This PDF file contains the front matter associated with SPIE Proceedings Volume 11451, including the Title Page, Copyright information and Table of Contents.

Japan Aerospace Exploration Agency (JAXA) promoted Hayabusa asteroid sample return mission, which was launched in 2003 and returned to Earth in 2010. The retrieved capsule contained many particles originating from the S-type asteroid Itokawa. Based on the Hayabusa's achievements, Hayabusa-2 succeeded to rendezvous with the C-type asteroid Ryugu in 2018. It has accomplished all of the proximity operations such as remote sensing, robot separations, crater formation, landings, and so on, and will come back to Earth in 2020. JAXA is now putting our space assets, from Mercury to Jupiter, which will make the JAXA Deep Space Fleet to elucidate 4.6-billion-year history of the Solar System.

The exoplanet science field has been growing quite rapidly while there are still a lot of unknowns on the nature of planetary systems. It is quite complex and challenging to understand how the present-day world with planetary systems emerged and how our solar system is unique among them. In this talk, we review the recent advances in astronomical observations at various wavelengths, which have brought us new insights such as about the exoplanet populations, atmospheric characteristics, and formation processes of planetary systems. This includes the science results from Atacama Large Millimeter/submillimeter Array which has yielded detailed and often unexpected images of protoplanetary disks. We will also try to present researchers’ wishes for future observing capabilities.

Progress in astronomy is largely driven by advances in technology. Yet, only a tiny fraction of the worldwide investment in technology directly targets the needs of astronomy, so astronomers must seek to leverage and harness technology advancements made for other purposes. In this presentation, I will explore the various ways this has been done for three major recent advances in the study of black holes: the detection of merging black holes with LIGO in 2016, the radio imaging of M87’s supermassive black hole with EHT in 2019, and the decades-long studies of stellar motions near the galactic center, which revealed the central black hole in our galaxy and is the subject of the 2020 Nobel prize. In each case, the technology story spans decades, and involves a combination of advances in several areas.

NASA’s Astrophysics Division (APD) funds development of cutting-edge technology to enable its missions to achieve ambitious and groundbreaking science goals. These technology development efforts are managed by the Physics of the Cosmos (PCOS), Cosmic Origins (COR), and Exoplanet Exploration (ExE) Programs. The NASA Strategic Astrophysics Technology (SAT) Program was established in 2009 as a new technology maturation program to fill the gap in the Technology Readiness Level (TRL) range from 3 to 6. Since program inception, 100 SAT grants have been openly competed and awarded, along with dozens of direct-funded projects, leading to a host of technologies advancing their TRLs and/or being infused into space and suborbital missions and ground-based projects. We present the portfolio distribution in terms of specific technology areas addressed, including optics, detectors, coatings, coronagraphs, starshades, lasers, electronics, cooling systems, and micro-thruster subsystems. We show an analysis of the rate of TRL advances, infusion success stories, and other benefits such as training the future astrophysics workforce, including students and postdoctoral fellows hired by projects. Finally, we present APD’s current strategic technology maturation priorities for investment, enabling a range of future strategic astrophysics missions.

The project NAIR "Novel Astronomical Instrumentation based on photonic light Reformating" is a DFG-funded collaboration to exploit the recognized potential of photonics solutions for a radically new approach to astronomical instrumentation for optical/infrared high precision spectroscopy and high angular resolution imaging. We present a project update, with the developments in our ULI waveguides and 3D printed structures for astronomical instrumentation and on sky testing results obtained at the WHT, Subaru (SCExAO) and LBT. This shows the NAIR project is helping to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation.

Heisenberg's uncertainty principle tells us that it’s impossible to determine simultaneously the position of a photon crossing a telescope's aperture as well as the angle of its momentum. A new technique suggests to overcome the diffraction limit via optical amplification. A number of entangled photons, created by amplification of a single photon, behaves as a single quantum system with respect to the uncertainty principle. Unfortunately, spontaneous emission contributes noise and negates the possible gain from this stimulated emission. The spontaneous photons guarantee the uncertainty principle. Thus the problem of low resolution is replaced by the problem of low SNR. The detection of spontaneous photons follows the same Poisson statistics in time and space. However, the stimulated photons are spatially and temporally coherent with the incoming photons. A pixel with additional hidden thermal signal will slightly modify the Poisson statistics, and only within the diffraction pattern of the photon packets. We characterise the average number of spontaneous photons in all pixels, and subtract it from the stimulated photons. This algorithm is applied on simulated detection events of an amplified signal. The reconstructed image is resolved beyond the limit of the same optical system in the absence of amplification. We produced a number of samples of a wide-band solid-state dye (DCM within PMMA), because the expected number of (stellar) photons is small, and a solid-state dye is easier to handle compared to a dye solution. Initial results with a white light source and a laser pump depict the parameters of the method.

The Calar Alto Schmidt-Lemaitre Explorer (CASTLE) is an innovative 35 cm robotic telescope aimed at demonstrating the impact and performance of curved detectors for astronomical observations. This telescope will use a spherically curved science-grade sensor matching its curved focal surface and it will be installed at the Calar Alto Observatory in Spain. In this paper we will show the design and we will present the status of the opto-mechanical design and construction. We will also show the preliminary results of the straylight analysis and the general plan towards commissioning and first light in 2021/2022.

The 3-concentric-spherical-mirror, object-centered, unit-magnification catoptric relay designed by Abe Offner combines excellent aberration correction with extreme optical simplicity. A drawback of the design for some applications is that for telecentric objects the pupil is located at the secondary mirror and for near-telecentric objects (such as are commonly produced by astronomical telescopes) the pupil lies in the confusion of rays near to the secondary mirror. Recently it has been “re-discovered” that the Offner design is a special case of a more general family of designs, all sharing exactly the same imaging characteristics and optical correction of 3rd and 5th order aberrations. We say “re-discovered” because Offner presented exactly this information in his 1973 patent, but this fact seems to have generally eluded the optical design community. The “generalized Offner” solutions differ from the well-known Offner relay in that the primary and tertiary mirrors have different radii, and there is no limitation in the ratio of these radii, other than that Petzval curvature be corrected. New relays constructed of monocentric triplets of object-centred mirrors are presented for which pupils are accessible without vignetting.

Additive Manufacturing (AM; 3D printing) for mirror fabrication allows for intricate designs that can combine lightweight structures and integrated mounting. Conventional lightweight structures utilise cubic or prismatic unit cells, which do not provide uniform support at the edge of curved mirrors. We present a new circular lattice based upon cylindrical coordinates and how this lattice has been incorporated within an 80 mm diameter mirror intended for use in a 3U CubeSat telescope. Several design iterations are explored, which include prototype mirrors produced in a titanium alloy and a finite element analysis of the one of the design iterations.

Recently, we have demonstrated the potential of a hybrid astrophotonic device, consisting of a multi-core fiber photonic lantern and a 3D waveguide reformatting component, to efficiently reformat the multimode point spread function of a telescope to a diffracted limited pseudo-slit. Here, we report on an investigation into the potential of this device to mitigate modal noise - one of the main hurdles of multi-mode fiber-fed spectrographs. The modal noise performance of the photonic reformatter and other fiber feeds was assessed using a bench-top spectrograph based on an echelle grating. In a first method of modal noise quantification, we used broadband light as the input, and assessed the modal noise performance based on the variations in the normalized spectrum as the input coupling to the fiber feed is varied. In a second method, we passed the broadband light through an etalon to generate a source with spectrally narrow peaks. We then used the spectral stability of these peaks as the input coupling to the fiber feed was varied as a proxy for the modal noise. Using both of these approaches we found that the photonic reformatter could significantly reduce modal noise compared to the multi-mode fiber feed, demonstrating the potential of photonic reformatters to mitigate modal noise for applications such as near-IR radial velocity measurements of M-dwarf stars.

