Members of the Astro2020 Decadal Report engage in discussion on the reporting process and results with the community and community representative.

Emergent technologies and their impact on future flagship missions: discussion with NASA Astrophysics Chief Technologist and technology leaders.

Being able to survey the sky at ultraviolet and near-infrared wavelengths, astronomical spectrographs can study objects almost impossible to see in visible light alone. Very low light intensities of small objects require single-photon sensitivity. The unique optical properties of fused silica, e.g., highest transmission, best homogeneity, and lowest absorption, enable next-generation observations, space-bound as well as earth-bound. However light absorption in the range >800 nm limits the performance of IR spectrographs when using standard fused silica. Metallic impurities and OH groups embedded in the fused silica matrix need to be reduced to achieve best operation. The MOONS camera system at ESO’s Very Large Telescope observes the universe in the NIR range using low OH synthetic fused silica lenses up to a diameter of 900 mm. In the UV spectrum of 160-350 nm, standard fused silica performs in earth-bound systems extremely well. However, in space-bound applications, radiation damage reduces the transmissivity over time. Radiation hard fused silica windows for the PLATO M3 mission of the European Space Agency (ESA) protect the sensitive telescope from damage due to solar protons. Key optical instruments require calibration of the light source to normalize spectroscopic measurements. A radiation resistant fused silica diffusor with Lambertian scattering behavior supports ESA’s Copernicus program in the Sentinel 5 satellite.

As next-generation precision space-based telescopes require larger and lighter weight mirrors, replicated composite optics are gaining increased attention as a route to decrease weight, cost, and manufacturing time over conventional glass mirrors. Despite numerous advantages of the replication technique, optical stability remains an issue since the optical layer is organic and susceptible to environmentally induced dimensional distortions that can cause deviations greater than SFE λ/20 (~32nm) allowed for these applications. Therefore, studies must be performed to investigate optical stability, specifically as a function of time due to long mission lifetimes. In this study, we manufactured replicated composite mirror coupons using a UV-cured epoxy at RT as a function of photoinitiator (PI) concentration. Subsequently, we monitored the optical changes of these mirrors as a function of time by laser interferometry. Residual stress was shown to be the primary driver in predicting optical performance since the magnitude of the stress was shown to be inversely related to replication quality. Changes occurring due to stress relaxation were also shown to negatively impact optical quality over time. A model was developed to optimize the final quality of the replication by controlling key processing parameters, including resin cure state, CFRP laminate thickness, and CFRP fiber stiffness. This understanding led to the development of a high-humidity cycling protocol used to eliminate all processing stress-induced distortions. This method has permitted the stabilization of replications for long-term storage by accelerating the stress relaxation phenomena and removing the driving force for optical change; the non-zero residual stress.

Spaceborne optical systems should ideally stay dimensionally stable under the conditions met over the orbital mission, e.g. stable under strong thermal gradients resulting from the instrument alternatively exposed to the sun then shielded by the terrestrial umbra and penumbra. In this situation, materials exhibiting a low coefficient of thermal expansion (CTE) mitigate the effect of thermal transients. However, deformations of the optics are not only due to thermal effects but can also be induced by environmental ionizing radiations resulting in compaction. Here we investigated the compaction of the glass-ceramic mirror material ZERODUR® under different types of ionizing radiations (γ, e-, H+) and for different deposited doses representative of the environment met over typical orbits. A phenomenological model binding the compaction to the estimated deposited dose based on the experimental data is proposed and compared with the data published.

Fe₃₉Co₄₉Cr₁₀Ni₂ has exhibited ultra-low thermal expansion with its average coefficient of approximately 0.13 × 10⁻⁶/K between 100 K and 300 K from cryogenic to room temperatures. This value is extremely close to that of the fused silica used for cameras and spectrographs mounted on astronomical telescopes. The nature of the low thermal expansion of Fe_39−xCo₄₉+xCr₁₀Ni₂ has been clarified by the measurements of the magnetic properties and the numerical simulations based on the path-integral effective-classical-potential theory. The saturation magnetization is found to decrease with increasing temperature in the range from 100 K to 350 K. The rate of decrease becomes larger at temperatures over 150 K. The numerical calculation has shown that this magnetic behavior is attributed to the electronic state changes from high spin state to low spin state in the Co atoms, which occurs at temperatures over 150 K. The electronic state changes in Co atoms are found to strongly contribute to the low thermal expansion for the first time in Fe_39−xCo₄₉+xCr₁₀Ni₂.

In space, planetary and astronomical observation systems are employed on spacecraft in orbit around the Earth, in halo orbits around the Earth-Sun, and in interplanetary missions. These instruments, in particular optical telescopes, can be strongly affected by the dust and micro-meteoroid environment and, for Earth orbits, by man-made space debris. Aside from the obvious catastrophic impact of large objects that may lead to the complete destruction of the optical surface, it is important to assess the impact hazards occurring with micron-sized and sub-mm particles, that might lead to nondestructive damage of the glass blank. This paper investigates the damage generated in a cometary coma particles environment by dust particles, as well as ice droplets; with respect to the latter case, in addition to the potential craterisation, the possible phase transition during the impact may cause sublimation/deposition and optics contamination. Both particle types can alter the cleanness level and the optical performances of the scientific cameras and bias the star tracker with fake detections caused by particles’ glitter. The purpose of this study is the definition of a methodology where the optical performance degradation of the instrument surfaces is expressed in terms of surface damage, coating ablation, stray light production and modulation transfer function deterioration. In addition, this methodology will provide the definition of the requirements for an impact test campaign to validate and extend the craterisation models available in the literature. The output will be used for the experimental activity to be performed with the light-gas gun currently under development at the Malta College of Arts, Science and Technology.

NASA has launched a long and storied series of ambitious strategic astrophysics missions, such as the Hubble Space Telescope, Compton Gamma Ray Observatory, Chandra X-ray Observatory, and Spitzer Space Telescope, with the recently launched James Webb Space Telescope set to join their ranks in producing incomparable science results. Continuing such a record of success requires ever-more-advanced technologies, as the science requirements of each new mission are more challenging than those of its predecessors. Maturing technologies across the mid-range of Technology Readiness Levels (TRLs), from 3 to 6, the so-called “Mid-TRL gap,” is crucial to developing indispensable components for such missions, a fact that was historically not appreciated. Recognizing this gap, in 2009, NASA’s Astrophysics Division established the Strategic Astrophysics Technology (SAT) Program. In the decade plus since, the SAT program, along with direct funding of certain technology-development efforts, has provided a wide array of benefits including significant milestones, such as TRL maturation, infusion of technologies into projects and missions, training the astrophysics workforce, and many more. The technology development and maturation projects funded by NASA Astrophysics are managed by the Cosmic Origins, Exoplanet Exploration, and Physics of the Cosmos Programs (COR, ExEP, and PCOS, respectively). Since 2009, over 140 projects have been funded on over 80 technology topics, with dozens advancing their TRL, and over 2/3 leading to technology infusions. We present the portfolio distribution in terms of specific technology areas addressed including optics, detectors, coatings, coronagraphs, starshade, lasers, electronics, cooling, etc. We show an analysis of the rate of TRL advances, infusion, and other benefits. Finally, we present Astrophysics Division’s strategic technology investment priorities following the recent release of the Decadal Survey, “Pathways to Discovery in Astronomy and Astrophysics for the 2020s” (Astro2020).

The ⨏ISPEx instrument is an innovative optical payload for planetary exploration which integrates a Color Camera and an Integral Field Spectrometer. The instrument uses a Three Mirror Afocal Telescope operating with a 2.4X beam reduction factor (input pupil diameter 84 mm, output pupil diameter 35 mm) to feed a collimated light beam to the camera's and spectrometers' entrance pupils. The FOVs of the two channels are aligned on a common boresight utilizing a beamsplitter and a dichroic filter. This solution, apart from saving mass by adopting a single telescope, allows operating the camera and imaging spectrometer in unison resulting in a great simplication of their operations. By selecting the polarization voltage applied to a Liquid Crystal Tunable Filter (LCTF) placed on the entrance pupil of the camera objective, it is possible to select the optical bandpass (between 0.42-0.73 μm) and the corresponding bandwidth (from 16 nm at 0.42 μm to 48 nm at 0.73 μm). This solution allows avoiding mechanical parts like in filters wheels while guaranteeing high spectral sampling. The camera optical design ensures a 3.3° x 1.6° FOV and an IFOV of 14 μrad corresponding to a spatial resolution of 0.14 m/px from a 10 km distance. The spectral channel is further divided in the VIS (0.4-1.05 μm spectral range, 3.2 nm/band sampling) and IR (0.95-5.0 μm, 8 nm/band) integral field imaging spectrometers. These channels use custom-made Coded Mask Optical Reformatters (CMOR) built from optical fibers bundles to collect the hyperspectral cube through a single acquisition. This innovative solution affords a great saving of the acquisition time with respect to instruments operating in whiskbroom or pushbroom mode. The FOV/IFOV are 0.4° (circular)/100 μrad and 0.28° x 0.28° (square)/225 μrad for the VIS and IR spectrometers, respectively. From a 10 km distance, the VIS spectrometer builds a 70 m-wide circular hyperspectral image made of 4000 pixels at 1 m/px resolution from 10 km distance; the IR channel acquires a 50x50 m image made of 484 pixels at 2.25 m/px. These requirements have been optimized for remote sensing of asteroids and comets from close distances but the instrument can be adapted also for other observation scenarios. In this respect, the ISPEx will offer the advantage to allow better exploitation of the data collected by the three channels resulting in a tremendous advantage for many scientific investigations. With this synergic approach, it will be possible to analyze high-resolution images to constrain morphology interpretation of a target, while hyperspectral data collected at the same time allow the retrieval of composition and physical properties. The availability of camera images makes it possible to apply sharpening algorithms to spectrometer ones. Apart from this, an integral-field spectrometer will keep the capabilities of more traditional whiskbroom and push broom spectrometers but it will overcome them when the target scene is rapidly evolving and changing during the acquisition: traditional instruments are limited by the fact that the cube acquisition process may take a time longer than the time scale of the investigated event. This is the case of observations enquiring into the dynamical evolution of planetary atmospheres, lightning events, outbursts, hyper-velocity impact, or fast-moving targets. By operating with fast readout detectors, an Integral Field spectrometer can adequately resolve the four dimensions of data (2D spatial, spectral and temporal) opening the possibility to perform time-resolved hyperspectral movies. Another substantial advantage of Integral Field spectrometers is their better operability during fast yby phases, where the distance from the target and illumination geometry are rapidly changing, resulting in limited periods suitable to observe the target with optimal conditions. By collecting the entire hyperspectral cube in a fraction of the time necessary to complete the scan for a traditional scanning spectrometer, an Integral Field spectrometer can reach a level of imaging exibility similar to the one achieved by a camera. Within this study, we are defining the configuration of the ISPEx5:0μm 0:4 space model operating in the 0.4-5.0 μm spectral range including optical performance analyses, and thermomechanical and electronic architecture. Moreover, we are realizing a development breadboard ISPEx1:0μm 0:4 limited to the 0.4-1.0 μm spectral range to conduct performance tests at the system level on LCTF and CMOR devices.

Image slicers are now common equipment in integral field spectrographs (IFSes) to detect and analyze distant galaxies and extra-solar planets. However, tilted surfaces cause vignetting between adjacent slices and off-axis optical aberrations. In this paper, we explore how the flexibility of metasurfaces can be used by adjusting the surface phases on single glass planes and mirrors. Therefore, each slice can be defined by its nanopillar spatial distribution, enabling control of slitlet positions and aberrations. We propose a conceptual design of a six-slices metasurface-based image slicer (MIS), optimized to operate in the J band (1.15-1.35 μm). In addition to resulting in diffraction-limited performance, the dispersion inherent to metasurfaces allows reaching a R = 2850 spectral resolving power in a short 273 mm-long design, with a transmission < 85%.

