telescope (SCT), featuring innovative dual-mirror optics. The SCT project has built and is currently operating a 9.7-m prototype SCT (pSCT) at the Fred Lawrence Whipple Observatory (FLWO); such optical design enables the use of a compact camera with state-of-the art silicon photomultiplier detectors. A partially-equipped camera has recently successfully detected the Crab Nebula with a statistical significance of 8.6 standard deviations. A funded upgrade of the pSCT focal plane sensors and electronics is currently ongoing, which will bring the total number of channels from 1600 to 11328 and the telescope field of view from about 2.7° to 8° . In this work, we will describe the technical and scientific performance of the pSCT.

We establish a viable laser payload design for the Orbiting Configurable Artificial Star (ORCAS) mission. We share observational considerations and derive the engineering requirements for the laser payload. Developed by general Atomics Electromagnetic Systems, the dual-wavelength laser will operate at 1064 nm and can be frequency-doubled to 532 nm, with two possible beam divergence modes and tunable power. The laser payload can be operated at pulse repetition rates greater than 10 kHz to enable compatibility with Adaptive Optics systems and to maintain pointing requirements. We show that such a laser payload can be constructed based upon a high-TRL amplified fiber laser Communication Terminal modified to meet the mission requirements.

The Polstar Mission seeks to study the evolution of massive stars including their effect on the interstellar medium and their behavior in binary systems using a 60 cm telescope with a UV Spectropolarimeter within MIDEX mission constraints on cost cap, throughput, coating requirements, and system-level dimensional stability. The mission is in a high-earth orbit and must ensure precise and repeatable polarimetric observations. Design-to-cost paradigms are exercised throughout all design phases and heritage approaches to structure and mirrors are evoked. In terms of classical error budgets, designing for diffraction-limited performance at 1.2 μm is sufficient, however, there are special design concerns at these wavelengths which require maximizing throughput of photons. Special coatings and minimum reflections are mandatory with meticulous attention to cleanliness throughout the entire mission life cycle. Decontamination heaters must be employed shortly after launch, prior to opening the door, and periodically throughout the mission lifetime. Additionally, spectropolarimetry requirements impose constraints on symmetry and control of phase and amplitude. The secondary mirror must have adjustment capability in three degrees of freedom (tip, tilt, and focus) to address drifts from thermal perturbations, aging, and possibly even spacecraft jitter. We present in-process design approach and analyses to meet the challenges of ultraviolet wavelengths and polarization stability..

From studying the fossil records of stars to exploring the circumgalactic medium, UV astronomy is a field rife with scientific opportunity. CETUS is a proposed next-generation UV space telescope equipped with a suite of instruments tailored to the study of UV phenomena in our galaxy. To achieve diffraction-limited imaging and spectroscopy performance at short wavelengths, a high-performance and resolution optical design is necessary. We describe the telescope design options including a trade study between a traditional on-axis TMA and freeform off-axis TMA solution considering their alignment sensitivity and tolerances. Different secondary support structures are explored for the on-axis design to analyze the irradiance distribution of the point-spread function (PSF) due to the pupil obscuration and how it influences the simulated starfield at the telescope focal planes. With rigorous analysis we aim to enable the next spaceborne observatory for UV astronomy.

The Laser Interferometer Space Antenna (LISA) is a large-scale space mission design to directly measure gravitational waves using laser interferometry techniques. The constellation of three spacecraft, each separated by 2:5 Gm, will follow a heliocentric orbit with a constant distance from Earth (~20°). Light exchanges between the spacecraft will be enabled by 300mm telescopes used to simultaneously transmit and receive. Each telescope is part of the interferometer, and each must meet tight requirements on its dimensional stability; below 1pm= pHz in the LISA band, μm-length stability over 10 years of mission duration, and below ppb backscatter of the transmitted light. Here, we present our progress in developing ground support equipment for the LISA telescope ground verification. We also report on recent experimental results of the dimensional stability for the telescope test structure; a key part of the ground support equipment, and simulations of the optical design and internal and external alignment tolerances of the test structure and the telescope within it.

A multiple-wave lateral shearing interferometer extends upon the traditional lateral shearing interferometry by producing multiple sheared copies of the incoming light. By using a special grating instead of a shear plate, it is able to produce fringes in multiple directions at the same time. This makes it possible to do single-shot reconstruction of both phase and amplitude aberrations. Instead of a surface relief grating, we propose to use a patterned half-wave plate manufactured using direct-write techniques that acts as a liquid-crystal geometric phase grating. We demonstrate its wavefront sensing capabilities with laboratory measurements with an ALPAO-97 deformable mirror in monochromatic light. Finally, we present on-sky measurements performed at the William Herschel Telescope, showing broadband operation in unpolarized light.