Safran Reosc has been awarded by ESO the manufacturing of the Secondary Mirror of the Extremely Large Telescope in 2016 [4]. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. We present the main ELT M2 manufacturing steps, namely adhesive qualification, pads bonding on the back and lateral sides, convex surface grinding and polishing. The necessary metrology instruments using interferometry and coordinate measurements are presented. The requirements driving the project with extraordinary means are presented, namely the schedule, the mirror volume, the convex surface figuring errors, the mirror lifetime of 30 years and the pad positioning accuracy (down to 0,125mm). We eventually present how these challenges are overcome: how the bonding lifetime was qualified, how Safran Reosc facility was re-configured to accommodate the polishing and control of 4-meter diameter mirrors. The polishing method (to accelerate the process and meet the requirements) are presented and discussed, including the presentation of the innovative double-head robot polishing station. We present the metrology devices adapted for the 4m mirrors and implemented in our factory. A focus on the 20-meter long interferometric bench designed and built by Safran Reosc is presented, to allow measuring the M2 convex surface and overcoming critical issues like birefringence and vibrations. This bench includes a convex-concave test plate together with a unique custom interferometer system, which are both presented.

CTA (Cherenkov Telescope Array) is a project for the next generation of the ground-based gamma-ray observatory. CTA will cover a wide energy range, 20 GeV and 300 TeV, by 3 types of the telescopes whose diameters are different. The Large-Sized Telescopes (LSTs) of the CTA are designed to cover the lowest energy range. We are working on mirrors for 4 LSTs which will be built at La Palma, Spain. The parabolic primary mirror of CTA-LST is 23 m diameter and its focal length is 28 m. The primary mirror consists of 198 segmented mirrors. Total effective reflective area is about 370 m2. Each mirror has a hexagonal shape of 1.51 m side-by-side size. It has a sandwich structure which consists of aluminum honeycomb (60 mm thickness) and two glass surfaces (2.7 mm) and the total weight is about 47 kg. We used a sputtering deposition technique to coat in the surface with 5 layer. The coated layers protect the surface for long operation such as 10 years at outside with a reflectance degradation of less than 1 % a year. The reflectance of the mirror reaches about 92 % at 400 nm and its resolution is 0.5 mrad in diameter that contains the 80 % light reflected by the mirror. The production technique based on cold slumping gave us the stable its production. We report on the design of the segment mirror and the production of about 950 hexagonal 2 m2 mirrors to achieve the parabolic shape of the optical system for 4 LSTs.

The primary mirrors of several large ground-based astronomical telescopes currently in the research or planning phases, such as TMT (Thirty Meter Telescope), ELT(Extremely Large Telescope) and LOT (Large Optical /infrared Telescope), take the form of a segmented array telescope. The fabrication of hundreds of off-axis aspheric segments is one of the key technologies for their development. Our proposed stressed mirror continuous polishing (SMCP) method combines the stressed mirror polishing (SMP) and continuous polishing (CP) technologies. The combined technologies deliver high-quality surface smoothness of the SMP technique with the high efficiency of continuous polishing. Nanjing Institute of Astronomical Optics & Technology (NIAOT) and TMT have carried out in-depth cooperation in the segment fabrication of the primary mirror. NIAOT is currently working on the development of off-axis aspheric segments. In this article, the progress for the 5m continuous polishing machine and ancillary equipment, the stress loading test on a 1.52 meter segment, the calibration of our 2-dimensional contact-metrology device and the reference mirror calibration are introduced here in detail.

The Thirty Meter Telescope (TMT) adopts a recently developed technology known as Stressed Mirror Polishing for the polishing of its 492 mirror segments. In this process, first the meniscus type spherical shape glass blanks are converted in to a desired aspheric shape by the application of forces around the edges using warping arms followed by spherical polishing in the stressed condition. After that, the blank edges will be cut in to its final hexagonal shape. These warping as well as the hex cutting process generate significant stress within the glass which in later stage, will cause the propagation of micro cracks and results in blank breakage. So prior and after the hex cutting process, it is essential to ensure that the glass blanks are free from stress accumulation. Hence the glass blanks need to be stress relieved before the hex cutting process. To achieve this stress relaxation, the glass blanks need to be kept over a platform or a support system which will provide a zero gravity condition for a time period of at least 48 hours. As a part of this, we designed, developed and analyzed a whiffletree based support system which will equally distribute the entire mirror blank mass into three points which are equally separated by 120⁰ from each other and thus balance itself as if it is in a floating condition. This support system which additionally gives optimized support for the glass blank which in turn minimizes the surface deformation due to its self weight sagging. This paper also discusses the positional sensitivity, reaction force sensitivity and alignment sensitivity analyses which are essential to obtain the tolerance values in the fabrication point of view.

Planet Formation research is blooming in an era where we are moving from speaking about “protoplanetary disks” to “planet forming disks” (1). However, this transition is still motivated by indirect (but convincing) hints. Up to date, the direct detection of planets “in the making” remains elusive with the remarkable exception of PDS 70 b and c (2; 3; 4). The scarcity of detections is attributable to technical challenges, and even for the rare jewels that we can detect, characterization is unachievable. The next step in this direction demands from near to mid-infrared interferometry to jump from ∼100 m baselines to ∼1 km, and from very few telescopes to 20 or more (PFI like concepts, (5)). This transition needs for more affordable near to mid-infrared telescopes to be designed. Since the driving cost for such telescopes resides on the primary mirror, in particular scaling with its diameter and weight, our approach to tackle this problem relies on the production of low-cost light mirrors

Millimetron Space Observatory is a 10-m aperture space telescope that will operate from millimeter to far-infrared wavelength range. To achieve an unprecedented sensitivity, the telescope with segmented primary mirror will be cooled down to a temperature less than 10K and the surface accuracy better than 10 μm will be provided. New materials and stateof- the-art technologies have been brought together to create the high precision and thermally stable primary mirror panels combining increased rigidity and low areal density (less than 9 kg/m²). Carbon fiber reinforced plastic was chosen as a material for the panels. It provides low coefficient of thermal expansion, high stiffness to weight ratio, and manufacturing flexibility. Nevertheless, it has significant drawbacks such as anisotropy, variation of mechanical properties of the ply, and microcracking that raise difficulties in achieving highly accurate and thermally stable design. The paper presents the design of the primary mirror panels and focuses on the most challenging steps of their development process. The main features of manufacturing based on replica technique and results of the numerical and experimental analyses are also discussed.

EUCLID is an optical/near-infrared survey mission to be launched in 2022 towards the L2 Lagrange point. It will aim at studying the dark universe and providing a better understanding of the origin of the accelerating expansion of the universe. By exerting cosmological sounding, it will investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of large structures formation. The EUCLID payload module (PLM) consists of a silicon carbide 1.2 m-class telescope and accommodates two instruments, the VISible Camera (VIS) and the Near Infrared Spectrometer and Photometer (NISP). As a subcontractor of AIRBUS Defence & Space, AMOS was responsible for the manufacturing of the secondary and tertiary mirrors of the telescope as well as for the flat folding mirror set within the focal plane arrangement of EUCLID telescope, which incorporates dedicated filtering functions. AMOS produced in addition the 1.3 m-class test collimator for the on-ground validation of the EUCLID instrument and the fused silica substrate for the dichroic beamsplitter.

We report a new manufacturing system for free-form large optics. The system enables both polishing and test on an optical surface by a robot arm. The robot arm drags sensors on the surface for measuring cross sections over the test surface. A new algorithm stitches the cross-sectional data to the two dimensional surface shape with improving the accuracy simultaneously. We used the system for an aspheric convex mirror (1 m in diameter) and a flat mirror (major axis of 1m); they are secondary and tertiary mirrors of Seimei telescope, respectively. The system took about one hour to obtain about 100 cross sections over the whole area of the mirror, and showed repeatability of better than 10 nm RMS. We installed the mirrors to Seimei without interferometric test. We confirmed performance of the system by obtaining a Hartmann constant of 0.26 arcsecond as combination of a M1 segment, secondary, and tertiary mirrors with observing natural stars.