The European Solar Telescope (EST) is a 4-m class solar telescope that will be installed at the Observatorio del Roque de los Muchachos in La Palma (Spain). Two important factors make EST different from other similar size telescopes: high thermal load due to solar radiation and open dome configuration during observation. Therefore, the entire telescope will be exposed to direct and indirect solar radiation, which requires a very demanding thermal management in order to minimize local seeing. In addition, the complete telescope will be exposed to wind, which has an important impact both in image stability and image quality. The EST M1 Assembly (primary mirror and cell) will have to cope with these critical issues, apart from other aspects common to primary mirrors of similar size. In 2020, in the scope of the EST Preliminary Design Phase, the development of the M1 Assembly was granted to the company SENER-Aeroespacial. The first step, before starting the design, was to evaluate different technologies and configurations for the following critical aspects: mirror (lightweight or not), number and type of axial actuators, cooling system, and number of interfaces with the elevation tube. A comparative study was carried out in order to define the baseline configuration for the M1 Assembly. This baseline configuration was then fully developed up to preliminary design level. This paper addresses the comparative study performed for selecting the most promising baseline configuration for the EST M1 Assembly.

The constantly increasing needs for astronomical imaging of ever fainter objects as well as for imaging the Earth from space require much higher angular resolution and dynamic range than current optical telescopes can deliver. Mirrors are the key elements of these systems; but they are technologically difficult to improve because they must maintain an exceedingly precise shape while resisting deformations (for example from gravity and/or variable wind loads) in the open environments in which they must operate. Our interdisciplinary novel technology will establish a new paradigm: we will shape thin, very smooth, “fire-polished,” lightweight glass to a predetermined curvature and generate dynamically controlled stiffness by using the addressable energy of electroactive polymers (EAPs) to resist environmental deformations – making what we call a “Live” Mirror.

The fact that SCHOTT has been awarded the ELT M1 production contract with the requirement to produce 949 1.5 m diameter mirror segment blanks until 2024, has not influenced the ZERODUR® production for industrial applications at all. ZERODUR® production for ELT is only a fraction of the total amount of material processed today. SCHOTT has expanded the production capacity and upgraded capabilities in time already many years ago reaching a significant milestone in 2019 with a brand new modern ZERODUR® dedicated CNC grinding facility to be ready for the future of ZERODUR® applications. Now, at about half time of the production of 949 ELT M1 segment blanks, the excellent results on the material properties and dimensional characteristics achieved on 486 ELT M1 segment blanks underline the high standard in ZERODUR® production. This paper gives an update on the achievements at half time of the project.

In 2017, the European Southern Observatory (ESO) awarded a contract for the manufacturing of the segments of the primary mirror for the Extremely Large Telescope (ELT) to Safran Reosc. Since then, we designed a production unit dedicated for ELT M1. From spherical mirror blanks, we first glue 39 pads per segment. We then shape, polish and cut the segments with 133 different off-axis aspheric definitions. We finish them to nanometer accuracy using ion beam figuring and a fully automated interferometric test bench. Now manufacturing has begun, we present the solutions and methods that we developed to manufacture more than one segment per day.

The European Extremely Large Telescope (ELT) Primary Mirror (M1) is a 39-m diameter mirror, compound of 798 segments. Individual Segment Assemblies need to be periodically extracted and re-integrated for off-line replacement of their reflective coating. Such replacement and alignment operation are to be performed at a rate of two segments per day. The Segment Manipulator is the handling tool allowing ELT M1 Segment Assemblies to be exchanged in-situ as part of routine ELT Telescope operations. It also enables the handling and positioning of associated generic payloads over the ELT M1. It includes as well, a permanent, general purpose long travel crane enabling the handling of equipment in and out of the ELT M1 Cell. In order to extract a segment from M1, the Segment Manipulator grabs the Segment Assembly by an interface based on 3 points at 120 deg located at its optomechanical support. To allow this operation the Segment Assembly has been moved up some centimeters from its nominal position in M1 by a dedicated Segment Extractor. Once the Segment Assembly is grabbed by the Segment Manipulator, it is moved out of M1 by the M1 Segment Crane and transferred to the unloading area, on the telescope Azimuth Floor, by the long vertical travel function provided by the Segment Manipulator. The system integrates different technologies (optics, vision, electronics, control, mechanics), facing several technological challenges, related to high seismic load, reduced mass budget and high ratio payload mass versus mass budget.

The European Southern Observatory (ESO) awarded Safran Reosc with the contracts for the manufacturing of the main optics for the Extremely Large Telescope (ELT) : in 2015 for the twelve ELT M4 Zerodur shells, in 2016 for the ELT M2 4.25-m diameter Zerodur convex mirror, in 2017 for the ELT M3 4-m diameter Zerodur concave mirror, and in 2019 for the ELT M5 2.7-m diameter Silicon Carbide flat mirror. We present for each of these optics their key requirements and challenges. The ELT M4 shells are thin glass sheets in Zerodur of 1.95 mm thickness with strong requirements on thickness uniformity lower than 15 μm and on surface deformation lower than 14 nm RMS. The ELT M2 and ELT M3 mirrors are large Zerodur mirrors of class 4-meter diameter with strong requirements on bonding and surface deformation, requiring dedicated WFE test benches. We present the main parameters to achieve and make an overview of their test benches developed in Safran Reosc facility. The ELT M5 mirror is a large piece of Silicon Carbide of dimension 2.7m x 2.2m that presents high challenges for its substrate manufacturing and surface deformation. We detail its main requirements and the test bench specially developed for this mirror. Finally, we summarize for each of these optics the main steps and results achieved, and the current progress status.

The Richard F. Caris Mirror Lab at the University of Arizona continues production of 8.4 m lightweight honeycomb segments for the primary mirror of the Giant Magellan Telescope. GMT’s 25 m primary mirror consists of a center segment surrounded by six off-axis segments, with an additional off-axis segment to allow continuous operation as segments are removed for coating. We cast the sixth segment (5 off-axis segments + center segment) in March 2021. As of June 2022 we have tentatively completed polishing of the third off-axis segment, and we are in the process of grinding the optical surface of off-axis Segment 5. For Segment 3, we improved the measurement accuracy for small-scale structure near the edge of the mirror, which has been a challenge for the off-axis segments. In addition to full-aperture interferometry and deflectometry, we used a 20 cm test plate to obtain high-resolution interferometric measurements of the edge.

The European Extremely Large Telescope (ELT) construction¹ is advancing and the M2, M3 and M5 cells are running the same path. The M2² and M3 mirrors are 4-meter class Zerodur® meniscus supported axially and laterally by mechanical whiffletrees. To allow low order corrections, axial support is provided with a warping harness system. Additionally, an electromechanical hexapod allows the movement in 5 degrees of freedom to optically align the telescope. The M2 Cell is ending the integration and test phase and approaching to the verification phase, where the highly demanding requirements will be checked. On the other hand, M3 cell is being assembled, taking advantage of the lessons learned during the M2 cell assembly. The M5 mirror is lightweighted silicon carbide elliptical flat mirror of 2.7 x 2.1 m², having the objective of folding the optical beams towards the Nasmyth platforms. Additionally, stabilizes the image movements induced by the telescope mechanisms and wind shaking vibrations. The M5 Cell is divided in two stages: the Tip-tilt and the Alignment Stage. The former allows to perform a fast-steering control while the latter provides active alignment capabilities to cope with the overall telescope structure misalignments The ELT M5 Cell is currently in the final design review, with qualification models for both stages successfully developed and tested. Preliminary results shown submicrometric resolutions for the Alignment Stage and around 5 nanometres for the Tip-Tilt Stage. This paper summarizes the design of the M2, M3 and M5 cells and presents its current development status.

TMT will be a telescope with 492 segments making up the primary mirror. Each of these segments is supported with a highly sophisticated Segment Support Assembly (SSA) and each of them is to be tested for acceptance using a tool called SMATT. This tool will be mounted on top of each SSA and provides the load distribution applied to its 27 support points. These loads will vary slightly due to mechanical tolerances which is perfectly acceptable and can be predicted using FE-analysis. However manufacturing errors and parasitic stiffness contributions may cause unacceptable deviations. This should be avoided. The article will describe the functioning of SMATT. The tool is designed such that the segment support loads can be measured in any position. In the SSA, 21 Warping Harnesses (WH) are implemented to actively compensate for Surface Form Errors (SFE). These WH can also be verified for proper functioning when using SMATT.

After a brief introduction of the SDSS-V optical instrumentation installed at the Apache Point 2.5m telescope, the presentation will be mainly focused onto the optical production and testing of the 3 large lenses (diameter 700-800mm) constituting the wide field optical corrector (WFC). Special emphasis will be made onto the measurement issues and solutions of the deep aspherical surfaces as well as the review of the specific anti-reflection (AR) coating with development of a dual band anti-reflective coating, carried out by Thales SESO-France. Before concluding, a dedicated paragraph will address on-sky imaging performances results with this new WFC. The presentation will conclude by a brief overview of the corresponding existing “state of the art” at Thales SESO for future manufacturing of similar or large optics for next generation of very wide field corrector.

Future NASA flagship missions will require the collecting area and resolving power of 6 meter or larger aperture telescopes. Due to limits in fairing size, highly accurate and stable segmented primary mirrors are desirable for achieving these apertures. Due to periodic discontinuities, hexagonally segmented mirrors have intrinsic diffraction grating-like structures, causing pronounced starburst point spread functions (PSFs). To mitigate unwanted image plane diffraction, we have designed and simulated a novel curved-edge segmentation method, called pinwheel segmentation, which more closely emulates a filled circular primary aperture. A parametric solution space for pinwheel segmentation has been developed and used to create in-house Python code which can be integrated with two optical propagation software: Physical Optics Propagation in Python (POPPY) and High Contrast Imaging in Python (HCIPy). Using HCIPy, we demonstrate optimized pinwheel design solutions which are less sensitive to realistic degradation scenarios on-orbit such as optical surface errors and beamwalk due to observatory pointing errors. Additionally, to demonstrate its potential benefits for high-contrast astrophysics, coronagraphy was compared using 6-meter class hexagonal and pinwheel segmented primary mirrors. Preliminary results demonstrate the advantages of alternative segmentation geometries when degraded PSFs are considered. The increased performance and robustness of pinwheel segmentation have the potential of significantly increasing science returns for future missions while reducing spacecraft operational constraints and cost.

X-ray optics, such as e.g. for ESA’s ATHENA (Advanced Telescope for High-ENergy Astrophysics) mission, often require that their alignment takes places under vacuum conditions, in particular when high precision and the compensation for vacuum-related distortion states is mandatory. In order to fixate the alignment state, standard bonding technologies for x-ray optics, such as clamping or adhesive bonding, only can be carried out at ambient conditions, causing a potential change of the alignment state when transiting from ambient condition to vacuum. In a study issued by ESA, the use of a specific laser-based soldering technology to be carried out under vacuum conditions was successfully conducted. The socalled “Solderjet Bumping” (SJB) technology combines solder alloy preform singulation and handling as well as laserbased reflow and placement at the desired, metallized bonding areas for x-ray optics. Achieving de-alignment less than a millirad during soldering under <1e-4 mbar vacuum condition is reported for complex and 3D x-ray optics geometries. Also, ca. twice the bonding strength was reached, compared to adhesive bonds carried out at ambient conditions.

