This PDF file contains the front matter associated with SPIE Proceedings Volume 11116, including the title page, copyright information, table of contents, and author and conference committee lists.

The Chandra X-ray Observatory has been operating successfully on orbit for 20 years, providing outstanding astrophysics data to the science community. The telescope’s mission life has been extended well past its required five years due to the robust system design as well as the mission planning skills of the engineers and scientists at the Chandra Operations Control Center (OCC) in Massachusetts. Lessons learned in designing and operating Chandra can significantly benefit the Lynx mission, one of the NASA Strategic Mission concepts under consideration by the 2020 Decadal Survey. This paper reviews the design features that have enabled Chandra’s long mission life, with emphasis on the performance of the thermal and mechanical designs. Impacts of the aging exterior thermal finish are discussed, including drifting internal temperatures and a corresponding loss of the 10C cold bias design surrounding the High Resolution Mirror Assembly (HRMA). Also addressed are the resulting changes to the radiative cold heat sink for the Aspect Determination System (ADS); the increased potential for contamination on optical and focal plane surfaces; and consequences of putting components close to their upper temperature limit. Actions to address the changing conditions by the flight operations team at the OCC are described, showing how thermal constraints have been accommodated. Lessons learned, optimized operations, and new thermal design options are presented as methods to ensure a long life for the Lynx mission.

Coronographic missions require ultra-stable mirror systems to achieve 10 E-10 contrast. The LUVOIR ULTRA study is assessing technological capabilities for a 15-meter telescope requiring sub nanometer optical stability. For ULTRA individual mirror stabilities at the picometer level are required. Thermal sensitivities for a proposed mirror design have been incorporated into a stability budget that indicates the level of stability required is attainable. Key factors to meeting stability allocations are an athermal design, acceptable mirror CTE homogeneity and mirror mount pad design. This paper will present the sensitivities and error budget used to predict the on-orbit mirror stability. The author requests that a paper given by Matt East from Harris come before this presentation if both are presented.

The Habitable Exoplanet Observatory (HabEx) is one of four mission concepts under study for the 2020 Astrophysics Decadal Survey. Its goal is to directly image and spectroscopically characterize planetary systems in the habitable zone around nearby sun-like stars. Additionally, HabEx will perform a broad range of general astrophysics science enabled by 115 to 2500 nm spectral range and 3 x 3 arc-minute FOV. Critical to achieving the HabEx science goals is a large, ultrastable UV/Optical/Near-IR (UVOIR) telescope. The baseline HabEx telescope is 4-meter off-axis unobscured, diffraction limited at 400 nm with wavefront stability on the order of a few 10s of picometers. The technology readiness level (TRL) to manufacture and test the HabEx baseline primary mirror is assessed to be at TRL-6 for all but two TRL-4 technologies: 1) non-destructive process to quantify CTE homogeneity of a 4-m mirror substrate with a spatial sampling of at least 100 x 100 to better than +/- 1 ppb/K; and, 2) process to quantify self-weight gravity deflection to better than 4-nm rms over a 100 x 100 spatial sampling. This paper reviews the technology needs to manufacture the HabEx primary mirror, assesses their TRL and proposes a roadmap to mature the two remaining technologies to TRL-6.

CHEOPS (Characterizing Exoplanets Satellite) is devoted to the characterization of known exoplanets orbiting bright stars, achieved through the precise measurement of exoplanet radii using the technique of transit photometry. CHEOPS was selected in October 2012 as the first Small-class mission (S1) within the Agency’s Scientific Programme, with the following programmatic requirements: science driven mission selected through an open Call; an implementation cycle, from the Call to launch, drastically shorter than for Medium-class (M) and Large-class (L) missions; a strict cost-cap to ESA, with possibly higher Member States involvement than for M or L missions. Following a phase A/B1 study, CHEOPS was adopted for implementation in February 2014 as a partnership between the ESA Science Programme and Switzerland, with a number of other Member States delivering significant contributions to the instrument development and to operations. The CHEOPS payload is a high precision photometer, with an optical Ritchey-Chrétien telescope with 300 mm effective aperture and a large external baffle to minimize straylight. The CHEOPS spacecraft (280 kg mass, 1.5 m size) is based on a flight-proven platform and will orbit the Earth in a dawn-dusk Sun Synchronous Orbit at 700 km altitude. CHEOPS completed the Preliminary Design Review at the end of September 2014, and passed the Critical Design Review in May 2016. In 2017, flight platform and payload have been separately integrated and tested, while satellite activities were completed by end 2018, allowing to reach flight readiness. CHEOPS is scheduled for launch on a shared Soyuz flight by the end of 2019.

The LUCI (LBT Utility Camera in the Infrared) instruments are a pair of near infrared (NIR) imagers and spectrographs for the Large Binocular Telescope (LBT) that include a set of cryogenic exchangeable focal plane masks. Although LUCI covers the NIR zJHK bands at different resolutions with existing gratings, it is not currently possible to get zJHK in a single exposure with a single LUCI which is required for some planetary science programs. To produce a simultaneous zJHK spectrum with a single LUCI, we designed a system consisting of small and simple optical elements to fit within the limited space in the focal plane mask frame to cross-disperse fixed short slits. This system, called MOBIUS (Mask-Oriented Breadboard Implementation for Unscrambling Spectra), consists of a double-folding mirror, a collimating spherical mirror with 180 mm radius of curvature, and a dispersing prism with the rear surface mirror-coated. MOBIUS disperses the input slit perpendicular to the dispersion direction of the gratings in LUCI. The resulting order separation is at least ∼2.7 arcsecond, allowing a slit length of up to ∼2.3 arcsec without mixing orders at the LUCI image plane. Since MOBIUS would be introduced into the existing light path via the exchangeable slit mask mechanism, no modification to the current LUCI instrument is needed. Eventually, binocular observations combining one of the Multi-Object Double Spectrographs (MODS) with LUCI+MOBIUS at the LBT will provide simultaneous coverage from 0.3 to 2.4 μm for studies of asteroids and other faint solar system bodies.