An adaptive secondary mirror (ASM) is currently being developed for the UH2.2 telescope, consisting of a slumped 620mm convex aspherical facesheet, manipulated by 210 variable-reluctance actuators and supported on a silicon aluminium alloy support structure. The total power dissipation of the actuators is expected to be under 3 Watts. The ASM will weigh around 55kg, which is about 15kg lighter than the original passive secondary mirror (M2). We present the design, breadboarding activities and manufacturing status of this adaptive mirror. The project is on track for delivery of the ASM in Hawaii in the middle of 2021.

The performance of astrophotonic instruments is determined by various factors including the quality of optical surfaces and the precise alignment of components. As instruments become more complex and compact, the manufacture and assembly of components is increasingly challenging. We propose that a laser-based glass microfabrication technique known as ultrafast-laser assisted etching (ULAE) is ideally suited to the manufacture of both existing and novel components for astrophotonic instruments. To demonstrate this potential, we will present ULAE manufactured microlenses with integrated passive alignment features for efficient optical fiber coupling. A full physical and optical characterization of the micro-lenses is given. These components have applications in fiber-fed multi-object spectrographs.

In April 2018, VDL signed the contract to deliver more than 900 Primary Mirror Support frames for the 39-meter diameter ELT to ESO. Obviously, the M1 structure is a rather complex module. Because of the extreme output specifications (25 nanometer surface form accuracy of 133 slightly different mirror shapes and positions), as well as the required serviceability and reliability over decades of planned operation, the module truly is an engineering masterpiece. This appears to be the first time in large infrastructure-based astronomy history that a supply chain is set up to manage an order of this magnitude of highly complex M1 support structures. VDL together with its development partners TNO and NOVA has taken up this challenge.

The coating plant for the M1 primary mirror segments of the Extremely Large Telescope (ELT) currently under construction in Cerro Armazones, Chile by the European Southern Observatory (ESO) will deposit a highly reflective protected-silver-based coating by magnetron sputtering technology. This paper describes how AGC Glass Europe SA, through its business unit AGC Plasma Technology Solutions, realized the design and manufacture of a highly automated coater based on its experience in building large area architectural glass coaters. In addition, some results of pre-acceptance testing in Lauenförde, Germany are reported.

Freeform optics can enhance optical performance by reducing the number of elements, enabling lighter and more efficient systems, and by reducing aberrations. Most traditional manufacturing techniques cannot yield polished freeform optical surfaces. Likewise, traditional metrology equipment has difficulty accurately measuring the deviation of freeform surfaces from their nominal shape, the surface form error. The freeform manufacturing process requires control of both the surface form and the surface location simultaneously. The combination of both surface location and form error is referred to as total error. Inclusion of mechanical fiducials on freeform optics allows for precise locating of all surfaces of an optic in reference to one another and provide a reference from which the freeform surface can be measured against. Alignment fiducials enable more precise measurement of total error and can also aid in alignment of the optic in the final assembly and test.

Interferometer for figure of large convex hyperboloid mirrors contains concentric meniscus lens and wavefront analyzer for study the wavefront reflected from measured hyperbolic mirror. Concentric lens rotates around the imaginary focus of the tested mirror to measure full surface of the hyperbolic mirror. Main advantage of proposed interferometer is the possibility of figuring the shape of extremely large convex hyperbolic mirrors with a diameter more than 2 meter. In addition, the interferometer may be applied for measure the hyperbolic mirrors with a very large aperture angles in the imaginary geometric focus, for example with the aperture angle of 180° or even more.

The highly aspheric secondary mirror M2 of ESO’s Extremely Large Telescope (ELT) is the largest convex mirror ever polished. We report on manufacturing and error analysis of the high accuracy computer-generated hologram (CGH) used as part of the test concept for the M2 mirror. In order to comply with the required measurement accuracies in the single nanometer range (RMS), existing error sources along the entire process chain need to be considered. Available characterization methods for measurement of fabrication errors are described, as well as modelling of wavefront errors resulting from the CGH manufacturing process itself. Additionally, a general approach to improve the transmission of CGHs applying an effective multi-level patterning realized by binary sub-wavelength structures is introduced.

Computer Generated Holograms (CGHs) are powerful optical elements used in many fields, such as wavefront shaping, quality testing of complex optics and anti-counterfeiting devices. Lee algorithm is the most used to generate binary amplitude Fourier holograms. Grayscale CGHs are known to give a higher reconstruction quality than binary holograms, but they usually require a cumbersome production process. A very simple and straightforward method of manufacturing rewritable grayscale CGHs is here proposed by taking advantage of two key components: a Digital Micro-mirror Device (DMDs) and a photochromic plate. An innovative algorithm, named Island algorithm, able to generate grayscale amplitude Fourier CGHs, is reported and compared with the standard Lee approach, based on 9 levels. A crucial advantage lies on the fact that the increase or decrease of the quantification does not affect the spatial resolution. In other words, the new coding leads to a higher spatial resolution (for a given CGH size) and a reconstructed image with an order of magnitude higher contrast with respect to the classical Lee-coded hologram. In order to show the large potential of our approach, we have investigated the coding of a grayscale image with a 201 levels Island hologram: it shows a much better reconstruction (resolution, fidelity, contrast) thanks to the quantification of the transparency than the Lee algorithm commonly used. By combining 3 grayscale images with RGB filters and adding them, we show the reconstruction of a full color image with high fidelity, high contrast and low noise.

Low expansion glass materials have been widely applied to space optics due to their extremely low thermal expansion properties and outstanding mirror polishing characteristics. Meanwhile, the authors have focused on cordierite ceramics as promising candidate materials for space optics because they have higher specific stiffness than the conventional low expansion glass materials, having extremly low coefficient of thermal expansion. In the previous study, we evaluated the material properties of cordierite ceramics and established the key technology for manufacturing lightweight structures and polishing precise mirror surfaces. However, a reliable strength evaluation on a large number of specimens with various surface conditions was insufficient. Because measurement of breakage stress is often susceptible to the method of evaluation, strength characteristics of the brittle material need to be evaluated using many specimens of different surface conditions and the same test standard and methodology. In this research, ring-on-ring test, an ISO standard (ISO EN 1288- 5), was selected as the strength test standard for brittle materials. The Weibull distribution was used as the extreme value distribution for the breakage analysis. The strength distribution of two cordierite ceramics (NEXCERA™ CD107 and CO720) with various surface conditions generated in grinding, polishing, and sintering processes were evaluated and compared with a low thermal expansion glass-ceramic (ZERODUR®). These results showed the cordierite ceramics tend to have higher strength and Weibull parameter than the glass-ceramic under some surface conditions. This paper describes the effect of the surface condition of cordierite ceramics on bending strength.

Additive manufacturing technology is expected to be the third processing method following cutting and machining. In recent years, there has been an increasing demand for higher precision in the fields for optical equipment. However, there are still few reports of additive manufacturing technology using zero thermal expansion material. The authors report the results for the purpose of developing additive manufacturing technology for zero thermal expansion metal.