Freeform optics have shown the ability to drastically increase optical design options, providing new solutions for space telescope and optics applications. Freeform optics manufacturing strives to provide designers this flexibility, but the requisite subaperture generation and polishing techniques often result in mid-spatial frequency (MSF) errors. Because these errors negatively impact final performance, they must be removed. However, mitigation options are costly and timeconsuming. The HERMES (High End Robotic MSF Elimination System) is a robotic platform that uses a machine learning algorithm and deflectometry data to smooth optics. This project has shown preliminary success, and began to forecast increases in form error to help balance smoothing time predictions. Other advancements are explored by the current work. First, the algorithm has been upgraded to include a force-controlled option that utilizes form data (from interferometry) to scale applied force during the smoothing process that is mapped to deviation data. This project also integrated an orthogonal (or raster) pattern of smoothing found in early human smoothing recordings. The various conditions were compared, and result metrics included the peak power spectral density (PSD) values of the frequency of interest as well as irregularity (form) error. All conditions decreased peak PSD values reliabilty, but irregularity data became more difficult to interpret. Findings and further work are discussed.

Wide field-of-view lens assemblies covering a full field of view of <100° across the diagonal dimension of the detector have been designed, built, and tested for spaceborne applications for various science objectives like landing operations, docking maneuvers, characterization of unknown regolith and other geological features. The all refractive lenses were designed for both telescopic and microscopic, high-resolution (≥227 l.p./mm), imaging scenarios using a nearly identical bill of materials (BOM). The optical designs have been achromatized and passively athermalized over the visible waveband for optimal performance across typical expected operating temperature range. The opto-mechanical designs have been ruggedized for severe spaceborne environmental conditions. This document demonstrates the design approach, assembly techniques, and test methodology of two different lens configurations using a nearly identical BOM. The first is a wide-angle telescope supporting a 5 Gpixel visible array with a nominal target range from 1m to infinity. The second is a close focus “microscopic” imager, with a nominal target distance of <100mm. A total of four lenses were built and tested; three of which were the telescopic configuration and one microscopic imager. The measured performance of the lens assemblies is near diffraction-limited and meets all flight system level requirements. This design methodology has resulted in significant realized cost reductions and time savings while simultaneously being able to push the envelope with next-generation optical solutions for complex spaceborne applications.

Image spectroscopy is an essential feature in spectroscopic observations. We realized a 30μm wide slicer by cutting in 2016, showing a new possibility of IFU by cutting. Our machine is capable of 3D machining and has the accuracy to machine gratings. By utilizing this function, it is possible to produce not only flat slicers but also slicers with free curved surfaces. We have fabricated a prototype of a slicing mirror with a free-form surface. Of course, the multi-faceted mirror is an integral part of the structure. It is not easy and innovative to realize such a structure with a wall between adjacent mirrors due to the difference in tilt angle. We can obtain a surface roughness of less than 2nm (RMS) by cutting alone. The cutting potential for innovative extension of IFU degrees of freedom is introduced in this presentation.

Conventionally, facesheets for deformable mirrors (DM) are manufactured through grinding, polishing and finishing of a piece of optical glass. Due to the amount of material that needs to be removed, the required surface form errors and the complex handling of the fragile product throughout all manufacturing steps, this is a lengthy, risky and expensive process. For the 620mm-diameter adaptive secondary mirror (ASM) of the University of Hawaii’s 2.2-meter telescope, we develop an alternative process where the facesheet is manufactured by hot forming (‘slumping’) of a flat glass pre-form in a mold directly to the proper shape. Since low-spatial surface form errors can be corrected with the DM actuators to a large extent, only the mid/high-spatial errors beyond the period of the actuator spacing are considered critical. Through accurate modeling, and an iterative approach of hot-forming, metrology and adjustment of process parameters, the forming process is optimized to obtain the near-final shape. This approach limits the amount of required finishing operations, resulting in a facesheet meeting specifications at significantly reduced cost. We present results for the modeling & simulations, and the progress in manufacturing of this ASM facesheet.

Adaptive Secondary Mirrors (ASM’s) provide telescope-integrated adaptive optics (AO) correction, potentially broadening the use of AO on the facility. However, unlike post-focal plane AO systems, ASM’s require large, curved, aspheric membranes to replicate the standard telescope secondary mirror that it replaces. Electromagnetic actuator technology has been developed to implement ASM’s, but the devices require 200-1500 mm diameter curved facesheets. We describe an approach for fabrication of these facesheets that uses a hot thermal forming process, or slumping. This technique traditionally relies on an accurate negative form to slump the glass into. We take a different approach of letting the glass slump freely within a support ring that is outside the final required optical diameter of the shell. Such an approach allows for creation of shells over a variety of radii of curvature, simply by changing details of the heating process. However, to create an approximately parabolic shape, an additional force is required. We model the expected deformation, showing that a top weight with a given force just outside the optical diameter creates close to the desired shape. Experimental verification of this has been carried out for shells 120 mm in diameter. We are exploring optimum forming fixtures for this approach and scaling this process to larger facesheets needed for upcoming ASM’s.

Lightweight, aluminum, freeform prototype mirrors have been designed and fabricated by a Thai led team, with UK support, for intended applications within the Thai Space Consortium (TSC) satellite series. The project motivation was to explore the different design strategies and fabrication steps enabled by both conventional (mill, drill, and lathe) and additive (3D printing) manufacture of the prototype substrates. Single Point Diamond Turning was used to convert the substrates into mirrors and optical metrology was used to evaluate the different mirror surfaces. The prototype criteria originated from the TSC-1 satellite tertiary mirror, which is designed to minimize the effect of Seidel aberrations before the beam enters the hyperspectral imager. To converge upon the prototype designs, Finite Element Analysis (FEA) was used to evaluate the different physical conditions experienced by the prototypes during manufacture and how these influence the optical performance. The selected designs satisfied the mass and surface displacement criteria of the prototype and were adapted to either the conventional or additive manufacturing process. This paper will present the prototype design process, substrate manufacture, optical fabrication, and an interferometric evaluation of the optical surfaces comparing the conventional and additive manufacturing processes.

Fabricating mirrors using additive manufacturing (AM; 3D printing) is a promising yet under-researched production route. There are several issues that need to be better understood before AM can be fully adopted to fabricate mirror substrates. A significant obstacle to AM adoption is the presence of porosity and the influence that has on the resultant optical proprieties. Several batches of high-silicon aluminium (AlSi10Mg) samples were created to investigate the relationships laser parameters, laser paths and build orientations have with the porosity. The results showed that eliminating defects relies on a complex interaction of the process parameters and material properties, with the residual heating from the laser proving to be a significant factor. In addition, the use of a hot isostatic press is investigated and some full prototypes of the Cassegrain CubeSat were produced.

Additive manufacture (AM), also known as 3D printing, builds an object, layer-by-layer, from a digital design file. The primary advantage of the layer-by-layer approach is the increase in design-space, which enables engineers and scientists to create structures and geometries that would not be practical, or possible, via conventional subtractive machining (mill, drill and lathe). AM provides more than prototyping solutions: there are a broad range of materials available (polymers, metals and ceramics); software capable of creating lightweight structures optimised for the physical environment; and numerous bureaux offering AM as a service on a par with subtractive machining. In addition, AM is an ideal method for bespoke, low-count parts, which are often the foundation of astronomical instrumentation. However, AM offers many challenges as well as benefits and, therefore, the goal of the OPTICON A2IM Cookbook is to provide the reader with a resource that outlines the scope of AM and how to adopt it within astronomical hardware, with an emphasis on the fabrication of lightweight mirrors. The Cookbook was an open access deliverable of the EU H2020 funded OPTICON (Optical Infrared Coordination Network for Astronomy; grant agreement #730890) A2IM (Additive Astronomy Integrated-component Manufacturing; PI H. Schnetler) work package and it was completed in June 2021. This paper will introduce the Cookbook, its scope and methodology, and highlight the paradigm shift required to design and AM lightweight mirrors for astronomy and space-science.

Recent developments of compliant mechanisms built by additive manufacturing confirm their potential for highprecision applications in harsh environments. Several projects are presented, from simple flexure pivots to complex 3D compliant mechanisms designs, to show the advantages and challenges of the design, manufacturing and testing of such systems. Focus is put on the specific design methodology, the integration of electrical functions, the guiding and lifetime performance results to highlight the potential for future high-precision applications such as high-precision, cryogenic or space. First, the redesign of simple flexure blades and crossed-blade pivots has been performed. The guiding performances have been compared to simulations. Then, more complex architectures of compliant mechanisms have been designed, manufactured and tested. For example, the European Space Agency (ESA) project Compliant Mechanisms based on Additive Manufacturing (COMAM) and the European H2020 project Prototype for Ultra Large Structure Assembly Robot (PULSAR). This project validates the positioning of tiles integrated on 3D printed flexible pivots and gimbals with a repeatability better than 1 μm. ESA selected CSEM for a new project, the development of a new compliant mechanism with a rotary to linear motion transformation. It is presented here with a novel 3D printed monolithic architecture made of stainless steel.

Additive Manufacturing provides the potential to manufacture complex geometries. It can be used for optomechanical parts such as housings for optical systems. They have to meet various mechanical specifications and often require a minimized mass. By using different optimization techniques, the design of an optical housing was generated with three approaches. In addition to the conventional design approach, topology optimization and foam-like lightweight structures were tested. In a proof-of-concept study, two designs were additively manufactured from an aluminum silicon alloy. These housings are used to verify the required post-process steps.

Additive Manufacturing (AM) has several potential advantages for astronomical instrumentation: particularly the ability to create custom parts with optimised geometries that cannot be produced with traditional manufacturing. The goal of the EU H2020 funded OPTICON (Optical Infrared Coordination Network for Astronomy; grant agreement 730890) A2IM (Additive Astronomy Integrated-component Manufacturing; PI H. Schnetler) project completed in June 2021, was to develop prototypes demonstrating these benefits. This paper presents the design and additive manufacture of a piezoelectric stack actuator driven, monolithic flexure for the active array of the Freeform Active Mirror Experiment (FAME). Flexure geometry had previously proved difficult to repeatedly produce and AM was considered as a potential solution. Two AM processes were used: powder bed fusion where metal powder is bonded using a laser, and binder jetting where powder is bonded using a polymer adhesive. A topology optimised, flexure hinged frame was designed based on the minimum feature size of each AM machine. This geometry was produced in Aluminium (AlSi10Mg), Titanium (Ti64Al4V) and Stainless Steel 316L. Porosity is a known issue with AM and Hot Isostatic Pressing (HIP): a post process whereby parts are subject to increased temperature and pressure was identified as a way of reducing this, thereby increasing the predictability of flexure behaviour and suitability for vacuum applications. Conformity of AM parts to their original geometry was assessed using external dimensional metrology. X-ray Computed Tomography (XCT) was used to identify internal porosity.

The European Solar Telescope (EST) is a 4-meter class solar telescope led by the Instituto Astrofísico de Canarias that will be erected in La Palma Island. The design process is now approaching the Preliminary Design Review. The EST, as solar telescope, will have singularities that make its design differ from other telescopes. Telescope configuration, high solar radiation, absence of dome during observation, besides other elements, will have a strongly impact to the system design. One of the telescope critical elements is the primary mirror and its support. Along the last months, the preliminary design of these items has been performed. Critical aspects as mirror heat rejection, surface temperature and homogeneity, shape error, wind perturbations, etc., have had strong influence in the design solutions, pushing to use, for example, the thinner solid meniscus mirror never manufactured supported by 80 tripods over electromechanical actuators. This paper presents the design of the primary mirror and its support, presenting the adopted design solutions to cope with the different difficulties as well as the results of some protypes manufactured to reduce the project risk.