The Strategic Astrophysics Technology (SAT) program has recently identified several critical technology gaps related to future missions that have direct relevance to thermal control methods for optical payloads: (1) thermally stable telescopes, (2) sensing and control at the nanometer level or better, and (3) sensing and control at the picometer level [1]. Implementing very tight control stability on optical payloads in the space environment to achieve precise line of sight and wavefront control is more than just a thermal problem. It is a combination of system design challenges implementing thermal, electronics, and control methods. These challenges are further complicated by size, weight, and power (SWAP) constraints for large-scale optical platforms due to both quantity of sensors and physical separation between sensing and control electronics. For thermal hardware, control errors arise from sensors indirectly coupled to the controlling heat source that may result from installation constraints, sensor or heater attachment methods, or poor thermal diffusivity. For electronics, control errors arise due to system resolution limitations which are dependent on bit accuracy over the temperature range of interest, current and voltage source accuracy, sensor self-heating, noise sources, sampling rates, and circuit averaging methods. Errors arise from limitations in the control methods such as over- and under-shoot with bang-bang (on/off) or proportional-integral-derivative control (PID). Within the PID control method, there are many nuances to the implementation as well, such as the need to tune each control zone for optimal control. This paper presents a description of the challenges and opportunities that come with high precision space telescope thermal control for candidate future astrophysics missions like LUVOIR or HABEX and provides examples of thermal design and analysis methodologies underway for the WFIRST program.

We describe a novel space observatory concept that is enabled by very large (8.5m-diameter), ultralight-weight multi-order diffractive lenses that can be cost-effectively replicated. The observatory utilizes an array of identical telescopes with a total combined light collecting area equivalent to that of a 50m-diameter telescope. Here we review the capabilities of a Nautilus unit telescope, the observatory concept, and the technology readiness of the key components. The Nautilus Observatory is capable of surveying a thousand transiting exo-earth candidates to 300 pc for biosignatures, enabling a rigorous statistical exploration of potentially life-bearing planets and the diversity of exo-earths.

The Schwarzschild-Couder Telescope (SCT) is a mid-size telescope proposed for the Cherenkov Telescope Array. In order to substantially improve the field of view and image resolution compared to i traditional Davies-Cotton telescopes, innovative solutions are foreseen in the design, like the use of Silicon Photomultipliers (SiPM) as light sensors and waveform digitizers for recording the fast light signals from atmospheric showers. A project is now underway to upgrade the camera by increasing its pixel count to 11,328 pixels and field of view of 8^o. The camera electronics has been completely redesigned by using new waveform digitizer and trigger ASICs with the final goal of lowering the gamma-ray energy threshold and therefore provide an excellent instrument tailored for extended sources investigations and multi-messenger astronomy.

THE MOST, The High Étendue Multiple Object Spectrographic Telescope, is a Primary Objective Grating (POG) telescope based on a Newtonian double dispersion architecture. Because it is inexpensive to scale up to large collecting area, this telescope could make an extraordinary spectroscopic survey instrument for astronomy. We describe here the laboratory tests on a scale model. We used two light sources to simulate starlight: The first was a blackbody incandescent filament in a flashlight. The other was a research-grade collimated laser. Both sources focused to 5 cm diameter circles with controlled angles of incidence. We tested two flat surface-relief reflection plane gratings as the POG in the scale model. The first POG was a conventionally ruled 4 cm², 1180 line/mm grating mounted on a glass plate. The other POG was a 750×300 mm holographically mastered 1600 line/mm embossed decorative crating on polyester film, sold commercially as Holosheen. We report on experiments that compared resolution in the first-order diffracted image with the zero-order reflection image. Our report provides empirical evidence that along the diffraction axis THE MOST figure tolerances are vastly relaxed compared to mirror and lens telescopes. We measure the throughput of the Holosheen decorative grating to be 14% in the first order. We estimate the rate of SDSS-like spectrum acquisition from THE MOST in comparison to that of the Sloan Digital Sky Survey (SDSS) as an example, though this survey is not optimized to take advantage of the unique capabilities of THE MOST.

National Space Organization (NSPO) has actively promoted local space industry and tried a promising approach for the new remote sensing satellite mission in the upcoming 10 years space program. NSPO executes a Micro-satellites program which promote the cooperation with domestic academia and industry to develop the space components, subsystems, system, and forming space industry in Taiwan. Most of the satellite key components will be designed and made in Taiwan. That will be the first space program in Taiwan aiming to the promotion of space industry locally. Development of a high quality proto-type multi-spectral filter is reported in this paper. This optical filter will be installed in front of CMOS image sensor to record multi-spectral images of the earth surface. The optical spectral design composes of five band-pass filters on single substrate, including three bands in visible range, one panchromatic band including whole visible spectrum and one band in near infrared. MORRISON Opto-Electronics (MOE) Ltd is responsible for spectral design and multi-layer thin film deposition. The optical filter has been implemented optical spectrum measurement and cross cut adhesion test. The environmental test, including high temperature and humidity, thermal cycling, and radiation tests are also performed to verify the validation of this device. A positive result shows that thin film structure and optical transmittance are acceptable and qualified for the space applications. According to the proto-type development experience, the fabrication of a space qualified multi-spectral filter which could fulfil the requirement for Micro-satellites mission is expected in near future.