Thermal expansion characteristics of Fe₃₉Co₅₁Cr₁₀ based stainless invar alloys were investigated with the aim of developing a novel structural material used at cryogenic temperature for infrared instruments mounted on astronomical telescopes. Ni-added stainless invar type alloys Fe₃₉Co₄₉Cr₁₀Ni₂ were found to exhibit anomalous thermal expansion behavior in the low temperature range between 100 K and room temperature. According to the measured thermal expansion curves, these alloys expanded with increasing temperature from approximately 100 K to 200 K and contracted from 200 K to 300 K. The coefficients of thermal expansion in each temperature range were estimated to be approximately 0.93 x 10^-6 /K and −0.71 x 10^-6 /K, respectively. As the results of thermal expansion measurements of Cr based alloys, similar negative thermal expansion behavior was observed in the same temperature range between 200 K and 300 K. These findings suggest that the Cr element plays a key role in the negative thermal expansion in the Fe₃₉Co₄₉Cr₁₀Ni₂ alloys. We concluded that the dimensional change between 100 K and room temperature could be precisely controlled in the extremely low thermal expansion range by controlling the negative thermal expansion. The coefficient of thermal expansion between 100 K and 300 K was achieved to be approximately 0.13 x 10^-6 /K in the Fe_38.4Co_49.7Cr_10.1Ni_1.8 alloys developed in this study. As this value was close to that of the fused silica used for infrared instruments, the developed alloys can be a powerful tool for high precision observation in astronomical telescopes.

Earth imaging satellites in geostationary orbit (GEO) are proving to be of great public utility such as disaster prevention and damage mitigation by providing satellite images. JAXA has been conducting research and development of an optical space telescope that allows ground observations with a ground sampling distance less than 10 meters at nadir from GEO. These optical systems must have stable thermo-optical properties even in long-term operation. The authors have been evaluating the resistance of candidate materials to the space environment as part of their investigation into suitable materials for optical system mirrors. Cordierite ceramics have become promising candidate materials in addition to glass materials since their excellent physical properties, such as a high Young’s modulus, low bulk density, high thermal conductivity, and a low coefficient of thermal expansion (CTE). Cordierite ceramics have not been used for space optics, so how their optical properties change in a space environment needs to be evaluated. Cordierite ceramics have not been used for space optics, so how their optical properties change in a space environment needs to be evaluated. The thermooptical properties of mirrors may be degraded by prolonged exposure to the unique conditions in space, such as ultraviolet light, radiation, and atomic oxygen. The authors evaluated the changes in thermo-optical properties due to these environments for three candidate materials. Changes in optical parameters will be used to design optical system. It was confirmed that samples with the SiO2 protective coating showed no significant changes in their resistance to radiation and ultraviolet light, even after irradiation equivalent to twenty years of exposure to the space environment. The thermo-optical properties of silver coated samples without SiO2 protective coatings showed a significant degradation under atomic oxygen, which will be present in low-orbit applications. This is due to the surface formation of silver oxide.

Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (Ariel) has been adopted as ESA “Cosmic Vision” M4 mission, with launch scheduled for 2029. Ariel is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 μm, operating in cryogenic conditions in the range 40–50 K. Aluminum has been chosen as baseline material for the telescope mirrors substrate, with a metallic coating to enhance reflectivity and protect from oxidation and corrosion. As part of Phase B1, leading to SRR and eventually mission adoption, a protected silver coating with space heritage has been selected and will undergo a qualification process. A fundamental part of this process is assuring the integrity of the coating layer and performance compliance in terms of reflectivity at the telescope operating temperature. To this purpose, a set of flat sample disks have been cut and polished from the same baseline aluminum alloy as the telescope mirror substrates, and the selected protected silver coating has been applied to them by magnetron sputtering. The disks have then been subjected to a series of cryogenic temperature cycles to assess coating performance stability. This study presents the results of visual inspection, reflectivity measurements and atomic force microscopy (AFM) on the sample disks before and after the cryogenic cycles.

The excellent homogeneity of the coefficient of thermal expansion (CTE) is the most advantageous property of ZERODUR^®. Many publications show that the typical CTE homogeneity is in the one digit ppb/K range for parts up to 4.2 m in diameter. Among these evaluations, the CTE homogeneity of two 1.5 m diameter blanks has been characterized with 90 CTE samples evenly spread on 10 cm spatial scale in 2008. Both measurements showed a CTE homogeneity of better 10 ppb/K. In 2016 SCHOTT introduced an advanced dilatometer setup with an improved repeatability. Therefore the extensive CTE evaluation from 2008 was repeated in 2019. The larger statistical reproducibility variations of the earlier dilatometers led to an obscuration of the real homogeneity values, which had been much better. With Monte Carlo simulations this can be demonstrated to be strongest for the most homogeneous items. The results are provided in this paper. SCHOTT started to produce 798 ZERODUR^® mirror blanks for the primary mirror of the ELT M1, 39 m in diameter, in 2019. In January 2019 SCHOTT has already successfully finished and delivered the ZERODUR^® mirror blank for the secondary mirror (M2) 4.25 m in diameter of the extremely large telescope (ELT) of ESO, followed by a ZERODUR^® mirror blank for the tertiary mirror (M3) 4 m in diameter in January 2020. All ELT mirror blanks have stringent CTE homogeneity requirements. Therefore, it is a good time to give an update on the CTE homogeneity of ZERODUR^® in the ELT century.

Canon succeeded in practical application of InP immersion rating for the first time in the world (Sukegawa et al., 2016). We have developed specialized material for an immersion grating in transmission characteristics and obtained a material comparable to Silicon. The InP immersion grating produced by this material and excellent cutting technology is shorter in wavelength and more practical than the initial InP immersion element and can completely cover the range covered by the Silicon immersion grating. In addition to the already proven around 2.2μm (K-band) and around 1.6μm (H-band), an immersion diffraction element having an ideal grating shape can be provided even at around 1.25μm (J-band). Of course, it is possible to produce not only the immersion grating but also the grism. In this paper, we will also show our latest machining optical devices such as micro-mirrors of Integral Field Units having the same structure as a groove of diffraction grating.

An Èchelle-grating specifically developed for the space borne measurement of earth’s atmospheric CO₂-concentration is presented. It has a line density of ~170L/mm and is manufactured on a thick crystalline silicon substrate using electron beam lithography. The echelle profile is realized using a highly anisotropic chemical etching process, which stops on the <111> crystallographic planes of the crystal. The established manufacturing process allows perfect linear grating facets with negligible corner rounding of the profile. The important property of showing a low polarization sensitivity of the diffraction efficiency is achieved by a special design and technology for applying the gold coating of the grating which intentionally leaves one grating facet uncoated.

We present the manufacturing and optical verification of the germanium immersed grating for the L/M band high resolution spectrograph (LMS). The LMS is one of the science subsystems of the Mid-infrared ELT Imager and Spectrograph, METIS. The immersed grating has very demanding requirement specifications: <100 nm RMS transmitted Wave Front Error (WFE) after double pass, and >70% peak throughput in all orders within the 2.9-5.3 μm wavelength range over the pupil. The grating has a period of 18.2 μm, a sawtooth groove profile with 89.6 degrees apex angle and a grating area of 150mm x 60mm. The germanium immersed grating was produced by Canon’s high precision mechanical cutting technology. We present the interferometric tests that were performed in order to verify WFE and two different measurements (based on cascade laser and Fourier Transform Spectrometer, respectively) for throughput verification.

Variable filters, in which the bandpass central wavelength shifts along a dimension of the component, is a promising way to simplify and to miniaturize imaging spectrophotometer systems for spatial observation. In this paper, we present a new method for the fabrication of variable filters with different spatial profiles based on plasma assisted reactive magnetron sputtering. Variable filters are based on the variation of the thickness of the layers deposited on the substrate. The layers are manufactured using a Bühler HELIOS machine. To insure a non-uniformity of the deposited layers, adapted masks are placed in front of the targets for the low and high refractive index materials. The uniformity of these layers is measured via a homemade set-up allowing a local measurement of the transmission over the component surface. We demonstrate the fabrication of a narrow variable filter where all the thicknesses of all layers are changing, resulting in an overall shift of the filter.