The Evanescent Wave Coronagraph (EvWaCo) is a coronagraph that utilizes the principle of Frustrated Total Internal Reflection (FTIR) to simultaneously collect both the starlight and the companion light by using a focal plane mask composed of a convex diopter and a prism placed in contact. The mask exhibits an achromatic behavior, and its size can be varied by adjusting the pressure at the contact area. The National Astronomical Research Institute of Thailand (NARIT) is developing a prototype to demonstrate on-sky the performance of EvWaCo. This prototype will be installed at the Thailand National Telescope (TNT). In this paper, the mechanical design of the EvWaCo prototype is documented. The mechanical requirements of this prototype include a maximum weight equal to 180 kg, a maximum deformation of 120 μm, and an average deformation of 100 μm for every optical component. To achieve this, the structural parts are designed to achieve the high directional stiffness, and the passive thermal compensation is conceptualized for athermalization. Then, the lightweight, high-performance materials are selected. The Finite Element Analysis (FEA) method is used to simulate the performance of the prototype under the realistic conditions. The prototype performs with an average deformation of 43 ± 15 μm and a maximum deformation of 63 ± 18 μm at the average thermal condition of ΔT = 13.6 ⁰C. The instrument performs with an average deformation of 67 ± 16 μm and a maximum deformation of 92 ± 19 μm at the worst thermal condition of ΔT = 25 ⁰C. This instrument design weights 175.7 kg.

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 40mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for Final Design Reviews. The Focal Plane Relay Sub-System (FPRS) relays the ELT's Nasymth A2 focal plane to the HARMONI IFS focal plane. The FPRS maintains a -15°C environment to reduce additional thermal noise and contains two Ø600 mm x 20 mm windows in a double-glazing arrangement at the entrance. The external window surface temperature will be below the ambient telescope temperature due to insulation limitations and could lead to condensation forming reducing optical throughput and increased scatter, in particular in the IR-bands. We present the design and testing of a window flushing solution utilizing air knife technology

The design of the Near Space Optical Survey-alpha (NSOS-α) telescope, encouraged by Korea Astronomy and Space Science Institute (KASI) aims to discover and catalog near-Earth asteroids, especially Potentially Hazardous Asteroids (PHAs). To achieve this goal, an optical telescope with a 1.5m class primary mirror and a 5 square degree field of view will be installed and operated at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The wide field telescope is under development and being designed by a collaboration between KASI and the NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab). A prime focus configuration was adopted and designed with a seven element wide field corrector (WFC). The main mechanical design philosophy was scaled, modified, and revised based on the Korea Microlensing Telescope Network (KMTNet). The NSOS-α telescope will be the first dedicated observation facility for asteroid survey in the southern hemisphere using a 1.5-meter class telescope. In this paper, a summary of the optical and mechanical assemblies designed for the NSOS-α telescope will be addressed.

We have developed photo-controlled deformable mirrors (PCDM) for spatial light modulation and wavefront control making use of three different photoconductor wafers: silicon (Si), zinc sulphide (ZnS) and cadmium sulfide (CdS). The devices are a membrane mirror based on a 1-inch monolithic non-pixelated photoconductive substrate with a transparent ITO electrode on one side coupled with a 5 μm thick nitrocellulose membrane. The devices are driven applying DC or AC voltage and are characterized by measuring the generated wavefront using a Shack-Hartmann wavefront sensor. The fabrication and characterization of these deformable mirrors are reported. The cadmium sulfide based PCDM did not work properly because has been noticed a memory effect after the illumination that faded in hours. For the zinc sulphide based PCDM the maximum deformation achieved is approximately 4.7 μm PtV (wavefront), with a deformation very stable, but considering that the ZnS photoconductor shows very low charge mobility and very high internal resistance it is suitable for making a PCDM working in DC with a memory effect that can be switched in a time frame of seconds and can carry the deformation for a long time compensating for long time wavefront drifts. For the silicon based PCDM, upon uniform illumination with a NIR LED, the deformation increases with the voltage applied, its frequency and the light intensity. The maximum deformation achieved is approximately 2 μm PtV. The repeatability of the induced deformation is also evaluated together with its time stability, proving that the device is reliable in the working conditions. The response time is evaluated and it is in the range of 10 ms, this device is suitable for an efficient adaptable focusing element with an important focus control.

A consortium of industrial and academic partners, coordinated by TNO, is working on the realization of a 620mm adaptive secondary mirror (ASM) for the University of Hawaii’s 2.2-meter telescope [1][2][3]. The ASM consists of a 620mm-diameter slumped convex aspherical mirror shell, manipulated by 210 variable-reluctance actuators mounted on a light-weighted support frame. The mirror shell is manufactured to the required accuracy at low cost through slumping. The actuators are driven by dedicated PWM current drivers and commanded through a real-time FPGA-based interface. After successful performance testing of several laboratory prototypes[4], this project will provide the definitive on-sky demonstration of this new technology. We report on the manufacturing and testing of the major subsystems, and on the integration status of the ASM as a whole.

The Vera Rubin Observatory hosts a large (8.4 meter) wide-field (3.5 degree) survey telescope⁴. The Secondary Mirror (M2) Assembly⁶ and Camera⁵ utilize large hexapods3 to facilitate optical positioning relative to the Primary/Tertiary Mirror. These hexapods were designed, fabricated, assembled, tested and met all their requirements¹. Unfortunately, both hexapods were damaged prior to integration. The camera hexapod was damaged from overheating induced separation of the low temperature grease into constituents. The M2 hexapod was damaged from water intrusion during shipping. In both cases the critical linear encoders/tapes interior to the hexapod actuators were affected. These encoders are used by the control system to determine the length of the actuator during hexapod operations. If these encoders require servicing while deployed on the telescope, the hexapod needs to be unloaded by removing its optical payload (camera or M2), and the hexapod disassembled. The hexapod actuator then needs to be disassembled and repaired. This procedure produces an unacceptable risk to equipment, and an excessive disruption of observing. To rectify this, the actuators were redesigned to allow on-telescope servicing of these encoders. The encoder to tape orientation was inverted, and an access cover was added. This facilitates servicing the encoder/tape while on the telescope, reducing the servicing time from days to minutes. To improve reliability, alterations were also applied to the electrical system. The limit switch wiring was rearranged, and the cabling to the hexapod legs was upgraded. Also, multiple software upgrades were incorporated to improve function, performance, and compatibility with the other observatory systems.

The 4-meter wide-field telescope VISTA will be upgraded with the 4MOST fiber-fed spectrograph. The future field diameter will be 2.5° (before 1.65°) with fast optics of F/3.3, therefore the mechanical alignment tolerances are tight. ESO is currently refurbishing the active optics system based on curvature wavefront sensing, using four technical cameras defocused by ±1.2 mm that image four different stars. We describe the algorithm that computes the telescope misalignment using least-squares fitting, the ESO-developed package Sensitizer that generates sensitivity matrices and we discuss maximum likelihood methods.

The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50m-diameter radio telescope for millimeter-wave astronomy. The performance of large radio telescopes like the LMT is often limited by their response to deformations caused by thermal gradients within the antenna structure. In this paper, we describe a development project to build a real time metrology system for the LMT. The Large Aperture Surface Error Recovery System (LASERS) will measure, on time scales of a minute in real time, the shape of the antenna's primary mirror and the location of its secondary mirror to high accuracy. The LMT's existing active systems may then be used to maintain precise alignment of the telescope’s primary surface and its secondary.

Hexapods are general solutions that provide movement with six degrees of freedom for instrument positioning, alignment, and support. In the case of the METIS instrument, the hexapod must satisfy the following stringent requirements: a) support the 11-ton weight of an instrument; b) allow alignment and provide position stability to the instrument to within a tenth of a millimeter; c) provide an adjustment range of about 20 cm; d) support the instrument allowing for accelerations of over 3 g in all directions; e) have the lowest mass possible. Commercial linear actuators that are generally used in such cases are designed for extended movement, include a complete set of bearings that constrain each actuator lateral displacements and a sophisticated central screw that defines only the longitudinal movement. These solutions tend to be heavy and costly if roller screws are used to avoid backslash. They encompass ranges that are a major fraction of the total length and are designed for fast movement. Both these characteristics exceed the requirements of the METIS application. We present an optimized design for the hexapod which includes a different, lightweight, sturdy, small-range, highprecision, no backslash, earthquake-proof actuator. The design of the hexapod is such that it can be used, in general, as a mass and vibration optimized solution for precision heavy instrument alignment.

Optical tests of large hyperbolic secondary mirrors has been always challenging. Many methods have been proposed and used in the past, including different combinations of auxiliary optics and configurations. Starting from a review of known methods, a new approach has been identified that offers a wide range of applicability, being modular and reusable for other applications. Two classes, one based on traditional null lens optics and a new one that relies on computer-generated holograms, are described.

The dimensions of optics for scientific and industrial applications are growing. Moreover, aspherical and freeform surface shapes are increasingly applied, both in centred and off-axis apertures. Measuring these optics in conventional interferometric setups is challenging: tall interferometric test towers are used and for cases other than off-axis parabolas, a CGH has to be used and setup size grows with the off-axis distance and radius of curvature. Measuring large convex surfaces is particularly difficult. High point density is necessary for measuring mid-spatial frequencies, which rules out coordinate measuring machines. Measurements of theses surfaces using the DUI NMF range of cylindrical non-contact measurement machines will be demonstrated, showing flexibility, high point density, short measurement times and no setup cost.

We describe the state of development of a white light interferometer to characterize the cryogenic mirrors for GW detector on operation. We include the first experimental results from the proof of concept of the metrology instrument. The instrument will characterize the topology as well as the vibration of the mirrors. This development takes place in the frame of the E-TEST project. E-TEST is one of the technology demonstrators for the future Einstein Telescope (ET). ET is dedicated to the measure and characterization of gravitational waves. The prototype built by E-TEST includes a large silicon mirror of 40 cm diameter suspended by innovative vibration isolation hanging modules. To reach the detection specification, the mirror is cooled down at cryogenic temperatures around 20 K. Nevertheless, even after the isolation, the mirror may not reach perfect stability once at cryogenic temperatures. Furthermore, the mirror may experience surface topology changes and wavefront deformation due to the extreme variations in temperature and gradient. With our metrology instrument, we can obtain on a single camera frame a set of interferogram maps of the area observed on the mirror at different optical path differences. To do this, we design an innovative phase mask for a white light low-coherence interferometer. In addition, we implement new algorithms for the white light interferogram analysis, avoiding the limitations of the conventional Phase Shifting Interferometry algorithms.

Space telescopes require always better stray light control. However conventional characterization methods rely on the same principles since several decades and we are reaching the limits of their capabilities. We have developed a disruptive approach which solves these limitations: stray light characterization by ultrafast time of flight imaging. By using a pulsed laser source and an ultrafast sensor, we are able to discriminate and identify individual stray light contributors, thus providing the ultimate understanding of the stray light origins in an instrument. In this paper, we present our approach and its potential for the future of space telescopes. We will present the experimental characterization of a refractive telescope with this method, using a streak camera and a fs laser. We will present how we use this approach was used to experimentally characterize, validate and improve a stray light measurement facility, using a SPAD sensor and a ps laser.