Space observatories have many advantages over ground-based telescopes. However, constructing and launching large space telescopes remains a significant challenge. A solution to this problem lies in autonomous, in-space assembly. To gain benefits from efficiencies of scale and mass production, a modular telescope assembled in space can be constructed from identical mirror segments. These identical segments must then be deformed to an appropriate shape in space. This work examines the optical feasibility of such a project, using a 31 meter Ritchey- Chrétien telescope composed of about 1,000 1-m mirrors as a case study. In particular, this work examines the shape of the telescope optics through Zernike decomposition and computes the physical optics propagation of such a system to analyze the resultant PSF with simulation in Zemax OpticStudio.

The Deformable Mirror Demonstration Mission (DeMi) is a 6U CubeSat that will characterize the on-orbit performance of a Microelectromechanical Systems (MEMS) deformable mirror (DM) with both an image plane wavefront sensor and a Shack-Hartmann wavefront sensor (SHWFS). Coronagraphs on future space telescopes will require precise wavefront control to detect and characterize Earth-like exoplanets. High-actuator count MEMS deformable mirrors can provide wavefront control with low size, weight, and power. The DeMi payload will characterize the on-orbit performance of a 140 actuator MEMS Deformable Mirror (DM) with 5.5 μm maximum stroke, with a goal of measuring individual actuator wavefront displacement contributions to a precision of 12 nm. The payload will be able to measure low order aberrations to λ/10 accuracy and λ/50 precision, and will correct static and dynamic wavefront phase errors to less than 100 nm RMS. We present an overview of the payload design, the assembly, integration, and test process, and report on the development and validation of an optical diffraction model of the payload. Launch is planned for late 2019.

We are developing an innovative compact coronagraph for studying the physical conditions in the solar wind acceleration region. This paper presents the new development of the compact coronagraph for the investigation of temperature and speed of electrons in the solar corona. The proposed compact coronagraph is a one stage externally occulted coronagraph without internal occulter or Lyot stop mask. The key aspect of the new idea is to set the inner field cutoff at External Occulter (EO) much smaller than the specified inner field cutoff. A second occulter on the surface of the detector array removes the remaining diffraction. The occulter on the detector surface functions similar to an internal occulter with the Inner Field of View Cutoff (IFoVC) exactly the same as specified. For BITSE, the desired inner field cutoff is 3 R⊕, but the cutoff at EO is only 1.5 R. The diffraction analysis shows that in the sensor plane, the diffraction intensity at the 3 R is not sensitive to the EO cutoff, either at 1.5 R or close to 3 R. The advantage of having a smaller EO cutoff is that the vignetting decreased for the Field of View (FoV) near 3 R, therefore, the signal increases. Meanwhile, the diffraction of Point Spread Function is much less in the radial direction, which not only increases the image quality around 3 R, but also increases the encircled energy and signal to noise ratio. In other words, the data is useful right at 3 R! The BITSE optical design and diffraction analysis will be presented in detail. The simulation shows the signal to noise ratio obtained from the diffraction and vignetting data enables corona temperature and speed measurement.

The Antarctica Plateau with high altitude, low water vapor and low thermal emission from the atmosphere is known as one of the best sites on the earth for conducting astronomical observations from the near infrared to the sub-millimeter. Many optical astronomical telescopes are proposed by Chinese astronomical society at present, such as Kunlun Dark Universe Survey Telescope (KDUST), 6.5-meter optical telescopes and 12-meter optical and infrared telescopes. Accurate estimation of the sky background brightness of proposed sites provides the scientific basis for instruments design and observatory site selection. Based on this requirement, a near-infrared sky brightness monitor (NISBM) based on InGaAs photoelectric diode is designed by using the method of chopper modulation and digital lock-in amplifier in the near infrared band of J, H, Ks. The adaptability of the monitor under extremely low temperature conditions in Antarctica is promoted by taking advantage of PID heating and fault detection system. Considering the weak signal of Ks band in Antarctica, a surface blackbody is equipped for real-time calibration. For the adverse circumstances to human, an EPICS and Web based Remote Control Software is implemented for unattended operation. The NISBM has been successfully installed in Dome A, Antarctica on January 2019.

An in-orbit calibration assembly is designed and tested to ensure the radiometric accuracy of Sentinel-4/UVN. One of the calibration method consists in measuring the Sun’s irradiance through a diffuser. A major challenge was the apparition of so-called spectral features, which come from speckle patterns due to the thin spectral bands of the instrument. A stack of diffuser was built to increase the average optical path difference above coherence length, thus decreasing the speckle contrast. A unique spectral feature test bench has been developed to validate the performance. A second calibration approach consists in illuminating the instrument with a white light source whose drift is characterized and limited. This paper discusses the design of the calibration assembly and the challenge which were encountered in the process. Also, on-ground tests results are presented, including the description of the test setup, in particular the spectral feature test bench.

Advances in our understanding of the Universe depend on improvements in sensitivity and angular resolution that can come only with larger telescopes. Telescope diameters increased by almost an order of magnitude in the last century, but that growth has been sporadic, limited mainly by the ability to make bigger mirrors that hold their shape against the dynamic effects of gravity, wind and temperature. Three major advances in mirror technology occurred in the 1980s, including the lightweight honeycomb mirrors made at the Richard F. Caris Mirror Lab at the University of Arizona. In this informal paper, I will describe these technologies and show how they enabled the current generation of 8- to 12-m telescopes and how they are now being used to build telescopes of 25 to 39 m.