We summarize the operational realities of re-aluminizing 8.4-meter primary mirrors in-situ on the Large Binocular Telescope. We review the evaporative coating system design, and summarize its performance in the 16 coatings since 2005. A mostly manual system with long-handled mops and traditional chemicals is used to remove the old coating and to clean the glass surface. After cleaning, the telescope is moved to horizon-pointing orientation and the aluminizing belljar is mounted to the primary mirror cell using the overhead crane internal to the enclosure. We report on the multi-year struggle to understand variations in deposition rate among the 28 crucibles that evaporate the aluminum. We describe the challenges of making operational improvements to a system that must reliably coat one of the two primary mirrors every year, and we report on some lessons learned along the way.

University of California Observatories has undertaken an effort to develop improved optical coatings for astronomy, with a particular emphasis on long-lived, highly efficient silver-based mirror coatings. We previously reported on a thin-film design including nickel oxide that showed significant improvement in accelerated aging tests, but had not yet undergone field testing. In this paper we briefly summarize our past work, and then discuss (a) two years’ field testing of our design with NiO; (b) efforts to find other materials that play the same role as NiO in improving durability; and (c) tests with ScF3 that show good results as a replacement for YF3/YbF3 but with improved optical properties. We also report on a long-term study of the effects of multiple cycles of stripping/re-coatings on various substrates, finding no significant substrate damage with aluminum, but some damage over time with our current protected-Ag coatings, particularly on borosilicate glass.

The Mid-infrared Imager and Spectrograph (METIS) is one of the first light instruments of the European Extremely Large Telescope (ELT). METIS optical design contains a first stage called common fore optics, which operates at cryogenic temperatures. Because of the azimuthal motions of the telescope, it integrates a field derotator, which has to run under vacuum and at 67 K. The French Alternative Energies and Atomic Energy Commission (CEA) is in charge of developing the cryogenic rotation stage, which actuates this field derotator. This is a kind of technological breakthrough as all the existing derotator systems operate outside the cryogenic vessels. This paper gives an overview of the derotator actuator design with the different trades that have been studied and some preliminary tests results.

Additive manufacturing (AM) methods and post processing techniques are promising methodologies considering that it is now possible to print in a wide variety of materials using processes much refined from those originally available twenty years ago. To date the uptake of AM in Astronomy is relatively low compared to other application areas, aviation being one such example. Due to the rapid progress made in additive manufacturing and the lack of its adoption in Astronomy, there are many opportunity to deploy new fabrication processes. In this paper, we outline the project and report the results of our investigation to make use of additive manufacturing and novel materials in the fabrication of multi-functional integrated components fit for use in astronomy instrumentation, which can operate in cryogenic environments and space application.

We recently started to investigate how liquid-crystal on silicon (LCOS) spatial light modulator (SLM) would perform as programmable focal-plane phase mask (FPM) coronagraphs. Such “adaptive coronagraphs” could potentially help adapt to observing conditions, but also tackle specific science cases (e.g. binary stars). Active FPMs may play a role in the context of segmented telescope pupils, or to implement synchronous coherent differential imaging (CDI). We present a status update on this work, notably early broadband contrast performance results using our new Swiss Wideband Active Testbed for High-contrast imaging (SWATCHi) facility. Finally, we unveil the upcoming near-infrared PLACID instrument, the Programmable Liquid-crystal Adaptive Coronagraphic Imager for the 4-m DAG observatory in Turkey, with a first light planned for the end of the year 2022.

High-resolution telescopic imaging is very important in astronomy. Super-resolution technology which breaks the diffraction limit of the imaging system can enhance the spatial resolution with compact setup and low cost. In this work, a novel super-resolution telescopic imaging method based on aperture modulation is proposed, and two different algorithms based on intensity extrapolation and image reconstruction for the recovery of super-resolution image are demonstrated respectively. With the help of aperture modulation, redundant information which contains high frequency components beyond the cut-off frequency of the imaging system is coded into the image sequence, and then they are extracted and used to reconstruct the super-resolution image by subsequent signal processing. Experimental results showed that the resolution was enhanced by 2.1 times for extended targets, and 3 times for point sources. Better performance is possible with the improvement of algorithm.

The present paper reports on the results of the validation test campaign of the ELT M1 Position Actuators (M1 PACTs). The main function of these actuators is positioning the 798 segments composing the primary mirror (M1) of the ELT with nanometric tracking error over the relatively large stroke of ten millimetres. In order to achieve this challenging goal the PACTs feature an hybrid actuation concept including a spindle drive for large and coarse movements in series with a piezo actuator for fine position adjustments. Active damping techniques are used to ensure robustness and benign dynamic response to perturbations transmitted through the supporting back structure. The design and manufacturing project of the M1 PACTs has recently entered in the final design phase. In this phase extensive validation testing is planned to verify that the final product is fit for purpose throughout its lifecycle. To this end the M1 PACT is first tested in stand-alone mode, with the objective of verifying its performances in a controlled and stable environment and deriving a reliable model of its dynamic response to be exploited for M1 performance estimation. Then several M1 PACTs are integrated in the M1 Test Stand, a small-scale mock-up of the primary mirror of the ELT. In this configuration multiple actuators are driven together to demonstrate relative position control between two segments. On the basis of the obtained results the paper discusses the expected M1 performance and possible control tuning strategies to be used during the commissioning of the ELT in Chile.

The desirability of making deployable mirrors for space telescopes has been known for years. Yet, once the wave lengths are longer than the short wave radio, progress has been slow. Therefore, we are exploring a new technology based on using a magnetic smart material film deposited on a shape-memory alloy (SMA) sheet. Our preliminary result is that when we apply an about 0.1-0.35 T in-plane magnetic field, we can produce greater than micron deflections in the SMA. Furthermore, we have shown the SMA can return to shape to within 1 micron. We will present an update on how long these deflections can be maintained and our progress toward making the SMA surface smooth enough to produce specular reflection. In addition, we assess the brightness and shininess of the NiTi piece before and after heating that is needed to produce a return to shape.

The Support Structure for the Thirty Meter Telescope (TMT) Infrared Imaging Spectrograph (IRIS) consists of 18 carbonfiber reinforced polymer (CFRP) struts, a CFRP ring and a metal interface frame. This ultra-stiff, lightweight structure suspends the five-ton IRIS Science Cryostat and Rotator below the Narrow Field Infrared Adaptive Optics System (NFIRAOS). Through comprehensive design and analysis driven by requirements for stiffness, optical alignment, adjustability, manufacturability, weight and space, much headway was made to bring this design to fruition. This work presents the current state of design, including material down-selection, adjuster design and strategies for fabrication, alignment and testing. It summarizes methodologies and simulation results examining stiffness, seismic and thermal loads and transmission of vibration between NFIRAOS and IRIS. A prototype strut is being developed and will undergo dynamic mechanical testing to characterize its performance.

We report a new route to mitigate focal-ratio degradation (FRD) in multimode optical fiber links. Our approach uses a custom multicore fiber (MCF) with dissimilar cores, which is then tapered at both ends to form multimode cores and create mode-selective photonic lantern (PL) transitions. The cores of the MCF are single-moded at 1550 nm and sufficiently spaced such that there is no observable core-to-core cross talk after a 7 m fiber length. The mode-selective PLs at each end of the MCF, in combinations with the low core-to-core cross talk of the MCF at 1550 nm, ensure that the relative amount of power in each spatial mode is preserved throughout the fibre link. As such, a PL-MCF-PL link using this approach has the potential to be resistant to FRD by inhibiting the coupling of light from lower-order to higher-order modes. We show that the full PL-MCF-PL link exhibits lower FRD than a custom multimode fiber that guides the same number of modes. A study of how the FRD behavior of the PL-MCF-PL link varies as a function of wavelength indicates some bandwidth limitations due to the wavelength-dependent properties of the PLs and the cross coupling in the MCF.