The Shack Hartmann wavefront sensor (SHWFS) is a reference in the field of optical metrology and laser diagnostic. Over the past 2 decades this wavefront sensing (WFS) technique has been used for the alignment of numerous instruments and the metrology of components. Imagine Optic SA has developed an approach to improve the LIFT technique developed at the ONERA that allows to increase the native spatial resolution of the SHWFS to achieve a factor 16 on the spatial resolution of the SHWFS without degrading dynamic, accuracy and linearity of the instrument. Furthermore this enhanced SHWFS can be associated with accessories such as self illuminated platform (R-Flex) to increase the versatility of these achromatic WFS.

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO and LTAO - or with NOAO. The Integral Field Unit (IFU) sub-system is part of an Integral Field Spectrograph cooled down at 130K. The IFU is in charge of splitting, slicing and rearranging the rectangular field of view into 4 long slits (~540mm) to feed 4 spectrographs. It consists of more than 60 mirrors blocks that have to be aligned and maintained inside the HARMONI cryostat. As the project prepares for Final Design Reviews, we present the design of the IFU mirror mounts as well as performance results obtained on engineering models.

The Multi-directional, Multi-polarization, Multi-spectral Imager (3MI) is one of the payloads that will be on board of the MetOp-SG “Satellite A”, developed to provide information on atmospheric aerosols. 3MI is a space-based, wide field-of-view polarimeter that is designed to acquire sequential images of the same ground target, which are then combined with multiple spectral views in both un-polarized and polarized channels. This article presents the on-ground calibration approach for the 3MI payload. The calibration methodology starts from the challenging high level specifications down to the specification of adequate Key Data Parameters (KDP) introduced in an instrument model. An error budget allows to evaluate the required accuracy on each KDP and, consequently, to specify the needed Ground System Equipments for the calibration. The paper will present the facility and the developed GSE used to measure these KDP. Preliminary test results of the EM calibration campaign are addressed

Thai Space Consortium (TSC) was originally a cooperation program by 13 leading Thailand research organizations that proposed to develop a microsatellite for a scientific mission; moreover, the aim of the TSC program includes sharing and enhancing an advanced engineering capacity in Thailand. Due to the National Astronomical Research Institute of Thailand (NARIT) being a part of the TSC contributor; therefore, the development of the star tracker is first promoted to be an astronomical-driven project through the eye of astronomical engineers. In practice, the knowledge of star trackers can apply to satellite attitude determination systems. It is the most precisely measured absolute three-axis orientation of a spacecraft when integrated with relative sensors. Therefore, in this work, the potential of using an 0.7-meter optical telescope-based for software experimentation as a function of stars pattern recognition and attitude determination algorithms according to the detected stars on astronomical images. The image is mapped to the guide star catalog to correct the magnification, shear, and rotation of the image sensor respective to the Cartesian coordinates as a function of astrometry engineering. The result of the method is a plate-constant that then corrects the positions of a target celestial body. An inertial measurement unit (IMU) was equipped with the telescope mounting to collect the local orientation of the telescope itself to demonstrate the results of attitude determination in both deterministic-TRIAD method and solving loss function based on Wahba’s problem. The proposed system integration method can be performed with negligible attitude determination error compared to the IMU-baseline attitude sensor. This contribution will present some of the experimental results and plans for further measurement campaigns.

The Laser Interferometer Space Antenna (LISA), with its extreme distance measurement requirements (pm over arm lengths of 2.5 Mio km), imposes many stringent requirements on the laser sources used for interferometry. Frequency and power stability, as well as the side band phase noise represent considerable technological challenges, that must be maintained over the full 12.5 years mission duration. These constraints demand a streamlined laser design and a particular attention to reliability and procurement strategy, which poses a significant challenge. The main requirements for the laser critical sub-system have been analyzed. The Centre Suisse d’Electronique et de Microtechnique (CSEM), in the frame of a European Space Agency activity, was mandated to demonstrate a laser head for the LISA mission based on an alternative laser oscillator approach that does not rely on the LISA-baseline technology (i.e. Nd:YAG NPRO laser). The activity was named MONALISA. After a presentation of the key laser head requirements, the laser head design is described. A comprehensive test campaign was performed, and test results are presented.

On-orbit optical stability of the Nancy Grace Roman Space Telescope (RST) is a key requirement that enables multiple science objectives and drives multiple aspects of telescope design and analysis. Thermoelastic changes are typically large contributors to optical instability, and both extremely low CTE materials and extremely stable temperatures are needed to achieve the RST optical stability requirements. We will present the results from a test that demonstrated the L3Harris capability to sense and control temperatures to milli-kelvin levels of stability across a range of operating temperatures.

The next generation of space telescopes will require large segmented apertures for observations in the near ultraviolet through mid and far-infrared regions to enable new science ranging from exoplanet characterization to precision astronomical observations that refine astrophysics models. Recent concept studies, such as LUVOIR, HabEx, and Origins, and the future IR/O/UV Large Strategic Mission telescope for exoplanet characterization and general astronomy discussed in the 2021 Decadal Survey "Pathways to Discovery in Astronomy and Astrophysics for the 2020s” include segmented telescopes that are capable of observations in UV through IR bands and thus drive the need for optical surface performance at cryogenic temperatures. These spaceborne mirror applications require precision control, and these segments will require actuators for controlled surface displacements capable of operation at cryogenic temperatures (<150 K). This paper presents a testbed mirror design to test out new actuators and control strategies. This work is directed at understanding the performance of piezoelectric multilayer stack actuator operation down to 100 K, which will provide actuator designers the critical information needed to model and predict performance. The data reported down to 100 K include: displacement/strain and capacitance as a function of applied voltage, stiffness, hysteresis, blocking force, DC resistance measurements, thermal strains, and the coefficients of thermal expansion as a function of the electrical boundary conditions. The actuators include a strain gauge to allow for closed loop control. This approach allows for a comparison of potential open-loop control drive strategies and associated errors reported in previous work. Surface actuation measurements using flexure-based actuators on aluminum mirror segments at room temperature will also be presented. In addition, we will present techniques to optimize displacement per voltage using amplified piezoelectric flexures and the potential to develop mirrors that can be operated at cryogenic temperatures by carefully choosing flexure materials and geometry.

The development of the optical components for METIS, which is the mid-infrared ELT imager and spectrograph, is a challenging task. On one hand, there are tight requirements regarding wavefront error, driven mostly by high contrast imaging modes. On the other hand, limited space envelopes are available. In this paper, we present the development of the largest flat aluminium mirror of derotator mechanism which is located within the Common Fore Optics (CFO) of METIS. The 240mm-diameter mirror is made of gold coated Rapidly Solidified Aluminium (RSA) 6061-T6 with the compact flexure-based mount directly embedded in the mirror body. This novel design includes mounting pads that are located at the front, inside the optical surface. This approach improves the universality of the proposed solution while introducing challenges from the manufacturing point of view, i.e. surface holes that make polishing more difficult. The mount design is a combination of leaf springs and rotational hinges and is optimised to reduce the aberrations due to gravity and assembly loads. Thanks to the in-house manufacturing process relying on 5-axes milling capabilities, it is possible to create such a complex integrated mirror mount with minimal impact of the holes on the quality of the optical surface. Numerical simulations of this mirror performed for various load cases contributing to surface aberrations show extremely low surface form error (< 15 nm RMS) of the metallic mirror.

METIS, the cryogenic mid-infrared imager and spectrograph designed for the ESO’s Extremely Large Telescope (ELT) in Chile, is designed to optimally utilize the high resolution provided by the large aperture of the ELT. To enable high contrast imaging, the wavefront errors in the Common Fore Optics (CFO) of METIS must be minimized. In this paper, we focus on the design, the development and verification of the largest, 260 mm diameter curved mirror (CM1) of the CFO. The mirror is made of Rapidly Solidified Aluminium (RSA) Al6061 T6 alloy to achieve a homogenous shrinkage of the instrument and as such facilitate room-temperature instrument alignment and verification. The strict requirement on the maximum surface form error of 30 nm RMS is challenging to meet with metallic mirrors. Thanks to the careful integral design of the mirror, its mount and its manufacturing, the sensitive degrees of freedom of the mirror surface are precisely tuned considering the various contributors. Using numerical optimization, the aberrations related to gravity, assembly loads and minute shrinkage differences are greatly reduced. Finally, by developing a manufacturing process based on inhouse milling, thermal treatments, diamond turning, in-house polishing and gold coating, the final mirror is manufactured. Interferometric tests show good agreement with the numerical predictions of the surface form error and confirm the excellent performance of the CM1 mirror. This is one of the state-of-art examples of metal mirrors designed, manufactured and assembled to such an accuracy.

The Gran Telescopio de Canarias Adaptive Optic System (GTCAO) is designed to provide nearly diffraction-limited images to GTC. GRANCAIN (GRAN CAmara INfrarroja) is a first light imaging instrument in J, H, and K infrared bands, and it will be integrated into the Nasmyth focus of GTCAO, contributing to carry out his acceptance tests. The instrument aims to capture NIR (Near-Infrared) diffraction limit images for a field of view of 22x22 arcsec operating up to seeing conditions of 1.5 arcsec and zenithal distances up to 60deg. The instrument has a telecentric optical design based on a collimator-camera with a 2:1 magnification, with a cold stop and the filters located in the collimated beam. The detector is a 4Mpx Hawaii-2 PACE Teledyne (H2P), which operates at 77K. The instrument is composed of a rectangular aluminum 6061-T6 cryostat cooled by a closed Gifford-McMahon helium cycle cryocooler with an optical bench where the entire optical train is mounted. The opto-mechanical system consists of two filter wheels, containing three wide and three narrowband filters in the J, H, and K bands in charge of selecting the wavelengths of the science images. Thanks to a wide background knowledge built up at the Instituto de Astrofísica de Canarias (IAC) and the use of commercial elements, the instrument development plan has been reduced to one year and a half. The article contains a general description of the design, fabrication, integration, and testing of the opto-mechanical elements, cryostat, cooling, and anti-vibration systems.

Astronomical mirror coatings are often metals protected by multiple layers of dielectrics. Varying the thickness and layering of dielectrics causes a significant dependence on the polarization properties (retardance, diattenuation, and depolarization) of reflected light across all wavelengths. Polarization further varies with angle of incidence and mirror shape. In models predicting polarization performance, assumptions on the properties and uniformity of coated optical surfaces are usually made. Here, we present how a non-uniformly applied coating affects polarization performance and causes depolarization across an aperture. We then assess the differences from assuming a uniform surface. Using the NSF’s Daniel K. Inouye Solar Telescope as an example of a complex, many-optic, articulated system, we also compare depolarization effects of mirror coating non-uniformity to other known sources of systematic polarization error on DKIST.

The imaging and characterization of a larger range of exoplanets, down to young Jupiters and exo-Earths will require accessing very high contrasts at small angular separations with an increased robustness to aberrations, three constraints that drive current instrumentation development. This goal relies on efficient coronagraphs set up on extremely large diameter telescopes such as the Thirty Meter Telescope (TMT), the Giant Magellan Telescope (GMT), or the Extremely Large Telescope (ELT). However, they tend to be subject to specific aberrations that drastically deteriorate the coronagraph performance: their primary mirror segmentation implies phasing errors or even missing segments, and the size of the telescope imposes large spiders, generating low-wind effect as already observed on the Very Large Telescope (VLT)/SPHERE instrument or at the Subaru telescope, or adaptive-optics-due petaling, studied in simulations in the ELT case. The ongoing development of coronagraphs has then to take into account their sensitivity to such errors. We propose an innovative method to generate coronagraphs robust to primary mirror phasing errors and low-wind and adaptive-optics-due petaling effect. This method is based on the apodization of the segment or petal instead of the entire pupil, this apodization being then repeated to mimic the pupil redundancy. We validate this so-called Redundant Apodized Pupil (RAP) method on a James Webb Space Telescope-like pupil composed of 18 hexagonal segments segments to align, and on the VLT architecture in the case of residual low-wind effect.