The vacuum of space can be used to fabricate functional reflective coatings in space via vacuum deposition. This approach allows for the direct deposition of reflective coatings onto mirror substrates in the vacuum environment of space. By coating mirror substrates in space, the sensitivity and reflectivity of visible, UV, FUV and X-RAY mirrors can be increased by eliminating the need to deposit oxidation protective coatings, as required on-earth. In addition, depositing mirror coatings on-orbit simplifies the handling and packaging of space mirrors and provides a low-cost capability to repair and restore mirror coatings on-orbit. In-space vacuum deposition has been successfully performed in Space during the Wake Shield Facility (WSF) demonstration missions (STS 60, 69 and 80) achieving a TRL of 7. The WSF Program was a space R&D program to develop in-space vacuum deposition technology in order to overcome the purity constraints found in terrestrial semi-conductor production. The WSF was deployed in the wake of the Space Shuttle at an orbital altitude of over 300 kilometers (186 mi), within the thermosphere, where the atmosphere is exceedingly tenuous. The forward edge of the WSF disk redirected atmospheric and other particles around the sides, leaving an "ultra-vacuum" in its wake. The experiments resulted in fabricating the highest quality GaAs semiconductors at the time and successfully proving the vacuum wake concept. By leveraging the WSF Program heritage, the fabrication of reflective coatings in-space can be directly applied to substrates to achieve the required performance which astrophysics requires for the next generation of space telescopes. Specifically, visible, UV and FUV mirrors coated with bare aluminum to achieve unparalleled reflectivity in space.

The next generations of Cosmic Microwave Background (CMB) polarimetry experiments will attempt to detect the faint primordial B-mode signal from gravitational waves. The increasing scale of photon-noise limited detector arrays of millimeter-wave astrophysics has led to the need for cryogenic refractive optics with large aperture, high dielectric constant, and low loss. Additionally, multiple frequency band observations for galactic foreground removal from CMB signal require broad bandwidth optics. Modern CMB polarimetry experiments use several cryogenically cooled refractive elements made of alumina or silicon. Their high dielectric constants require multiple layers of anti-reflection (AR) coating with different dielectric constants to minimize reflection at the dielectric boundaries. We have developed an AR coating technology for millimeter-wave optics which achieves minimal dissipative loss and broad bandwidth with a simple and accurate fabrication process. Ceramic coatings are applied using a standard plasma spray system. We tune the dielectric constant of the coating by mixing hollow ceramic microspheres with alumina powder as the base material or varying the parameters of the plasma system. By spraying low loss ceramic materials with a tunable dielectric constant, we can apply multiple layers of AR coating for broadband millimeter-wave detection. The ceramic coating also has matching coefficient of thermal contraction with alumina and silicon for robustness to cryogenic delamination. We report on the design, fabrication methodology, and measurement of coating uniformity, repeatability, and transmission at room and cryogenic temperatures. This technology is applicable from submillimeter to millimeter wavelengths for coatings with greater than octave bandwidth.

Optical coatings and materials need to be qualified against space environmental agents, such as protons, helium ions and electrons. The induced damage effects are studied in dependence on ion species, energy, flux and fluence. Results show that damages can be dramatically dependent not only by fluence, but also by ions energy, so that qualification should take this factor into account. The significance of results obtained by using gamma-rays in place of ions irradiation to qualify components is briefly discussed. A systematic experimental plan for an extensively study of the irradiation effects induced in a large number of different sample coatings and materials is presented.

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF₂, are used to protect the of aluminum mirrors against oxidation since aluminum oxide absorbs short wavelength light. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and ellipsometric (SE) studies of aluminum oxidation as a function of MgF₂ over a range of layer thickness (0-6 nm). We also show for the first time, dynamic SE data which, with appropriate modeling, tracks the extent of oxide growth every few seconds over a period of several hours after the evaporated Al + MgF₂ bilayer is removed from the deposition chamber, exposing it to the air. For each SE data set, because the optical constants of ultrathin metals films depend strongly on deposition conditions and their thickness, the optical constants for Al, as well as the Al and Al₂O₃ thicknesses, were fit. SE trends were confirmed by X-ray photoelectron spectroscopy. There is a chemical shift in the Al 2s electron emission peak toward higher binding energy as the metal oxidizes to Al⁺³. The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF₂ layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF₂ as a function of MgF₂ layer thickness. Here, k is a factor which depends only on MgF₂ thickness, and decreases with increasing MgF₂ thickness. The techniques developed can illuminate other protected mirror systems.

A review of extreme ultraviolet (EUV) multilayers for solar physics applications is presented. Several promising material couples, as well as different optimization strategies, have been explored to optimize the peak reflectance at different wavelengths. For example, ad-hoc capping layers can be employed both to protect the structure underneath and to enhance the reflectance performance at a specific wavelength or to achieve other features such as spectral width and purity. Wide-band and multiband EUV coatings can be achieved by using a-periodic designs, which are still largely unexplored. Aperiodic structures provide novel solutions to be considered in view of future space missions. All the proposed designs are discussed considering their stability in time and along space missions.

This paper presents modeling results for coating thickness as a function of position, for aluminum films made with a hexagonal array of evaporation sources. The computer simulation is based on measured plume data from a single evaporation source. The model is used to determine optimum source spacing for a given plume shape. The analysis revealed that arrangement of multiple sources in a hexagonal array can produce uniform coatings while utilizing a reasonable number of evaporation sources per square meter of coating area. Monte Carlo simulations followed by gradient descent optimization methods were used to determine optimal flatness solutions for groups of deposition sources with varied deposition times. Thin aluminum films with exceptional coating flatness are needed to meet the wavefront error requirements of future space-based telescope concepts such as HabEx, LUVOIR, CETUS and others.