The optical community frequently relies on optical adhesives to join multi-component optical assemblies. However, options for bonding infrared materials are limited, since most optical adhesives are organic which means poor index matching to high index materials, and the introduction of their own IR absorption bands. Contact bonding is a potential solution, as it provides adhesion through intermolecular forces, capillary forces, and covalent bonds between bonding surfaces. However, to be successful, a reliable non-destructive characterization technique must be designed to evaluate bond quality. Many optical applications require low temperature processing to avoid degradation of any optical coatings. At room temperature, trapped water in the interface is a dominant contributor to the strength of contact bonded assemblies. Therefore, evaluating the interface is a crucial portion of determining bond quality. This report will analyze the potential of acoustic microscopy as a non-destructive technique to probe trapped water in the bonded interface. Several experiments were conducted to ascertain the sensitivity of acoustic microscopy to trapped water: (i) FTIR spectroscopy was used to provide a benchmark, (ii) bulk silicon samples were bonded in ambient atmosphere and aged over several weeks, (iii) and various annealing procedures were also performed to demonstrate water content evolution. A spring model of the bonded interface is used to quantify the amount of water in these studies through interfacial stiffness. Through these experiments, acoustic microscopy is shown to be an effective non-destructive technique to observe changes in interfacial water and may be used alongside other methods to screen high-value optical assemblies.

LISA is a space-based gravitational wave observatory aimed at detecting gravitational waves in the frequency range of 0.1 mHz to 0.1 Hz. The observatory is composed of three spacecraft, each separated by 2.5 million km in an equilateral triangle formation, trailing the Earth in a heliocentric orbit. One of the many crucial components to the mission is the LISA telescope, a bidirectional component used to expand an outgoing laser beam to the far spacecraft as well as compress a large incoming beam to a diameter of a few mm at the optical bench. Since the telescope is in the path of the long-baseline interferometer, its structure must be dimensionally stable at the pm/√Hz level at mHz frequencies. A way to measure the stability of the LISA telescope is with a compact optical truss interferometer (OTI), consisting of three Fabry-Perot cavities mounted along the telescope to monitor structural distortions over time. All three cavities are operated with a common laser source, and each cavity is equipped with an acousto-optic modulator to shift the nominal laser frequency as well as an electro-optical modulator to modulate the laser phase for Pound-Drever-Hall locking. Variations in each cavity’s length create variations in their corresponding laser frequency, which can be measured against a reference frequency that is locked to an external ultra-stable cavity. We will present the design and preliminary results in the fabrication and testing of firstgeneration OTI prototypes.

A systematic characterization of dielectric coatings for space application is carried out to evaluate the performance drop upon protons exposure. Different energy levels and fluence values are tested. Also, since the irradiation experiments are performed in large scale facilities, different flux rates are tested to establish the operational parameters that better mimic the real irradiation conditions. The induced damage associated with each irradiation session is characterized for a selection of single and bi-layer coatings. Results show that the performance drop is highly dependent on the fluence and the implantation energy.

Verification of thermal-mechanical-optical design for optical instruments in space exploration is highly significant due to large temperature variation and exposure to high shock and vibration levels. Such instruments must be completely robust to these harsh environments, as there are usually no options for realignment. The JAXA Martian Moons eXploration (MMX) Mission is set for launch in 2024 with main objectives to study the Martian moons, Deimos and Phobos. A rover will acquire for the first time Raman spectra of the Phobos surface using the Raman Spectrometer for MMX (RAX) developed at DLR. The Structural-Thermal-Model (STM) of RAX presented an early opportunity to evaluate the robustness of the instrument optical alignment to thermal and mechanical environments. An interferometric method implementing dummy objectives with cross hairs was developed to enable inline six-DOF measurements at critical places within the STM before development model (DM) optics were manufactured.

Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a space-based, MIDEX-class mission concept that employs a 17-meter diameter inflatable aperture with cryogenic heterodyne receivers, enabling high sensitivity and high spectral resolution (resolving power ≥10⁶) observations at terahertz frequencies. OASIS science is targeting submillimeter and far-infrared transitions of H₂O and its isotopologues, as well as deuterated molecular hydrogen (HD) and other molecular species from 660 to 80 μm, which are inaccessible to ground-based telescopes due to the opacity of Earth’s atmosphere. OASIS will have <20x the collecting area and ~5x the angular resolution of Herschel, and it complements the shorter wavelength capabilities of the James Webb Space Telescope. With its large collecting area and suite of terahertz heterodyne receivers, OASIS will have the sensitivity to follow the water trail from galaxies to oceans, as well as directly measure gas mass in a wide variety of astrophysical objects from observations of the ground-state HD line. OASIS will operate in a Sun-Earth L1 halo orbit that enables observations of large numbers of galaxies, protoplanetary systems, and solar system objects during the course of its 1-year baseline mission. OASIS embraces an overarching science theme of “following water from galaxies, through protostellar systems, to oceans.” This theme resonates with the NASA Astrophysics Roadmap and the 2010 Astrophysics Decadal Survey, and it is also highly complementary to the proposed Origins Space Telescope’s objectives.

The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a 17 meter class space observatory concept that will perform heterodyne and high spectral resolution observations at terahertz wavelength ranging from 81 to 659 micrometers to observe the transition of water and its isotopologues and other molecular species[1]. The baseline design, in particular with an inflatable primary antenna achieves orders of magnitude larger photon collection area >120 m² and diffraction limited performance at field of view (FOV) of +/- 0.05 deg with a simple tip/tilt scanner and over 0.2 degrees with an advanced scanning field lens design. The THz observatory with such an inflatable primary system involves an interesting challenge in optical design. The surface shape of the inflatable primary antenna, known as Hencky surface, induces 4th or higher order deformation of reflector surface which is corrected by following 3-mirror correction optics, with a power arrangement which is similar to Offner’s null corrector optics. The same optical architecture is also applicable for more parabola like inflatable antenna shape. The diffraction limited intermediate image field is scanned by a mirror tip-tilt scanner, alternatively for a larger FOV scanning a field lens, refractive or reflective, rigidly connected to the scanning mirror is employed. The design with thin reflective field lens or all refractive design increases overall photon throughput while accommodating broad band spectral range. Along with the 1st and 3rd order optical design procedure, in this presentation, We address challenges in optical design of such a large and inflatable antenna based photon collection system in THz astronomy, including correction of aberration from a membrane antenna, and relay optics to match mode field of antenna to that of THz heterodyne detectors.

OASIS (Orbiting Astronomical Satellite for Investigating Stellar Systems) is a space-based observatory with a large inflatable primary reflector that will perform high spectral resolution observations at terahertz frequencies. An inflatable metallized polymer membrane serves as the primary antenna with large photon collecting area, followed by aberration correction mirror pair that enables a large field of regards of 0.1 degrees while achieving diffraction limited performance over a wide terahertz wavelength ranging from 80 μm to 660 μm. An analytical model is developed to define a solution space based on the profile of primary reflector which is a function of pressure. The photon collecting area, size and weight of the correction mirror pair, and optical aberrations are governed by a 1st order power arrangement of the telescope and is a function of base radius and clear aperture of the primary reflector. Based on the parametric design study, the figure of merit for the profile of the primary reflector is discussed and a baseline design satisfying the scientific and system requirements is proposed.

Deformable reflector technology has mainly been used for observations at visible and infrared wavelengths but has yet to be utilized for terahertz wavefront correction. We present an actuator for deformable reflectors that overcomes challenges particular to this wavelength such as a millimeter-scale stroke requirement. Bending moment actuators are used in both the radial and tangential directions to correct low-order wavefront aberrations. Strong and flexible materials such as Delrin are used for the reflector material. Such a deformable antenna can be used to correct wavefronts on future large radio antennae such as the Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS). This antenna uses a 20-meter thin membrane as its primary radio wave collector. A deformable reflector may be added to this system to allow for looser tolerances on the primary antenna shape and correct for wavefront errors inherent in an inflatable optic. To predict the wavefront errors that may be expected when using this type of thin membrane primary reflector, TVAC (Thermal Vacuum Chamber) test methods are also presented in these proceedings.

Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a mission concept being developed in preparation for the 2021 MidEX Announcement of Opportunity. This paper describes the key features of the OASIS architecture as they are currently understood. OASIS’s choice of a large inflatable primary reflector results in large collection areas at very high mass efficiency enabling the science mission. We describe the spacecraft bus, based on Northrop Grumman’s LEOstar-2, and the receiver, a heritage design based on the GUSTO balloon heterodyne system. We also discuss the observing strategy and pointing requirements from its planned L1 location. Particular emphasis is placed on challenges to the design, such as momentum management, balancing consumable mass allocations, thermal management, and testing.