The applicability of optical fibers to astronomical spectroscopy in the infrared depends heavily on the fiber's wavelength-dependent transmission properties. Standard silica fibers typically allow transmission at wavelengths as red as 1550nm but absorb strongly at redder wavelengths, limiting their use for the infrared instruments. Fibers using fluoride glasses transmit redward of this range, opening up the infrared K band, corresponding to a window of transmission in the Earth's atmosphere. In order to further assess their feasibility for astronomical instrumentation, the characteristics of these fibers must be well-known. Here initial measurements and analysis of the focal ratio degradation (FRD) of two types of fluoride fibers are reported.

Atmospheric emission limits observation with ground-based telescopes in the NIR. Those emission lines are very narrow in bandwidth and thus enable for filtering them with fiber Bragg gratings to utilize the light passing in-between. We present a scheme for the compact inscription of multiple notch filters with a semi-aperiodic fiber Bragg grating. Herein, we utilize the interference of neighboring grating sections of a phase mask which then produces the desired resonances inside a fiber. Our current design addresses up to ten wavelengths which are matched with an accuracy better than 175pm.

We present a method of assembling a fiber-optic pseudo-slit, inside a custom FC connector. 19 SMFs with 80 μm cladding diameters are arranged in a 1,511 μm pseudoslit, held in the center of a connector ferrule. The SMFs in the pseudo-slit are well positioned and well ordered, having an average core separation in the ‘long’ direction of 79.5 μm and an StDev in the ‘narrow’ direction of 2.68 μm. The nearfield output distribution of the pseudo-slit was measured under 615-730 nm light, finding an FWHM intensity distribution ratio between the two directions of 1 : 21.9. This method could be used with other types of optical connector, allowing pseudo-slits to be used conveniently with existing optical instruments.

The Multi-Core Integral-Field Unit (MCIFU) is a diffraction-limited near-infrared integral-field spectrograph designed to detect and characterise exoplanets and disks in combination with extreme adaptive optics (xAO) instruments. It has been developed by an extended consortium as an experimental path finder for medium resolution spectroscopic upgrades for xAO systems. To allow it to achieve its goals we manufactured a fibre link system composed of a custom integrated fiber, with 3D printed microlenses and an ultrafast laser inscribed reformatter. Here we detail the specific requirements of the fibre link, from its design parameters, through its manufacture the laboratory performance and discuss upgrades for the future.

This paper presents the design and performance of an easily scalable Theta/Phi fiber positioner for Multi-subject Spectrographs. It has been developed by Ecole Polytechnique Federale de Lausanne (EPFL) together with the Swiss manufacturer of miniature microsystems MPS Microsystems. The positioner has been designed with the purpose of adapting readily to the requirements of any Multi-Object Spectrograph, which requires precise positioning of optical fibers. Whether MOONS, COBRA, MEGARA, or DESI, each project has so far developed its own fiber positioner from scratch, with all of the risks linked to such a complete new development. The positioner presented in the current paper intends to significantly reduce the development time, costs, and eliminate unforeseen risks. The design is easily scalable in size, length, and shape. For validation, we built eight prototypes with a diameter of 9.5mm which are capable to reach any target within five micrometers with just two correction moves.

The Extremely Large Telescopes (ELTs) are promising a profound transformation of our understanding of the universe through large-scale surveys of a myriad of previously unseen astronomical objects across cosmic space-time. For such surveys, the ELTs will need highly multiplexed spectrographs that can super-efficiently target multiple objects at a variety of spatial scales across broad spectral ranges. This can be enabled by (or at least can benefit from) a dedicated device that streamlines this “mix and match” process between different spatial samples at the telescope focus and different kinds of back-end spectrographs by splitting wavelengths to appropriate channels, scrambling beams to maximum spectral PSF stability, and switching between fibers or fiber-integral-field-units. The Beam-Switch Module (BSM), being developed for the VIRUS2 instrument at the McDonald Observatory, is one possible blueprint of such a system and this paper details its design and recent prototype results.

MOONS (Multi-Object Optical and Near-infrared Spectrograph) is a third-generation visible and near-infrared spectrograph for the ESO Very Large Telescope currently under construction. The instrument’s spectroscopic capabilities are multiplexed via a fibre positioning module (FPM) which configures the location of 1001 fibres. The fibre positioning units (FPUs) have been jointly developed by the UK Astronomy Technology Centre (UKATC) and MPS Microsystems (MPS) to optimise instrument efficiency by providing excellent transmission and an open-loop positioning strategy, allowing a tightly packed focal plane to be rapidly reconfigured. The mechanism geometry enables all positions in the focal plane to be observed in conjunction with a companion sky fibre at close separation. A description of the as manufactured design and production process of the FPUs is presented, along with a discussion of the performance proven to date, including achievement of the critical pupil alignment and positional repeatability requirements. An overview of the custom testing rig built to automate the characterisation and calibration process is also presented.

Absolute metrology techniques have been proposed for and used in a variety of optical systems as a means of precisely measuring distances. Typically the metrology concepts utilize frequency stabilized multiple wavelength lasers. We describe a multi-wavelength approach that utilizes the strengths of Photonic Integrated Circuits (PIC) to implement an absolute metrology sensor. The metrology sensor is a self-contained system that forms a multi-wavelength distance measuring interferometer. The PIC includes the waveguide circuitry, laser source, detectors and a National Institute of Standards and Technology (NIST) traceable reference in a compact form. The PIC is a hybrid combination of Indium Phosphide circuitry with active elements (detectors and laser) and a passive Silicon Nitride circuit of waveguides. The PIC implementation is presented along with measurement results showing an unambiguous range of 30 mm with nanometer precision.

Current high-resolution spectrometers have been designed for seeing-limited sources. Designing a spectrometer for diffraction-limited sources makes it possible to significantly improves its compacity and cost, but it also opens up new concepts, including better efficiency, and adaptability to various spectral domains, and up to very high resolution (several 10^5).  A novel, near-IR,  R~80000 spectrometer has been developed at IPAG to characterize two sources at once in the H or K bands. Its design is based on a virtually imaged phased array instead of an échelle grating, which allows the spectrometer to fit inside a 0.2m3 cryostat, and results in a gain in throughput with respect to usual échelle spectrographs. One specific science case that can benefit from this new type of design is the characterization of exoplanets' atmosphere. This paper presents the results of its test in the laboratory, as well as the preparation for an on-sky demonstration tentatively scheduled for summer 2020.

With the recent advancement of ultra-high-precision micro-optics fabrication technologies, it is now possible to fabricate IFUs based on machined image slicers with slicer width of 30 um or less. We have developed a machined image slicer IFU (MISI) to replace the BiFOIS IFU of the DL-NIRSP Instrument, one of the first light instrument for the DKIST Telescope. Overall, the IFU contains a total of 336 micro mirrors and a common collimator. The large number of micro optical elements of MISI present a great challenge to the optical and mechanical design process of the IFU. Typical design method that model and position each optical element manually would result in a tedious and lengthy mechanical design process. This paper describes the CAD design process of MISI using Solidworks macros that were developed to automatically generate all the optical surfaces based on the ZEMAX model data output and also simulating the tool shape and path to generate a precise model of the IFU.

We demonstrate a key step along a technical route to achieving cm/s scale accuracy for astronomical spectrographs over long (multi-year) time scales, which is critical for the Doppler characterization of earth sized exoplanets, and measurement of small cosmic redshift drift over many years. This same technique is also en- abling for searching exoplanet atmospheres for biosignficant molecules in direct planet imaging using otherwise insufficiently low resolution and drift prone dispersive (grating or prism) spectrographs. Using a new method called crossfading" for externally dispersed interferometers (EDI) to get highly robust spectra, we recently demonstrated a factor of 1000x reduction in the net shift of an EDI measured ThAr line to a deliberate simulated wavelength translation of the detector. This 1000x gain in disperser stability can be combined with conventional stability gains afforded by fiber scramblers, vacuum tanks, and thermal control, to provide an additional 1 to 3 orders of magnitude reduction in the net PSF shift drift. Crossfading combines high and low delay fringing signals that react oppositely in phase to cancel their net reaction to a detector wavelength drift. This can be implemented by an interferometer addition to a facility spectrograph.