The RISTRETTO instrument is a high-contrast high-resolution integral field spectrograph, designed to observe and characterize nearby exoplanets located at 2 lambda/D from their host stars. RISTRETTO’s spectrograph will be fed by a high-contrast IFU consisting of 7 single-mode fiber (SMF) spaxels. In this context, the coronagraph is specified to provide at least 10-4 raw contrast at 2 l/D across the I-band. Two coronagraph designs are presented: (i) an amplitude ring-apodizer mask, and (ii) a PIAA-derived solution with improved throughput. Simulations including realistic XAO residuals and early laboratory results are shown. The foreseen SMF injection unit design is also detailed.

A next-generation instrument named, Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES), is being planned for the W. M. Keck Observatory. SCALES will have an integral field spectrograph (IFS) and a diffraction-limited imaging channel to discover and spectrally characterize the directly imaged exoplanets. Operating at thermal infrared wavelengths (1-5 μm, and a goal of 0.6-5 μm), the imaging channel of the SCALES is designed to cover a 12′′ × 12′′ field of view with low distortions and high throughput. Apart from expanding the mid-infrared science cases and providing a potential upgrade/alternative for the NIRC2, the H2RG detector of the imaging channel can take high-resolution images of the pupil to aid the alignment process. Further, the imaging camera would also assist in small field acquisition for the IFS arm. In this work, we present the optomechanical design of the imager and evaluate its capabilities and performances.

We present aluminum (Al) mirrors protected with a flash lithium fluoride (LiF) overcoat. Each of these Al and LiF layers are produced with a novel room-temperature reactive Physical Vapor Deposition (rPVD) process that consists of exposing these films growth to a fluorine-containing xenon di-fluoride (XeF2) gas. We report two sets of Al/LiF mirrors produced with this rPVD process. The first set is optimized at a wavelength of 121.6 nm and presents an unprecedented reflectance of 92.6% at this wavelength. The second set is optimized at shorter wavelengths by reducing the thickness of the LiF overcoat to have a more balance reflectance performance in the far-ultraviolet (FUV) spectral range from 90-200 nm. This new process is observed to produce more durable and less hygroscopic mirrors than those fabricated with standard PVD process, and has utility in realizing an intrinsic high reflectance of aluminum in the critical FUV spectral range.

Several telescopes like VISTA or the ELT are using or will use silver coatings, replacing aluminum (Al). The advantage of silver is a higher overall reflectivity, in particular around 825 nm. Yet, silver must be protected by covering layer(s), which lowering the reflectivity in the blue/UV region. Therefore, ESO completed a 2-year development with Fraunhofer IOF under the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability. The developed coatings have been examined by standardized tests like scratching sensitivity, peeling, salt mist and H₂S gas. The quasi-standard in silver coatings for telescope mirrors was developed for the Gemini observatory and it uses nickel chromium nitride (NiCrN_x) as interlayer between silver and the protective top layer(s), finished by a hard silicon nitride (Si3N4) layer of ~ 15 nm thickness. We replaced the NiCrNx by aluminium oxide (AlO_x) and it led to higher reflectivity but low durability towards the salt mist test. This test is relevant for many large telescopes because of their proximity to oceans, causing salty airborne dust. By replacing the NiCrN_x by a ruthenium-based layer and by optimizing the protection, the objective of extending the wavelength range of high reflectivity to shorter wavelengths without decreasing the coating durability could be achieved.

Various layer schemes are used to protect thin film silver mirrors from tarnish, corrosion, and degradation. One common design is based on the coating currently utilized by the twin Gemini astronomical telescopes, consisting of two very thin layers of nickel-chromium nitride (NiCrN_x) on either side of the silver layer followed by a protective overcoat of silicon nitride (SiN_x). Variations of this coating design have employed assorted NiCr- or Cr-based layers of varying thicknesses on either side of the silver, in efforts to achieve both high reflectivity and good durability. Generally, however, it is believed that reflectivity comes at the expense of durability. Ongoing work in our laboratory has explored the durability and corrosion behavior of various Gemini-style silver mirrors with NiCr- and Cr-based layers. Here, we present recent work on varying the Ni/Cr ratio in the NiCrNx adhesion layer above the silver and the effect on mirror durability, in terms of reflectance, scatter, and corrosion feature development during accelerated environmental exposure testing. By varying the adhesion layer Ni/Cr ratio from Ni-rich (90/10 Ni/Cr) to Cr-rich (0/100 Ni/Cr), it is possible to control the initial optical performance of the mirrors, as Ni-content is inversely related to reflectance. By varying the Ni/Cr ratio, it is also possible to control mirror durability, including the mechanism of corrosion feature development. This suggests that there may be an intermediate Ni/Cr composition with reduced Ni-content that improves reflectance without sacrificing durability.

Telescope mirrors typically consist of glass-ceramic substrates coated with a thin layer of aluminum or protected silver. Airborne contaminants on such surfaces can significantly degrade their reflectivity, IR emissivity and light scattering properties and cause damage such as pinholes. After exposing mirror samples near the Very Large Telescope (VLT) on Cerro Paranal, we investigated the contaminant formation at the microstructural level using electron microscopy. We show contaminant damage mechanisms on aluminum coated compared to protected silver mirror samples.

The development of high reflectivity, protective mirror coatings for the Lyman UV (λ < 120 nm) is essential for enabling both CubeSats and large UVOIR missions to tap this rich bandpass. Aluminum mirrors with fluoride-based protective layers are the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with long flight heritage. However, despite their overall acceptable optical performance, the efficiency of current state-of-theart Al coatings is still insufficient in the 100-120 nm wavelength range for optical systems in which several reflections are required. Optimizing the efficiency down to 100 nm is essential for the viability and scientific return of the next generation of future FUV-sensitive space telescopes, as was called out in the 2020 astrophysics decadal survey. In this proceeding, we will present recent advances in the development of environmentally stable Al+eLiF+MgF₂ mirrors deposited on substrates with increasing thicknesses, developed for the SPRITE CubeSat. SPRITE is projected to be the most sensitive instrument to date in the 100 - 120 nm range and the first orbital imaging spectrograph for this bandpass with sub-arcminute resolution.

Narrow-band filters are often used to constrain the chemical composition of astronomical objects through photometry. A challenge to derive accurate photometry is that narrow-band filters are based on interference of multiple reflections and refractions between thin layers of transparent dielectric material. When the light rays reach the surface of a filter not perpendicular to it, they cross the layers obliquely travelling a path longer than the thickness of the layers and different for each inclination. This results in a blue-shift of the central wavelength and a distortion of the transmission curve. Hence, particular care should be taken when narrow band filters are used in presence of small f-numbers and large non-telecentric angles, as frequent in the large field of view (FoV) instruments. Sometimes, the broadening and central wavelength shift of the transmission curve are considered and compensated in the design of filters for instruments with a small f-number. Here we consider the combined effect of small f-number, non-telecentricity and large FoV. Where single spectral lines are considered, a shift in central wavelength or a change in the shape of the transmission curve may introduce an instrumental dispersion in luminosity and in the linked color indices. We found that transmission curves of narrow band filters can be significantly different in shape than the nominal ones. The bottom limits for filters’ effective FWHM for each f-number; the monotonic behavior of the blue-shift with distance from the center of FoV; the monotonic quality decrease of the transmission curves and the photometric dispersion introduced by the filters are computationally estimated. This work could represent a useful tool to evaluate the fitness of a particular filter at a particular facility.

Volume phase holographic gratings (VPHGs) have gained popularity for use in astronomical spectrographs due to their high first order diffraction efficiency, angle tuning capabilities, low scatter, durability, and ability to be manufactured in large formats; gratings up to 30 cm across are routinely achievable. The nature of the VPHG manufacturing process offers optical and instrument designers a tremendous amount of flexibility to optimize performance for a specific application or need. In this talk, we will explore VPHGs from a grating designer’s perspective, aiming to give insight to spectrograph designers as they develop grating requirements for new spectrographs. We will address considerations and limitations with respect to grating size, angle of incidence (AOI), dispersion, wavelength of operation, and bandwidth. We will look at how AOI affects grating design and efficiency, stepping through several AOI regimes to explore the diffraction efficiency and spectral profiles which can be achieved, as well as the physics behind them – from required film thickness to the relative efficiency of s- and p- polarization as index modulation is varied in each regime. We will also touch upon the advantages offered by use of a grism configuration (GRating + prISM), and provide examples of both routine and unusual examples of grism implementation.

Volume Phase Holographic Gratings (VPHGs) are optical element widely used in astronomical spectrographs as main disperser or cross-disperser in high resolution echelle instruments. In spite of the fact that other technologies are available on the market, the VPH technology remain a key one. In the EU funded OPTICON project, different activities were carried out in order to consolidate the design and production of VPHGs for astronomy. In particular: i) a production process based on innovative high performance holographic materials (by COVESTRO AG) has been defined; ii) high quality VPHGs ⪆ 170 mm in diameter were manufactured; iii) innovative configurations, such as multiorder and multiplexed gratings were proposed and the devices realized. Now, more than 10 devices based on this technology are mounted on observing facilities and several more are in development or planned. Here, we retrace the achievement of the VPH activities in the last years and we propose our roadmap for future improvements in the VPHG design and production for supporting the requests of the astronomical community starting from the ORP EU project.

In next generation space and ground-based instrumentation for Earth and Universe Observation, new instrument concepts include often non planar gratings. Their realization is complex and costly. We propose a new technology for designing and realizing convex blazed gratings for high throughput spectrographs. For this purpose, the requirements are driven by a Digital-Micromirror-Device-based (DMD) MOS instrument we are developing, called BATMAN. The two-arm instrument is providing in parallel imaging and spectroscopic capabilities. The objects/field selector is a 2048 x 1080 micromirrors DMD, placed at the focal plane of the telescope; it is used as a programmable multi-slit mask at the entrance of the spectrograph. The compact Offner-type spectrograph design contains a low density convex grating to disperse light. For optimization of the spectrograph efficiency, this convex grating must be blazed. A blazed reflective grating has been designed with a period of 3300 nm and a blaze angle of 5.04°, and fabricated into convex substrates with 225 mm radius of curvature and a footprint diameter of 63.5 mm. The blaze is optimized for the center wavelength of 580 nm within the spectral range of 400 – 800 nm. Such grating has been fabricated by using lithography, angular Ar ion etching, transfer of the blazed grating from a flat surface onto a convex substrate with a flexible stamp, etched into the substrate by RIE etching. and finally coated with a silver-based layer. With a final 7° blaze angle over the whole surface, efficiency close to 90% on the 1st diffraction order at 700nm has been obtained, measured on BATMAN spectroscopic arm. An optimized device with the exact required blaze angle would reach the same efficiency and be centered on the mid of 400-800nm wavelength band: its realization is on-going. The wavefront error of the diffracted beam will also be optimized. The grating brings a significant contribution in the total amount of straylight at instrument level. Their straylight level remains a critical issue, and its reduction by specific and controlled implementation of improvements in manufacturing process is a challenge to tackle. Straylight measurement has been done and shows a BRDF cosq values of 10-8 sr-1 on the optical surface and 10-7 sr-1 on the structured features. This new type of non-planar reflective gratings will be the key component for future high throughput spectrographs in space missions.