As part of leading research for future space missions, we have been conducting a feasibility study on an optical imager system assumed to be mounted on a geostationary satellite for instantaneous Earth observation as needed. The target for ground sampling resolution was less than 10 m from geostationary orbit, and the primary mirror was set to a diameter of 3.5 m based on our previous conceptual study. Moreover, the primary mirror was conceptually designed with cuttingedge technologies such as segmented mirror technology for scalability to larger sizes in the future. The main technical challenges in achieving such a large optical system entailed reducing the primary mirror weight and minimizing dimensional changes in the space environment. Optical material selection was a particularly key consideration in defining the optical system performance. Therefore, a trade-off study was conducted on the selection of materials for the segmented primary mirror. The thermal deformation behaviors of certain low thermal expansion materials as mirror substrates were analytically compared under the assumed unsteady and inhomogeneous thermal conditions in geostationary orbit, in consideration of the deterioration induced by radiation.

This study examines optical materials for use in a geostationary Earth observation satellite. Cordierite ceramics are promising materials for mirror substrates because they have excellent physical properties such as a high elastic modulus, low bulk density, high thermal conductivity, and low coefficient of thermal expansion (CTE). Since cordierite ceramics have not been used in a space equipment, the resistances of their physical and optical properties to space environments are unknown and must be evaluated. Long-term exposure to radiation in space may change physical properties of materials that could degrade telescope performance. Changes in a parameter can also be used to analyze the performance of these mirrors. Therefore, the authors tested two cordierite ceramics, and three traditional glasses for comparison. Irradiation used an electron beam of 10 MeV to apply a dose of about 7 MGy, which corresponds to a total cumulative dose received over 20 years by an observation satellite in a geostationary Earth orbit (GEO). The elastic modulus did not change significantly in any material, and the CTEs of the two glass materials at around room temperature increased significantly after irradiation. This CTE deterioration may have been induced by the excessively accelerated test conditions, so the dose-rate dependence of the deterioration was also evaluated by gamma ray irradiation. Although the dose rate under the gamma ray irradiation was the three orders of magnitude lower than that under electron beam irradiation, changes in the CTEs of the two glass materials were measured.

Recent development in coating deposition processes for aluminum (Al) mirrors that are protected with a metal-fluoride overcoat (such as LiF, MgF₂, or AlF₃) have improved reflectance performance particularly in the far- ultraviolet (FUV) part of the optical spectrum. The active research in this area is motivated by the fact that these gains in reflectance are expected to significantly increase the throughput of any future FUV sensitive NASA missions into the Lyman Ultraviolet. These reflectance improvements are attributed, in part, by performing the metal-fluoride overcoat depositions with the substrates at an elevated temperature as high as 250 °C. ZERODUR® is a widely used material as a mirror substrate because, among other things, it exhibits a low coefficient of thermal expansion (CTE) over a wide range of temperatures. Moreover, ZERODUR® has recently been proposed for several future NASA concept missions where this improved FUV mirror coating may be used. Given the elevated temperature at which these improved FUV coatings are produced, it is imperative to make sure that heating of the substrate will not significantly impact the final figure of the coated mirror. In this paper, we will study and report the effects of heating ZERODUR® up to the highest temperature mentioned above (250 °C) during a simulated coating process. These studies are relevant since it has been reported the CTE will change if ZERODUR® is cooled down from application temperatures between 130°C and 320°C with rates that differ from the initial production annealing rate of 3°C/hr.

The Predictive Thermal Control Technology (PTCT) development project is a multiyear effort initiated in Fiscal Year (FY) 2017, to mature the Technology Readiness Level (TRL) of critical technologies required to enable ultra-thermallystable ultraviolet/optical/infrared (UVOIR) space telescope primary-mirror assemblies for ultra-high-contrast observations of exoplanets. Key accomplishments of 2017 to 2019 include: creating a high-fidelity STOP model of the AMTD-2 1.5-m Ultra-Low Expansion (ULE®) mirror (manufactured by Harris Corp) by merging 3D X-Ray computed tomography data of the ‘as-built’ mirror and coefficient of thermal expansion (CTE) data maps for each of the 18 core elements; partially validating this model by measuring the mirror’s response to bulk temperature changes and lateral thermal gradients; designed and built (with PTC partner Harris Corp) a 1.5-m enclosure with 26 actively-control thermal zones; and defined specifications for a potential 4-m primary mirror thermal enclosure for the Habitable Exoplanet (HabEx) Imager mission.

We present an optical metrology instrument for measuring both transmitted and reflected wavefront error (TWE and RWE) of coated or uncoated optics over a diameter of 5 inches. Depending on the coating transmittance and reflectance, the measurements have to be done at different wavelengths. Interferometer is a standard instrument to measure the TWE and RWE of uncoated optics. But in the case of coated optics (bandpass filters for example) measurement of TWE is not possible because the optics may not transmit the interferometer laser light. The chosen solution is based on a quadriwave lateral shearing interferometer (QWLSI) wavefront sensor. QWLSI is an achromatic technique, meaning that it measures OPD at any wavelength without any need for recalibration at specific wavelengths. Consequently, various sources at different wavelengths can be used with the same instrument and metrology bench. In addition, QWLSI measures the derivative of phase contrary to interferometer that measures phase. Therefore, QWLSI has by design a better WFE dynamic range for TWE and RWE measurement. Moreover accuracy (below 15nm RMS) and repeatability (below 2nm RMS) is perfectly adapted to optical metrology measurement. The optical solution is a standard double pass configuration composed of a collimator and a beam expander to adapt the size of the beam to the wavefront sensor aperture. We use LED sources to avoid any noise due to interferences within the optics, which occur with coherent light. We can use different wavelength between 400nm and 1100nm. We can optimize the longitudinal chromatic aberration by moving a lens from the beam expander. We characterized the bench according to the ISO 5725 standard for different wavelengths. Its precision was tested with different samples (filters and mirror). The precision on TWE was found to be below 2nm RMS.