The 60-meters submillimeter telescope is a large astronomical facility under proposed with the advantages of high sensitivity and large field of view, which mainly planned to observe the low temperature gas and dust in the universe. The aperture of primary reflector is 60 meters, and the requirement of surface error is 30μm rms which enables it to observe in the wavelength of 0.65mm to 3mm. In order to achieve the high accuracy requirements of telescope, the primary reflector is design to be composed of CFRP segments in one of the conceptual design schemes, similar to Large Millimeter Telescope (LMT). The natural limit of surface accuracy and pointing accuracy of telescope will be improved after the application of CFRP with the advantages of high specific stiffness and low thermal expansion, etc. This paper describes the initial development of a 3 meters prototype CFRP reflector segment, mainly including the design process of support truss and manufacture status of panel and truss.

The surface quality of replicated CFRP mirrors is ideally expected to be as good as the mandrel from which they are manufactured. In practice, a number of factors produce surface imperfections in the final mirrors at different scales. To understand where this errors come from, and develop improvements to the manufacturing process accordingly, a wide range of metrology techniques and quality control methods must be adopted. Mechanical and optical instruments are employed to characterise glass mandrels and CFRP replicas at different spatial frequency ranges. Modal analysis is used to identify large scale aberrations, complemented with a spectral analysis at medium and small scales. It is seen that astigmatism is the dominant aberration in the CFRP replicas. On the medium and small scales, we have observed that fiber print-through and surface roughness can be improved significantly by an extra resin layer over the replica's surface, but still some residual irregularities are present.

In the manufacturing process of Carbon Fiber Reinforced Polymer (CFRP) mirrors (replicated from a mandrel) the orientation of the unidirectional carbon fiber layers (layup) has a direct in influence on different aspects of the final product, like its general (large scale) shape and local deformations. In particular, optical methods used to evaluate the surface's quality, can reveal the presence of print-through, a very common issue in CFPR manufacture. In practical terms, the surface's irregularities induced, among other artifacts, by print-through, produce unwanted scattering effects, which are usually mitigated applying extra layers of different materials to the surface. Since one of the main goals of CFPR mirrors is to decrease the final weight of the whole mirror system, adding more material goes in the opposite direction of that. For this reason a different layup method is being developed with the goal of decreasing print-through and improving sphericity while maintaining mechanical qualities and without the addition of extra material in the process.

The space mission SVOM is dedicated to the detection of gamma ray bursts attributed to the explosion of massive stars or the merger of neutron stars. Gamma-ray bursts are a powerful tool to probe the Universe and to study physics in extreme conditions. Inside of the satellite, MXT is a X-ray telescope which contributes to localize such events. MXT camera is enclosed in an aluminum shielding which is supported by three small support feet made of titanium alloy. Although the supports mounted in the satellite will be built using conventional techniques, CEA/Irfu has worked in parallel on several ‘concurrent’ designs based on structural optimization. They have been drafted and fabricated by additive manufacturing. The main input constraints are related to space environment: low temperature, limited thermal conduction, limited mass, resistance to high vibration levels corresponding to the takeoff of the mission. In this paper, the optimization process and the numerical tools involved are described. Feedback is provided on the modeling, drafting and manufacturing steps, including additive manufacturing and conventional re-machining. Finally, the experimental vibration tests are described and the results are compared with numerical finite elements models in order to qualify the components similarly to the nominal parts.

A possible answer to the challenge brought by the construction of the next generation 40 m-class telescopes was the case study of FAME (Freeform Active Mirror Experiment). As the new instruments typically increased in both volume and complexity, the use of highly aspherical freeform surfaces could be a great solution as these systems are built up from fewer parts and can achieve higher performance. The idea of FAME was to create a thin face sheet which is then deformed to the nominal shape utilizing actuators mounted on the back of the mirror and acting parallel to the optical surface. The test phase of the FAME prototype revealed the complexity of the design and its sensitivity to manufacturing and assembly processes. As part of the characterization it was very difficult to predict correctly how the system behaves which is due to the several interfaces between the actuators and the face sheet. These experiences led to the development of a new structure that eliminates the strict tolerance chain obtained from a variety of components mounted on one another. It also means that the design for conventional manufacturing technologies should be left behind, and additive manufacturing must be introduced. This paper gives a brief overview how the lessons learned from the previous development is matched with the new design approach of the same component using topology optimization, additive manufacturing of metals and post processing of 3D printed parts. This work is funded under the OPTICON H2020 INFRAIA-2016-2017/H2020-INFRAIA-2016-1 Grant Agreement 730890.

Based on a new concept to build flexible structures by metallic 3D printing that has been developed at CSEM, several compliant mechanisms have been redesigned for Additive Manufacturing (AM). In addition to the new geometric possibilities offered by AM, the needs for machining and assembly after printing are drastically reduced. Support structures under flexure blades are thus minimized and the overall process becomes more streamlined. Moreover, this idea allows us to advantageously design and produce monolithic cross blade flexure pivots with interlocked flexures. Thanks to this concept, CSEM is now developing and testing new architectures of Compliant Mechanisms based on Additive Manufacturing (COMAM) for the European Space Agency (ESA) in the frame of a GSTP research project.

Optical materials are key components for scientific instruments in space missions. SCHOTT is providing optical materials for space applications since many years. The most current example are radiation resistant optical glasses for the OSIRISRex Camera Suite OCAMS. SCHOTT optical glasses offers high homogeneity and transmittance even for extreme optical positions to enable lowest wavefront distortions in scientific instruments. Optical materials have to withstand harsh environment, many kinds of different radiation and extreme temperature changes. Radiation resistant glasses do not change transmittance properties by gamma or electron radiation. Measurements on chalcogenide glass (IRG), used as lens material for infrared applications from part of the visible to infrared, indicate a very high radiation resistance. Lead containing radiation shielding glass used for nuclear power plants can be used as a window to protect sensitive optics in harsh environments of space applications. This paper gives an overview of the different materials and aspects.

This paper outlines an overview of the mechanical design of the support structure and optical bench, for ELT (Extremely Large Telescope) class of instrument utilizing standard materials. It also provides a presentation of a possible strategy to lighten this mechanical design. The mass requirements for the new class of astronomical instruments, for the Extremely Large Telescope, put tight constraints for the mechanical design of said instruments. While aluminum or steel are still the main materials that are usually employed for main support structures, carbon fiber reinforced polymer (CFRP) offers a wide range of promising possibilities. The interesting properties of carbon fibers make composite materials reinforced with carbon fibers ideal for numerous applications with high mechanical performance and low weight requirements. Carbon fibers have high strength and low weight (high strength-to-weight ratio). Furthermore, carbon fibers have a high modulus of elasticity and low thermal expansion, they are corrosion resistant and their density is lower than that of aluminum: thus, they are ideal for "light applications". In this paper two possible mechanical designs for a "large optical bench", based on different materials, are presented and compared: structural steel and carbon fiber reinforced polymer (CFRP). The models are validated through FEM Analysis approach, utilizing CAD and CAE software.

We report on an expanded catalog of total reflectance measurements of various common (and uncommon) materials used in the construction and/or baffling of optical systems. Total reflectance is measured over a broad wavelength range (250 nm < λ <2500 nm) that is applicable to ultraviolet, visible, and near-infrared instrumentation.