This paper discusses pressure control for the OASIS primary antenna element, A1. This discussion is centered around the evaluation of pressure changes and what might drive them. A1 is created from thin polyimide film and from its orbital position near Sun-Earth L1, is subject to many environmental effects, the solar wind, radiation pressure, charging and micrometeoroids. This paper begins by describing the architecture of the pressure control system. We show that the solar wind and radiation pressure are too small to impact A1’s performance. We also discuss the need to connect the A1 to system ground for solid technical and programmatic reasons. A large section discusses the micrometeoroid environment and how recent mission data shows that the flux faced by OASIS is likely larger by factor of ~3 than might be expected from naïve application of the traditional models.

The uninflated shape configurations of parabolic and spherical membrane mirrors were calculated by solving the inverse problem, i.e., given the design inflation pressure, the membrane material and geometric properties, what must be the initial uninflated shape such that on inflation to the design pressure, the exact desired surface of revolution is obtained. The resulting first order nonlinear differential equation was numerically integrated using the boundary conditions. The initial uninflated shape was then subjected to a forward transformation using FAIM, a proprietary geometric nonlinear membrane finite element code. FAIM has been validated against exact analytical solutions for both small and extremely large deformations that are up to eight orders of magnitude larger compared with the starting undeflected shape. Simulations reveal that to fabricate a very accurate and precise inflated membrane mirror relative to the design parameters, one must not only accurately measure and input the moduli in both meridional and hoop directions but an accurately measured Poisson’s ratio as well. The code was used to guide the membrane mirror design. For very small aperture diameters, the initial uninflated shape may be fabricated by thermo-forming the membrane. For aperture diameters exceeding one meter however, the membrane mirror is built with discrete gores that are joined together with tapes at the seams. This provided the impetus to write a companion computer code FLATE, to calculate the gore shapes using a slight modification of the solution to the inverse transformation equation to account for the presence of the seam tapes. After the gores were determined, the resulting final inflated shape was calculated and verified using FAIM. Sensitivity analyses can now be carried out to determine the resulting surface shape as a function of the different sources of error: gore width, gore length, perimeter attachment uncertainties, thermal effects, variation of material properties over the membrane continuum and inflation pressure changes. The code has been shown to be more robust than equivalent commercial analytical packages in so far as membrane, cable and space-frame element combinations are concerned. In particular, the analytical and finite element codes were used in the preliminary assessment of a membrane optic for the OASIS Mission (Orbiting Astronomical Satellite for Investigating Stellar Systems) [1]. The OASIS is a 20-meter class space observatory operating at high spectral resolution in the terahertz frequencies. Over its nominal 2-year mission it will probe conditions and search for biogenic molecules on hundreds of protoplanetary disks and other solar system objects.

Inflatable membrane primary optics for space telescopes are a smart approach in the context of saving flight payload weight and volume. The Orbiting Astronomical Satellite for Investigating Stellar systems (OASIS) adopted the membrane architecture for primary optics (primary antenna, A1) to have 20 meter diameter collection area with operation bands at the terahertz frequency. The membrane is made of Kapton or Mylar film with an aluminized surface, and the balloon (transparent surface + aluminized surface) is inflated to work as the convex mirror. In order to leverage the carrying volume advantage of inflatable optics, it must be folded during launch and deployed in orbit. The thin membrane film can crumple easily when it is folded, and it should be ironed out when the telescope is deployed for observation. We studied the microroughness and mid-to-high spatial frequency characteristics of the membrane via optical metrology to evaluate the surface properties. Because it is not of traditional shape and material, it is impossible to test with an offthe- shelf interferometer and profilometer. Moreover, the defect spatial frequency of interest is a few hundred microns to millimeters range, so the measurable field and dynamic range need to be in range of a few centimeters with microns resolution. To meet those requirements for metrology, we developed a flexible optics testbed utilizing deflectometry. The microroughness and mid-to-high frequency properties are measured with a white light interferometer and proposed methodology. The test results show that the candidate membrane is suitable for OASIS and this reliable test will guide the further design study of A1 assembly and optical system error budget.