Euclid is a space telescope currently developed in the framework of the ESA Cosmic Vision 2015-2025 Program. It addresses fundamental cosmological questions related to dark matter and dark energy. The lens system of one of the two scientific key instruments [a combined near-infrared spectrometer and photometer (NISP)] was designed, built-up and tested at the Max Planck Institute for Extraterrestrial Physics (MPE). We present the final imaging quality of this diffraction-limited optical assembly with two complementary approaches, namely a point-spread function- and a Shack- Hartmann sensor-based wavefront measurement. The tests are performed under space operating conditions within a cryostat. The large field of view of Euclid’s wide-angle objective is sampled with a pivot arm, carrying a measurement telescope and the sensors. A sequence of highly accurate movements to several field positions is carried out by a large computer controlled hexapod. Both measurement approaches are compared among one another and with the corresponding simulations. They demonstrate in good agreement a solely diffraction limited optical performance over the entire field of view.

Direct imaging of exoearths with high-contrast internal coronagraphs depends on ultra-stable opto-mechanical systems. Ultra-stable mirror assemblies enable decadal survey missions like LUVOIR and HabEx. To precisely define the necessary level of stability, the essential first step is to budget the maximum allowable disturbances for each optic in the system. Ideally, allocations are budgeted with respect to spatial- and time-domain frequencies. If allocations do not span these domains, the optic assembly designer cannot take advantage of frequency bands where requirements are looser because of assumptions about telescope control systems and internal coronagraph filtering. This paper explores how mirror assembly technologies and designs are predicted to impact stability, especially within the frequency bands that drive coronagraph contrast performance.

The current trend for higher resolution and sensitivity for Astronomy and Earth Observation space missions is leading to larger entrance apertures for future optical payloads, often requiring challenging and ultra-stable optical performances driving the instrument design and implementation. The level of complexity for such large systems requires a multidisciplinary approach and technological developments in cross-sectorial areas such as optics, structures, pointing accuracy, control, mechanisms… We present here a range of ESA R&D developments related to future large ultra-stable optical instrument architectures, providing perspectives on the identification of enabling technologies in view of current and future optical missions as well as and a wayforward to maturity for implementation within potential future missions.

Thales Alenia Space is designing and developing space observation instruments since more than 40 years. This paper explains why active optics is needed for next generation of instruments for Earth observation. It describes what kind of solution is preferred and gives an overview of the development status on the associated technologies. Indeed, the future missions will have to deal with better performance, better optical quality while from manufacturing point of view, the total mass, the development schedule and the final cost have to be reduced. These constraints induce a new generation of solutions based on large entrance optics associated to high lightweight ratio which naturally provide solutions sensitive to gravity deformation. In these conditions, the enhancement of the final performance can only be guaranteed by using active optics in flight. A deformable mirror is therefore foreseen to be implemented in future large telescopes in order to correct manufacturing residues and ground/flight evolution, including gravity. Moreover, low mass and low cost require more compact designs which entail solutions more sensitive to misalignment. An active positioning mechanism is then also needed in order to correct the telescope alignment during operation conditions. Thales Alenia Space has been selected by CNES to develop and qualify active optics building blocks and then to test and demonstrate the improvement that new active technologies can bring in a full size instrument representative of the next generation of observation instruments. An overview of the current development status and the achieved performances is given for each building block (Primary Mirror, deformable mirror, 6-dof mechanism, wavefront sensor).

ZERODUR® has been selected as substrate material for the M1 to M4 of the ELT, the world largest ground-based telescope. Reliable serial production of 949 segments started:18 verification blanks have been finished. Mirror substrates as large as 4.25m in diameter are required. This is in the context of precision instrumentation markets outside of astronomical telescopes traditionally dominating ZERODUR® production. SCHOTT has extended its capacity to the extent that there is little or no impact on ZERODUR® availability in the various established markets of ZERODUR®, including other space and ground astronomical applications. Capacity related investment of several 10 m€ along the entire ZERODUR® process chain will eliminate any bottleneck.

A significant portion toward understanding evolution of the universe comes from x-ray astronomy since many astronomical objects of interest, such as black holes, supernovae and distant galaxies, emit radiation in the x-ray band. As basically all materials have almost unity refractive index for x-rays focusing x-ray beams is only possible by reflection at grazing incidence. Due to the low photon flux of the objects under study (few photons per hour) each individual photon is of particular interest. Thus, the collective area of x-ray telescopes needs to be as large as possible which is achieved by a large amount of concentrically nested x-ray mirrors whose shape needs to be ideal fitting to the designed geometry for high image quality. Due to the mirrors' curvature even state of the art mechanical machining and chemical mechanical polishing processes leave a residual surface error of several hundred nanometers. Those residual errors can be significantly reduced by ion beam figuring.

The conventional state-of-the-art manufacturing processes of aspheric reflective optics normally consists of the following steps: • Production of the glass blank; • Machining and grinding of the blank to approximate shape, inclusive of backside lightening; • Deterministic figuring and polishing. Deterministic figuring and polishing are iterative processes performed by IRP (Intelligent Robot Polishing) or MRF (Magneto-Rheological Figuring) or IBF (Ion-Beam Figuring), which iteratively converge to the targeted performance with guidance from accurate metrology information. The process capability of these one-off methods is well established, but hardly cost-effective for any small/large series production because of the need to repeat the entire process for each product unit. Differently from the conventional methods, the Cold Shaping Optics manufacturing technology consists of precisely shaping an inexpensive thin glass sheet (⪆ 1 mm) over a high precision mandrel and freezing its shape over a low-cost substrate by means of an epoxy adhesive layer. The mandrel must have the same surface shape accuracy specified for the desired optics. However, in a mini production series, the Cold Shaping Optics technique can reduce the recurrent production costs by amortizing the cost of the re-usable shaping mandrel over multiple product units allowing the manufacture of high-performance reflective optics at a fraction of the cost of traditional grinding and polishing methods. In addition to that, the possibility of actively changing the shape of the mandrel allows the series production of optics with different shapes from the same mandrel, hence further reducing the cost paradigm. In this paper we report the results obtained during the development of first prototype mirrors of 380 mm diameter.