THESEUS mission aims to detect and observe transient sources in the X-ray and Gamma-Ray bands, with a follow-up in the infrared band. The primary objectives are to provide real time trigger and accurate location of GRBs and to discover new high-energy transients. THESEUS InfraRed Telescope (IRT) is, with the Soft X-ray Imager (SXI) and the X-Gamma ray Imaging Spectrometer (XGIS), one on the 3 instruments onboard THESEUS satellite. The IRT (0.7-1.8 μm) is a 0.7 m class IR telescope with 15x15 arcmin FOV, for fast response, with both imaging and spectroscopy capabilities. The most critical component for the spectroscopic mode is the grism. This grism is a complex component with multi - function properties: (i) fixing the line of sight towards the detector; (ii) selecting the correct spectral band with interferential filter; (iii) compensating for aberrations (both geometrical and chromatic) to reach near diffraction-limit image quality; (iiii) dispersing the wavelengths with a resolving power around 400. Such grisms have been developed by our laboratory for the NISP instrument of the EUCLID mission, with industry partnership from Silios Technologies. These grisms, with curved lines, are manufactured by photolithography. In the context of THESEUS IRT grisms, new R&D activities are mandatory to validate the manufacturability of the gratings since they have a groove density 3 times higher than NISP grisms. In this article, we will present the results we obtained on grating prototypes developed during the phase A of the project. Profilometric measurements of the groove profile and diffraction efficiency measurement are analyzed.

We fabricated nanoscale random anti-reflective surface structures on a prototype Reactive-Ion Plasma-Etched (RIPLE) grating. The prototype is a binary-profile, 1767 l/mm linear structure, over a 25x25 mm² area on a ¼-inch thick Quartz plate, E-beam patterned, imbedded into the substrate by a Reactive-Ion Etching process. We achieved an 8:1 aspect ratio between the grating’s phase depth and space-width (187nm wide). The measured 1st-order unpolarized diffraction efficiency reaches 94.5%~96% and stays greater than 70% across a 200nm bandwidth centered at 560nm wavelength. The non-patterned side was populated with AR nanostructures. We detail the fabrication and evaluation of this complete prototype RIPLE grating.

The Cassegrain U-Band Efficient Spectrograph (CUBES) is a high-efficiency spectrograph designed for observations from 305 to 400nm. It will be integrated at a Cassegrain focus of the Very Large Telescope (VLT). The image slicer technology is applied to reformat the field of view reducing the spectrograph entrance slit etendue and minimising the spectrograph volume and weight without slit losses. Two image slicers will provide CUBES with two spectral resolving powers: R≥20,000 for high resolution (HR) and R≥5,000 for low resolution (LR). Both image slicers are composed of two arrays of six spherical mirrors. For the HR mode, a rectangular field of view of 1.5arcsec by 10arcsec is reorganised into a slit of 0.19mm × 88mm; for the LR mode, a field of view of 6arcsec by 10arcsec is reformatted into a slit of 0.77mm × 88mm, with slicer mirrors of width 0.5mm and 2mm, respectively. CUBES is currently in the Preliminary Design Phase (Phase B). This communication presents the Conceptual (Phase A) design and the main performance for the HR and LR image slicers addressing the following technological challenges: compact layout with the minimum number of optical components to optimise throughput, near diffraction limited optical quality, telecentric design with overlapped exit pupils for all slices of the field of view, distribution of the slicer mirrors to reduce shadows and selection of the best substrate for the very short wavelengths at which CUBES will operate.

With the advancements of ultra-high-precision micro-optics fabrication technologies, it is now possible to fabricate integral field units (IFUs) with slicer mirror width of 30 m or less. This paper describes a 36-um machined image slicer IFU (MISI-36) for the Diffraction-Limited near-IR Spectropolarimeter (DL-NIRSP) of the Daniel K. Inouye Solar Telescope (DKIST). MISI-36 has a unique 2-section image slicer design, and is consists of a monolithic image slicer block with 112 micro slicer mirrors, a parabolic collimator, a monolithic flat mirror array consists of 112 fold mirrors, and a monolithic spherical mirror array consists of 112 spherical mirrors. We have successfully fabricated a prototype device using Canon Inc.’s diamond-cutting CNC, and conducted a preliminary performance evaluation using an experimental bench-top spectrograph similar to the spectrograph of DL-NIRSP. We will present the optical design and optical performances of the MISI-36 prototype.

MOONS (Multi-Object Optical and Near-infrared Spectrograph) is a third-generation visible and near-infrared spectrograph for the ESO Very Large Telescope, currently nearing the end of the assembly phase. The three channel spectrograph is fed via a fibre positioning module (FPM) which configures the location of 1001 fibres. The robotic fibre positioning units (FPUs) have been jointly developed by the UK Astronomy Technology Centre (UKATC) and MPS Microsystems (MPS) and provide a high-performance multiplexed focal plane with excellent transmission characteristics. An overview of the as-built mechanisms and supporting infrastructure is presented, with details on the extensive calibration process carried out. The integration process to date will be described, including a discussion of key lessons learned.

The 4-meter Multi-Object Spectroscopic Telescope (4MOST) instrument uses 2436 individually positioned optical fibres to couple the light of targets into its spectrographs. The AESOP fibre positioner is mounted at the Cassegrain focus of the VISTA telescope, which houses the fibres in a hexagon-like structure with a diameter of 535 mm that covers a 2.5 deg diameter field of view on the sky. Fibres are positioned relative to fixed fiducial fibres. The metrology system determines the position of the fibres on the focal surface of the telescope relative to the fiducial fibres. The location of the fibres needs to be measured to better than 3 micron RMS in the focal surface, approximately 0.05 arc seconds on sky. Four imaging cameras are mounted on the VISTA spider vanes that look through the entire optical train, including primary and secondary mirror as well as the wide field corrector (WFC) / atmospheric dispersion compensator (ADC) unit. We recreated the setup for the metrology system in the lab with similar dynamic behavior but different optical design due to the lack of the VISTA telescope. We demonstrate the metrology system measurement accuracy in lab conditions on the full scale test stand. We also show how we measure distortions induced by optical path and the calibration procedure as a precursor for commissioning on the telescope. In particular, we present a method how to measure the surface shape of any optical surface with approx. 10 nm accuracy over its entire optically active surface.

Two unique wide-field and high-accuracy polarimeters named WALOP (Wide-Area Linear Optical Polarimeter)- North and WALOP-South are currently under development at the Inter-University Center for Astronomy and Astrophysics (IUCAA), India, to create a large area optical polarization map of the sky for the upcoming PASIPHAE sky survey. These instruments are designed to achieve a linear polarimetric measurement accuracy of 0.1% across a field of view (FoV) of 30×30 arcminutes. The WALOP-South instrument will be installed first on a 1 m telescope at the Sutherland Observatory, where the temperatures during the night can vary between 10 to -5°C. These temperature variations and the instrument’s pointing to various non-zenithal positions in the sky can introduce stress birefringence in the lenses, leading to time-varying instrumental polarization. This work estimates stress-induced birefringence due to thermal, and gravity stresses on WALOP-South lenses. Using the optomechanical model of the WALOP-South, we carried out Finite Element Analysis (FEA) simulations in SolidWorks software to estimate the stresses for various scenarios of temperature, telescope pointing airmass, and lens mount material (aluminum and titanium). Further, we use the stress tensor analysis to estimate the principal stresses and their directions and consequent birefringence and retardance introduced in the lenses. The stressinduced birefringence will change the optical path length for orthogonal polarization states of the beam passing through the lenses and introduce phase retardation. Overall, with the lens mount design of the instrument, we find that the retardation and consequent instrumental polarization will be within the instrumental accuracy requirements. Additionally, the stress birefringence is found to be higher for aluminum compared to titanium mounts. We further incorporated this retardance in the instrument Mueller matrix estimation to understand its effects on the polarization measurements.

The large sizes of conventional astronomical spectrographs pose a challenge with their thermo-mechanical stability, which in turn adversely affects the spectral precision of high-resolution spectrographs, particularly for the upcoming extremely large telescopes. Astrophotonic spectrographs can resolve this issue by enabling compact, single-mode fiber-fed diffraction-limited spectroscopy on a chip. Here we demonstrate a continuous-output, curved focal-plane photonic Arrayed Waveguide Grating (AWG) chip which allows a continuous sampling of the spectrum, thus alleviating the issues of discrete-output AWGs. This chip was fabricated using a commercial Silicon-nitride platform with a square waveguide cross-section of 800 × 800 nm. The AWG has a resolving power of 12,000, a free-spectral range of 18 nm, and a wide operational band of 1425-1650 nm. The chip is highly compact with a size of only 8.75 × 3 mm. Notably, the AWG chip is cleaved along its curved focal plane (i.e. Rowland curvature) to avoid defocusing of the spectrum due to planar cleaving. The curved focal plane can be imaged onto a planar detector using focusing optics. However, in this paper, the focal plane was sampled by translating a single-mode fiber across it. While this device highlights the capability of the commercial high-contrast SiN platform for building compact, high-resolution, and continuoussampling spectrographs for astronomy, it also underlines the key challenges in imaging / characterizing the high numerical aperture chip output at low loss. We discuss these challenges and the way forward.

Efficiently coupling light from large telescopes to photonic devices is challenging. However, overcoming this challenge would enable diffraction-limited instruments, which offer significant miniaturization and advantages in thermo-mechanical stability. By coupling photonic lanterns with high performance adaptive optics systems, we recently demonstrated through simulation that high throughput diffraction-limited instruments are possible (Lin et al., Applied Optics, 2021). Here we build on that work and present initial results from validation experiments in the near-infrared to corroborate those simulations in the laboratory. Our experiments are conducted using a 19-port photonic lantern coupled to the state-of-the-art SCExAO instrument at the Subaru Telescope. The SCExAO instrument allows us to vary the alignment and focal ratio of the lantern injection, as well as the Strehl ratio and amount of tip/tilt jitter in the beam. In this work, we present experimental characterizations against the aforementioned parameters, in order to compare with previous simulations and elucidate optimal architectures for lantern-fed spectrographs.

Integrated optics are used to achieve astronomical interferometry inside robust and compact materials, improving the instruments stability and sensitivity. In order to perform differential phase measurements at the Hα line (656.3nm) with the 600-800nm spectro-interferometer FIRST, a photonic integrated circuit (PIC) is being developed. This PIC performs the visible combination of the beams coming from the telescope pupil sub-apertures. In this work, TEEM Photonics waveguides fabricated by K+ : Na+ ion exchange in glass are characterized in terms of single-mode range and mode field diameter. The waveguide diffused index profile is modeled on Beamprop software. FIRST beam combiner building blocks are simulated, especially achromatic directional couplers and passive π/2 phase shifters for a potential ABCD interferometric combination.

Details of a programme to investigate the outgassing rate of additively manufactured (AM) aluminium alloys are presented. AM has significant potential benefits to applications in ground- and space-based instrumentation, particularly in mass optimisation, part consolidation and increased design freedom. However, its use in high-risk projects is often curtailed by lack of heritage and an imperfect understanding of the materials. The programme goal was to address one of the most significant topics preventing wider adoption of AM technology in cryogenic and space-based applications; uncertainty about material outgassing. The sensitivity of outgassing rates to various key parameters was characterised, including print method, post-processing and geometrical complexity. Correlation of outgassing rates against other measurable properties, such as sample porosity and surface roughness, was also investigated via the use of X-ray computed tomography and profilometry. Finally, the test apparatus, experimental design and implications of the findings on design and process control are discussed.