The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a 20-meter class proposed space terahertz observatory supported by an inflatable membrane architecture. To measure 150 mm and 1m models of the A1 reflective membrane antenna, two deflectometry configurations were designed. The smaller assembly and its corresponding deflectometer were simulated, built in our laboratory, and produce a reconstructable signal for clocked measurements of the highly-sloped pneumatic surface. We use non-sequential raytracing simulation to bound the maximum contributions of all shape errors and suggest the N-Rotations algorithm to remove the remaining radially asymmetric errors. Then, the 1m prototype assembly was tested inside a thermal vacuum chamber (TVAC). Differential deflectometry measurements tracked the 1m surface shape changes as it was subjected to a variety of environmental setpoints, cycled between three inflation gases, and also during controlled puncture. We summarize our development and results for absolute measurements as well as from TVAC testing.

Spaceborne optical lens assemblies serve a wide range of applications, including deep-space exploratory missions, earth weather imagery, satellite reconnaissance, and surveillance. These applications demand operation over a wide range of optical performance metrics and environmental conditions. Systems deployed in space cannot afford to use the limited battery or solar power to energize motors required for maintaining focus and diffraction-limited image quality. Therefore, it is vital to develop and characterize passively athermalized solutions for spaceborne optical assemblies that are isolated from direct human contact or have limited access to power. These multi-lens element systems of varying glass materials are susceptible to small optical property changes between glass melt lots and highly dependent on the opto-mechanical assembly method; this creates large sensitivities within the system design that can result in changes in system level performance parameters. Accurate quantification of the optical system performance over the specified operating temperature range prior to flight is critical for mission success. Collins Aerospace, Mission Systems Optronics has developed an environmental test setup and interferometric wavefront measurement approach capable of quantifying the optical performance over temperature of passively athermalized spaceborne optical assemblies. Interferometric wavefront measurements are performed across the operating temperature range; resulting interferograms are analyzed and decomposed in terms of thermal defocus, low-order aberrations, and RMS wavefront error to assess final system compliance. The laboratory prototype system consists of a 633nm common-path interferometer, nitrogen-purged environmental test chamber, and retro-null reflecting pin. We use an f/3 aerospace lens, intended for deep-space application, to demonstrate these measurement techniques.

Adaptive optics (AO) offers an opportunity to stabilize an image and maximize the spatial resolution achievable by ground based telescopes by removing the distortions due to the atmosphere. Typically, the deformable mirror in an AO system is integrated into the optical path between the secondary mirror and science instrument; in some cases, the deformable mirror is integrated into the telescope itself as an adaptive secondary mirror. However including the deformable mirror as the primary mirror of the telescope has been left largely unexplored due to the previous cost and complexity of large-format deformable mirror technology. In recent years this technology has improved, leaving deformable primary mirrors as a viable avenue towards higher actuator density and a simplification in testing and deploying adaptive optics systems. We present a case study to explore the benefits and trade-offs of integrating an adaptive optics system using the primary mirror of the telescope in small-to-mid-sized telescopes.

Spectroscopic observations in the vacuum (VUV, 115-200 nm) and extreme ultraviolet (EUV, 40-115 nm) is of fundamental importance in solar physics, in the physics of interstellar medium, in the study of planetary exospheres. The PLUS project is focused on the development of a high performance spectrograph for the observations of planetary exospheres in the 55-200 nm range. The instrument layout is based on a two channels (VUV/EUV) design. It will be characterized by improved detection limit, shorter observations integration time and unprecedented performance in terms of dynamic range. Such characteristics will be obtained thanks to the development and combination of two key technologies: high efficiency optical components optimized for each channel and high resolution/dynamic range solar blind photon counting detector. The photon counting detector will be based on a Micro-Channel Plate coupled with ROIC ASIC read out system.

Previously, we presented an optical design of a wideband spectrometer for science observations using space-borne telescopes. Therein, a multichannel configuration was adopted to cover a wide wavelength region from visible to midinfrared, by low, medium, and high dispersion spectrometers, and the influence of defocusing was considered. Using the optical designs of each channel as independent spectrometers was also possible. Herein, we report some progress on works for the optical design of a spectrometer for space-borne telescopes.

Multiband optical cameras have played important roles in science observations for many planetary explorations. To ensure high-quality imaging observations using a camera, it is crucial to reduce stray light, which is defined as light reaching an imaging detector via an irregular path in this study. Multiple reflections between a detector and a bandpass filter are often the major sources of stray light in planetary camera optics. In this article, we present a study for camera optics using tilted bandpass filters to reduce stray light caused by multiple reflections between a detector and bandpass filters. A comparative study for four cases of optical design, which have different configurations of the bandpass filter, was conducted. A case of optical design adopting a combination of X- and Y-tilted filters provided high performance. The results of this study are potentially applicable not only for cameras for planetary exploration but also for various other optics.