From the systems perspective, best mirror substrate production will also concurrently streamline subsequent optical finishing. SCHOTT, recognizing that machining processes are substantially faster at material removal than optical processes, now generates aspheric surfaces to high accuracy, while reducing patterned errors from tool path, and also substantially reduces sub-surface damage from fixed-abrasive grinding processes. In many cases, the optical fabricator can now take a SCHOTT generated ZERODUR® substrate directly into small-tool deterministic optical finishing. This is especially important for lightweight mirrors where in the past large optical tools would be used to generate the aspheric and to remove subsurface damage. In the process of large optical tool work, the optical fabricator would induce mid-spatial frequency quilting, which subsequently would need to be removed by small tools. Substantial time-savings for the optical fabricator are now available with ZERODUR®.

Additive manufacturing (AM; 3D printing) is a fabrication process that builds an object layer-upon-layer and promotes the use of structures that would not be possible via subtractive machining. Prototype AM metal mirrors are increasingly being studied in order to exploit the advantage of the broad AM design-space to develop intricate lightweight structures that are more optimised for function than traditional open-back mirror lightweighting. This paper describes a UK Space Agency funded project to design and manufacture a series of lightweighted AM mirrors to fit within a 3U CubeSat chassis. Six AM mirrors of identical design will be presented: two in aluminium (AlSi₁₀Mg), two in nickel phosphorous (NiP) coated AlSi₁₀Mg, and two in titanium (Ti64). For each material mirror pair, one is hand-polished and the other is diamond turned. Metrology data, surface form error and surface roughness, will be presented to compare and contrast the different materials and post-processing methods. To assess the presence of porosity, a frequent concern for AM materials, X-ray computed tomography measurements will be presented to highlight the location and density of pores within the mirror substrates; methods to mitigate the distribution of pores near the optical surface will be described. As a metric for success the AlSi₁₀Mg + NiP and AlSi₁₀Mg mirrors should be suitable for visible and infrared applications respectively.

Design for additive manufacture (AM; 3D printing) is significantly different than design for subtractive machining. Although there are some limitations on the designs that can be printed, the increase in the AM design-space removes some of the existing challenges faced by the traditional lightweight mirror designs; for example, sandwich mirrors are just as easy to fabricate as open-back mirrors via AM, and they provide an improvement in structural rigidity. However, the ability to print a sandwich mirror as a single component does come with extra considerations; such as orientation upon the build plate and access to remove any temporary support material. This paper describes the iterations in optimisation applied to the lightweighting of a small, 84mm diameter by 20mm height, spherical concave mirror intended for CubeSat applications. The initial design, which was fabricated, is discussed in terms of the internal lightweighting design and the design constraints that were imposed by printing and post-processing. Iterations on the initial design are presented; these include the use of topology optimisation to minimise the total internal strain energy during mirror polishing and the use of lattices combined with thickness variation i.e. having a thicker lattice in strategic support locations. To assess the suitability of each design, finite element analysis is presented to quantify the print-through of the lightweighting upon the optical surface for a given mass reduction.

The ESA Cosmic Vision “Euclid" mission will conduct a 6-years long survey of 15,000 square degrees of the sky to a look-back time of 10 billion years, with the aim of characterizing the matter-energy content of the Universe and to better understand the dark energy responsible for the acceleration of its expansion. The Euclid payload consists of a wide field 1.2m aperture telescope equipped with two instruments that simultaneously observe patches of < 0.5 square degree on the sky: the visible light camera (VIS, and the near-infrared spectrometer and photometer (NISP). These two instruments are separated by a dichroic plate splitting the beams around a wavelength of 920 nm. The NISP large field of view (FoV) - larger than the full moon disk - together with high demands on the optical performance and strong requirements on in-flight stability, lead to very tight and challenging specifications on the alignment and positioning of the NISP optical assembly (NI-OA). This required an extensive tolerance analysis at system level during the design phase. The hardware is now completed and went through all optical tests at assembly level. In this paper we present the strategy and results of the warm optical test. In this test, we measured the length of the optical axis behind the NI-OA - or back focal distance (BFD) - using a novel combination of computer generated hologram (CGH) and a coordinate measuring machine (CMM). The agreement between the predicted and measured BFD values is excellent, within 1 μm. In addition, we measured the system wave-front error under warm conditions in double path and found diffraction limited performance on- and off-axis all over the field of view. These warm tests validated the anticipated performance of the NI-OA and allowed us to prepare the time-consuming and risky cryogenic tests with a high level of confidence.

The National Astronomical Research Institute of Thailand (NARIT) is currently developing a new kind of focal reducer for the 2.4 m Thai National Telescope (TNT). The objective is to image a circular Field Of View (FOV) of 15 arcminute diameter with an image quality close to the seeing limit over the spectral bands B, V, R and I. This focal reducer comprises one doublet lens L1 located on one robotic rail mounted on the telescope fork and one triplet lens L2 mounted on the instrument cube in front of the camera. First, we remind the specifications and the optical design of the instrument. Second, we present the method used to assemble the lenses inside the barrels. Third, we describe the procedure we have used to integrate the focal reducer on the TNT. We describe the robotic rail on which L1 is mounted and we present the results of the wavefront measurement performed to verify the optical quality of the TNT equipped with L1. We also describe the operations of the installation of L2, the filter wheel and the camera on the telescope. Fourth, we present the preliminary performance of the focal reducer measured on-sky with the TNT. We show that the focal reducer provides a resolution close to 1.5’’ over a FOV equal to 12’x12’ limited by the dimensions of the current filters. We also show that the plate scale is equal to 0.6’’/pixel and is stable over the B, V, R and I bands.