New Heavy Metal Oxide Glasses have been analysed for their excellent optical properties and their application in nonlinear optics. More specifically, chloro-antimonite glasses have been obtained in the Sb2O3-PbCl2-AgCl ternary system. Thermal, optical and mechanical properties have been studied. The silver chloride concentration was increased at the expense of antimony oxide according to the following composition rules: (80-x)Sb2O3-20PbCl2-xAgCl ; (70- x)Sb2O3-30PbCl2-xAgCl where x is the AgCl concentration. Optical transmission range extends from 400 nm in the visible spectrum to 7 m in the infrared. Refractive index is close to 2.

Silicon and germanium are important and popular, transmissive, mid-wave infrared (MWIR) and long-wave-infrared (LWIR) optical materials used in a very wide variety of applications including scientific instruments, thermal imaging, and defense infrared systems. Prior to the present study, accurate, cryogenic, refractive index measurements of these two materials using the Cryogenic High Accuracy Refraction Measuring System (CHARMS) at the Goddard Space Flight Center (GSFC) were last completed for NASA’s James Webb Space Telescope program and published around 2006. Numerous infrared optical systems have since been successfully designed and built based on these previous CHARMS index data. A new, more accurate study of cryogenic refractive indices of silicon and germanium has been carried out for sample materials produced in the 2019-2020 time period. This paper presents findings for about a half dozen samples of each material from different boules and, including results from 2006, represents the first significant study of interspecimen index variability of Si and Ge using CHARMS. These new measurements spanned the wavelength region from 1.11-5.43 μm for Si and 1.90-5.43 μm for Ge over the temperature range from 100-310 K. Updated measurement uncertainties for these materials in CHARMS are presented reflecting improvements made to CHARMS since 2006.

This paper reports on absolute refractive index measurements for the S-FPL55, S-LAL8, and S-BAH28 glass materials that are used in the visible channel of the HARMONI integral field spectrograph of the Extreme Large Telescope (ELT). These measurements were carried out by using the world-class Cryogenic High Accuracy Refraction Measurement System (CHARMS) refractometer (located at the Goddard Space Flight Center) in the 0.420−3.5 μm wavelength range and in the cryogenic temperature range of 105−310K. We also report on the validity of a cold-lens system design that is based on extrapolated 300 K refractive index data in comparison with directly measured cryogenic data obtained with the CHARMS refractometer.

Many components of our STELLA telescopes located on Tenerife, which were built by Halfmann in the 2000s have reached the end of their life with no replacement parts available. A solution was necessary to guarantee continuous operation and support for the next ten years. The prerequisite for the retrofit, however, was that the mechanical components remain largely untouched in order to simplify the upgrade. We decided to remove all the existing electronics in the main control cabinet. In order to avoid electronic interference in the scientific instruments, we took several precautions. This included an isolating transformer, line filters and power chokes for the servo drivers. All of the control electronics as well as the sensory inputs is now handled by Beckhoff components. A Beckhoff PLC CX5140 is the new ”electronic brain” replacing a Linux computer running the telescope control firmware. The new telescope control firmware written in TwinCAT3 is available as open source. MQTT messages are used to command the telescope and report sensor values and position information. Sensor measurements and the state of the telescope are logged in an Influx∗-database and visualized using Grafana†. Future enhancements include an improved guiding of the telescope using machine vision and a GigE camera in a closed loop on the PLC.

Wide field-of-view (WFOV) lenses covering a full field of view of <120° have been designed and analyzed for spaceborne applications for use in severe environments. The optical designs have been passively athermalized and achromatized over the visible waveband for use during operating conditions and the opto-mechanical designs have been ruggedized for survivability in severe environmental conditions with random vibration levels exceeding 2000 Grms and cryogenic temperature conditions reaching -175°C. To characterize the performance and survivability of the lens assembly for such environments, image quality and random vibration testing was performed on representative optical assemblies, with previous flight experience, before and after exposure to cryogenic liquid nitrogen temperatures. The dynamic test environment consists of low-level random vibration testing in each of the three orthogonal axes (X, Y & Z) and the image quality testing performed includes resolution (MTF), distortion, and effective focal (EFL). Finite element analysis (FEA) was performed to verify survivability to full random vibration spectrums and shock levels; external testing was performed with flight hardware and results were validated. High correlation of characterization measurements before and after cryogenic exposure indicate the optical assemblies' ability to withstand severe temperature environments without any impact to functionality or performance. This document provides an overview of the optical and opto-mechanical lens designs for severe space environments, a comprehensive look into component material characteristics over temperature for various space-grade adhesives used to mechanically retain lens elements, and detailed explanations of the cryogenic test methodology using a custom laboratory test setup to characterize the performance and survivability of lens assemblies and their various components.

Innovative optical designs combing freeform optics and curved freeform sensors will be key elements to improve the optical performance and reduce the overall manufacturing and testing pressure of optical complex systems. In this paper, three version of unobscured all-reflective TMA-type telescopes (D=10cm, FOV=7°×7°) will be designed separately with flat sensor, spherical sensor and freeform sensor. Then, we will analyze and compare their imaging quality (RMS spot radius) and surface complexity (PV sag departure and Max slope departure) to demonstrate the advantage of freeform sensor applied in the off-axis optical system. Finally, we make comparison on sensitivity analysis of surface error between mirror surfaces and freeform sensor surfaces to prove that the manufacturing pressure of the sensor did not increase even though its surface complexity increased. We expect that such a design has a great potential to be applied in the space weather monitoring and solar system planetary exploration drones.

In 2015, the pGCT, a telescope prototype planned for the Cherenkov Telescope Array (CTA) gets its first Cherenkov light in the Meudon site of the Observatoire de Paris. As a part of the small-sized telescopes of the CTA, this telescope was designed to detect showers of secondary particles produced when very high energy gamma rays and cosmic rays enter in the upper atmosphere and interact with the atmospheric gas. It is now dedicated to a test bench for Cherenkov observation and to educational purposes. Within this last framework and in order to propose to the general public an easy way to observe high energy particles, we started in 2021 the development of a Langsdorf cloud chamber based on a previous model using Peltier cells and developed for students in Tours. Cloud chambers provide a convenient way to observe signatures of charged particles related to cosmic rays since they allow a direct detection also during daytime and are easier to use for the visits by the general public to concretely illustrate the existence of these high energy particles. This paper describes mechanical and electronic designs of this cloud chamber. Some results for educational purposes are also given.

JSC LZOS has been using diffraction optical elements (DOE) or Computer Generated Holograms (CGH) for many years to test the surfaces of large-sized optical mirrors for astronomical and space purposes. They are used for aspherical surface figure testing; testing of aspherical surface vertex position relative to the mirror geometric center, calculation of distortion in interferogram image, mutual adjustment of mirrors in testing scheme, etc. Thus, CGH have become an integral part of the up-to-date testing of large-sized optical mirrors and optical systems aspherical surfaces.

The Fred Young Submillimeter Telescope (FYST) is a 6-meter diameter telescope with a surface accuracy of 10.7 microns, operating at submillimeter to millimeter wavelengths (100 GHz – 1.5 THz). It will be located at 5600 meters elevation on Cerro Chajnantor in the Atacama desert of northern Chile overlooking the ALMA site. Its novel optical “crossed-Dragone” design will deliver a high-throughput, wide field-of-view telescope capable of mapping the sky very rapidly and efficiently. This paper discusses the mirror panel production and its contribution to the overall half wave front error of the telescope. The first half details the panel manufacturing precision. The effect of panel production quality on the beam shape and beam quality is presented. The second half of the paper looks at the local surface roughness of a mirror panel. Surface roughness data for a machined panel with an etched surface are presented. Some non-ideal surface features for an etched panel are discussed.

Integral fields units such as lenslet-arrays, fibre-bundles or image-slicers all suffer from inefficient over-sampling of the spatial PSF unless anamorphic magnifying optics are used, which adds optical aberrations, and reduces optical throughput. In 2014 we presented the novel twisted image slicer concept that produces fully Nyquist-sampled data cubes without the use of anamorphic optics; in this paper we present the opto-mechanical design of a prototype twisted image slicer, its nominal performance, and preliminary characterisation results. The image slicer, manufactured by Canon Inc., will be used with a test-bench integral field spectrograph as part of the R&D activities for the Planetary Camera and Spectrograph (PCS) for the Extremely Large Telescope (ELT). First interferometric measurements indicate a very good surface figure with a figure error smaller than 100 nm P-V. Preliminary spectra taken with the test-bench spectrograph suggest that the twisted slicer concept does indeed achieve Nyquist sampling in both spatial directions as well as along the spectral axis.

FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument’s stability and sensitivity, we have designed and fabricated a 3D laser-written optical chip 5T beam combiner. The multi-aperture beam combiner consists of 5 input waveguides spaced by 250um. Each input is split into 4 waveguides. A pairwise recombination scheme with Yjunctions produces/leads to 10 outputs (127um separation, compatible with commercial V-grooves). In this work, we present the experimental characterization of the chip: transmission performance, polarization issues and single mode spectral range. The targeted single mode spectral range must cover 550-800nm. Different optical powers for laser writing are used to finely tune the ideal single-mode behavior of the waveguides, ranging from 220 to 270mW. A straight waveguide was used as a reference, imprinted close to the first interferometric channel. Using different lasers (635nm, 780nm and 980nm) as well as wideband sources, we have been able to characterize the spectral transmission, the polarization behavior (TE/TM) and the interferometric contrast. The chip was inserted in the FIRST/SUBARU optical bench simulator at LESIA, in order to inject the 5 inputs simultaneously and scan the fringes using 4 independent MEMS, inducing a relative OPD modulation. Preliminary results show very good transmission for such a complex chip: all input channels are above 45% at 635nm (comparing the injected single mode with the sum of the 4 corresponding outputs), with two inputs reaching 80%. A huge advantage of this technology is to avoid the crosstalk due to in-plane waveguide crossings, and we show that no crosstalk is indeed observed. Both polarizations are transmitted, without noticeable birefringence. However, perfect vertical alignment of the outputs is difficult to obtain, and must be optimized prior to any connectorization to fiber bundles.

This conference presentation was prepared for the Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V conference at SPIE Astronomical Telescopes + Instrumentation, 2022.

This conference presentation was prepared for the Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V conference at SPIE Astronomical Telescopes + Instrumentation, 2022.

For the first time in Russia, a technology has been developed, using computer-controlled automatic system and a set of special tools, that allows testing and aspherization of off-axis large-sized optical elements of astronomical and space telescopes with a deviation from the nearest sphere up to 20 mm. Computer-controlled process of optical surface figuring is carried out at all stages – milling, grinding and polishing.

A two-in-one optical element consisting of a planar pick-off mirror (16×14 mm²) and a slice mirror array (twentysix 0.52 mm-wide and 18 mm-long mirror facets) was developed as a component for a near-infrared integral field unit: SWIMS-IFU for a near-infrared imager and spectrograph SWIMS for TAO 6.5 m telescope. The two optical entities were formed on a single bulk of aluminum alloy using a 5-axis ultraprecision machine and a specialized pair of diamond cutting tools. The relative position and orientation of the two optical entities were controlled to satisfy the tight optical tolerance, by precise measurement of tool tip positions with respect to the axes of the machine tool. Regarding measurement, an X-ray CT scanner was introduced as a solution to a major problem in the evaluation of complex monolithic optics, where some facets cannot be measured due to geometric constraints. The diamond-machining and measurement methods proposed in this study will be useful also for more complex monolithic optics to be developed in the future.