We present our development of high-efficiency reflective grating development by holographic processing. Its primary objective is to carry out exoplanet science studies in the ultraviolet (UV) wavelength region using space-borne telescopes. While the final development goal is aspheric grating, in this study, we manufactured planar grating samples with laminar and blazed grooves for our first step in order to establish processing conditions and to evaluate characteristics of each grating. Geometry of the manufactured gratings is 30×30×10 mm, and their groove density is 2400/mm. It was confirmed by Atomic Force Microscope (AFM) evaluation that laminar and blazed grooves were constructed on the surface of each grating. The measured absolute diffraction efficiency achieved by the brazed grating is 40.2% and 44.1% at wavelengths of 122 nm and 131 nm, respectively. These values are higher than values of the laminar grating by factor of ∼1.5.

Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (ARIEL) is the M4 ESA mission to launch in 2028. ARIEL is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 μm and 7.8 μm (main instrument), operating in cryogenic conditions in the range 50 - 60 K. For the main mirror substrate, the Aluminum 6061 alloy has been chosen as baseline material after a trade- off. The large size of the mirror however (0.6 square meters) presents specific production challenges concerning opto-mechanical stability in cryogenic applications. To minimize risk, the machining, polishing, thermal treatments and coating processes will first be tested on flat samples of 150 mm of diameter and then applied to a full-size demonstrator mirror, before finalizing the design and producing the flight mirror. This study, following a review of existing literature on fabrication of Al 6061 mirrors for spaceborne IR applications will characterize the optical properties of the samples after each phase of thermal treatment with the goal of determining an optimal process for material stress release, figuring and surface finishing and final optical stability in the operating cryogenic environment.

Optical imaging has been widely used in many fields as a tool for acquiring images, which can be used in microscopic observation, medical analysis, remote sensing, and astronomical observation. In the field of astronomical observations, it is very important to adjust the focus of the coronagraph. Due to the significant differences of the intensity between the corona and the solar photosphere, the ground-based coronagraph usually adopt an occulter, which is a little larger than the solar disk, to shield the light from the solar photosphere. In absence of the Sun as reference, focusing of the coronagraph is much harder than usual optical system. For the ground-based coronagraph, we developed a method of focusing using the solar disk image behind this disk occulter. Based on a series of images collected by shifting the center of the solar disk, we extract edge information of the solar disk and use the edge gradient algorithm to fit the focal length. This method can reduce the error of manual focusing. We can precisely find out the coronagraph focal point and obtain a clear coronal image, which lays a foundation for the technical support about remote control system of the coronagraph.

Mt. Wumingshan (Mt. WMS) is an outstanding astronomical site recently discovered by Yunnan observatories, Chinese Academy of Sciences during the survey of potential sites for China's next-generation ground-based large-aperture Solar telescopes. Currently, two temporary observation platforms have been built in Mt. WMS. Moreover, the continuous observations have also been conducted with the accumulation of numerous data. This paper describes the overview of the site, the observation platform and the monitor instrument. Besides,the preliminary statistical results are presented.

Commercially available optical design software lacks support for optimizing X-ray instruments for astronomy, which can feature thousands of optical surfaces operating at grazing incidence. To address this need for software, we present arcusTrace, a modular, Python-based raytracing software developed in support of the Arcus soft X-ray spectrometer. Each modular package models the behavior of an optical component in the spectrometer, allowing raytracing of single components up to the entire instrument. In addition, we leverage Python's built-in class structure to define common objects and simplify function calls. We have employed arcusTrace to predict the performance of the Arcus spectrometer and validated the software's performance by comparing its output to X-ray measurements conducted at the PANTER X-ray test facility. The resulting optical design for Arcus yields a high-resolution, high effective area spectrometer uniquely capable of addressing outstanding science questions in large scale structure formation, feedback, and stellar astrophysics.

The Exoplanet High-Resolution Spectrograph (EXOhSPEC) is a high-resolution spectrograph for the characterisation of exoplanets with the Thai National Telescope. The folded version of this instrument comprises one triplet lens to collimate the beam incident on the grating and to focus the beam reflected by the grating onto the camera. This collimator comprises three lenses L1, L2 and L3 of diameter varying between 50 mm and 60 mm. We specified the barrel to guarantee a maximum decenter of the lenses equal to 25 μm. The maximum error in the orientation of each single lens is specified to be lower than 0.03º. The proposed concept is based on a semi-kinematic mounting which is used to restrain these lenses with 6 and 30 N of preloads on the axial and lateral directions to ensure their stability. These preloads are applied to the lenses using the elastic pushing force of silicone elastomers and spring force from ball-plungers. We present the design of the collimator and the assembly method. Our Finite Element Analyses show that the maximum surface error induced by the preloads is lower than 60 nm Peak-To-Valley on each optical surface of L1, L2, and L3. We describe our manufacturing process using NARIT’s CNC machine and its validation using our Coordinate-Measuring Machine.

Many of NASA’s direct imaging of exoplanet missions and projects require fabricated coronagraph masks to control scattering and diffraction of light. The designed, patterned mask intended for the coronagraphic testbeds are highly absorptive in the visible range on non-metallic regions. In this work, we employed the cryogenic etching process to fabricate black silicon (BSi) to achieve a high aspect ratio (HAR) structures with higher etch rate than conventional reactive ion etching (REI). Recent bidirectional reflectance distribution function (BRDF) measurements of uniformly etched BSi on silicon wafer show highly diffusive BSi with a specular reflective component in the orders of seven magnitudes lower than the total hemispherical reflectance when the polarized or non-polarized incident beam is used.