Cordierite is a ceramic with low thermal expansion properties similar to Zerodure and ULE glasses in large-mirror telescopes used in Astronomy. Coefficient of thermal expansion of Cordierite is not only comparable to low thermal expansion glasses, but also has additional advantages with minimal dimensional deformation with time (~3.8nm/m/year), high Young's modulus (~145GPa), and high thermal conductivity (~4W/mK). However, Cordierite mirror is susceptible to voids generated by crushed grains leading to tetrahedral gaps from sphere packing of raw powder material during CIP (Cold Isostatic Pressing) process. These voids often appear with mirror polishing of Cordierite and may become a source of light scattering. To solve this problem, we propose Flexible Molding (Fmolding) as our new fabrication method for Cordierite. With F-molding, raw material powder is converted to liquid slurry which is then poured into a resin mold and solidified directly into near net shape of a mirror. Liquid slurry of Fmolding are thus cured in forming without pressurized sphere packing which generates void defects. In addition, the mold prepared from F-molding is an effective tool for processing multiple volumes of identical structures and is an ideal process for segmented mirror designs. To confirm the feasibility of F-molding for large mirrors used in Astronomy, we prepared a light-weighted flat mirror (270 mm in diameter) using the method with our Cordierite. In this paper, we introduce details of our F-molding mirror fabrication method, and report on measurement results of its optical performances.

We are developing an image-slicer type integral field unit (IFU), SWIMS-IFU, for SWIMS (Simultaneous-color Wide-field Infrared Multi-object Spectrograph), a near-infrared instrument for TAO 6.5 m telescope. SWIMS- IFU divides a field-of-view of 16:006 12:008 into 26 slices with a width of 0:005, which is the largest FoV among near-infrared IFUs on 8 m class telescopes. It is also capable of obtaining entire near-infrared spectra from 0.9 to 2.5 m with R1000 with a single exposure. Because of limitations of space in SWIMS, SWIMS-IFU should fit in a volume of 170 x 220 x 60 mm3, which results in small and complicatedly aligned mirror facets. To reduce alignment procedures, we adopt an ultra-precision cutting technique to fabricate mirror arrays monolithically. We have completed one of the mirror arrays, the slit-mirror array which consists of 26 spherical mirror facets, and confirmed both their surface roughness and shape errors satisfy the requirements. We also have fabricated a prototype of the pupil-mirror array including some elliptical mirror facets and confirmed that the elliptical mirrors have enough surface qualities and produce better image quality than spherical ones by a pinhole imaging test.

The Thirty Meter Telescope (TMT) is a joint venture of scientific institutions in Canada, China, India, Japan and the United States to build a 30 m diameter Ritchey-Chretien optical-infrared telescope. The large aperture size and field of view, with its adaptive optics (AO) systems will enable imaging at the diffraction limit, precision astrometry, high contrast imaging and spectroscopy from the ultraviolet through the mid-infrared. Its 30 m primary mirror consists of 492 off-axis aspheric segmented hexagonal mirrors, each having a diameter of 1.44 m across corners. The manufacture of the segments is distributed amongst India, USA, China and Japan. KiwiStar Optics (business unit of Callaghan Innovation), New Zealand manufactured a reference sphere for India TMT Co-Ordination Centre (ITCC). This paper presents polishing, testing procedures and metrology challenges of a 61.7 m long radius of curvature reference sphere. The radius of curvature and surface figure interferometric testing error budget is discussed in detail.

NIRPS (Near Infra Red Planet Searcher) is a near-infrared, fiber-fed, high-resolution, high precision radial velocity (pRV) spectrograph to be installed at ESO 3.6m telescope in La Silla Observatory in Chile. High precision radial velocity measurements require to have a very stable optical assembly. The gluing of optical elements in their mounts with A12 epoxy was selected as bonding process to minimize kinematic motion and optimize stability. However, coefficient of thermal expansion (CTE) mismatch between optical elements, their mountings and the glue may produce large local mechanical stress. Finite element analysis (FEA) was performed to estimate the thermal stress at room temperature and cryogenic temperature (80K). The selection of suitable bonding parameters (gluing setup, glue thickness, etc.) was a challenge given the CTE difference of optical elements (ZnSe and ZerodurTM) and holding flexures (SS304 and InvarTM). Extensive tests were performed to find a suitable bonding strategy. Gluing samples were tested under cryogenic temperature during several weeks. Mechanical shear stress tests were also performed to show that glued assembly could survive a 12g vertical load.

High-resolution spectropolarimetry is a technique used to study many astronomical objects including stellar magnetic fields. It has mainly been used on ground for optical and, more recently, infrared (IR) observations. Space mission projects including ultra-violet (UV) high-resolution spectropolarimetry, such as Pollux onboard LUVOIR proposed to NASA, are being studied in Europe under CNES leadership. Bringing a spectropolarimeter into space means that the instrument should be prepared for space environment including temperatures. The UV polarimeter we are considering is composed by a rotating modulator and an analyzer. Both components are made of magnesium fluoride (MgF₂). The modulator is a rotating block of waveplates in molecular adhesion, each plate having its own fast axis. The analyzer is a Wollaston prism, also made with molecular adhesion. MgF₂ being birefringent, the plates and prism are anisotropic and will dilate and retract due to thermal changes differently along their fast and slow axes. Each plate having its own fast axis, the thermal changes will create stress at the interfaces, i.e. at the molecular adhesion between the plates. This study focuses on the most critical part: the plates of the modulator. To demonstrate the resistance of the modulator and increase its technological readiness level (TRL), an optical bench including interferometry has been set at the Paris Observatory. It allows us to observe in real time the state of the molecular adhesion between plates as they are submitted to thermal changes in a vacuum chamber. Additional samples have been tested in a thermal vacuum chamber at CNES. This article describes the modulator using molecular adhesion, the test experiments, and the conclusion of this thermal study. Although molecular adhesion broke in 2 samples during thermal cycling, most samples survived which provides encouraging results for this technique.

HARMONI is the ELT first light visible and near-infrared (0.45 to 2.45 μm) integral field spectrograph over a range of resolving power from R~3500 to R~18000. It will provide 4 different spatial scales from 4*4mas to 30*60mas. 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. This paper depicts the final design of the Integral Field Unit in charge of splitting, slicing and rearranging the rectangular field of view into 4 long slits (~540mm) to feed the 4 spectrographs. The manufacturing of sensitive optics such as the field splitter or the image slicer are detailed. The mechanical and alignment design strategy is explained as well as the design of some dedicated integration tools, developed for cryogenic environment. The performances of the full unit are deeply discussed in terms of image quality, straylight analysis, tolerancing.

HARMONI is the first light visible and near-infrared (NIR) integral field spectrograph for the Extremely Large Telescope(ELT). The HARMONI spectrograph will have four near-infrared cameras and two visible, both with seven lenses of various materials and diameters ranging from 286 to 152 mm. The lens mounts design has been optimized for each lens material to compensate for thermal stresses and maintain lens alignment at the operational temperature of 130 K. We discuss their design and mounting concept, as well as assembly and verification steps. We show initial results from two prototypes and outline improvements in the mounting procedures to reach tighter lens alignments. To conclude, we present a description of our future work to measure the decentering of the lenses when cooled down and settled.

The 4-meter Multi-Object Spectroscopic Telescope (4MOST) is a wide-field, high-multiplex spectroscopic survey facility under development for the Visible and Infrared Survey Telescope for Astronomy (VISTA) of the European Southern Observatory (ESO). The 4MOST Wide Field Corrector and Atmospheric Dispersion Corrector (ADC) lenses have been manufactured and tested at KiwiStar Optics, New Zealand. The two ADC wedged doublets form a Risley Prism Pair; are both 650 mm in diameter and relatively thin; and have been cemented using Sylgard 184 silicone elastomer. The cementing procedure and its challenges are reported here. Interferometric measurements of the single surfaces, glass blanks and the finished doublets have been compared in order to assess the effect of the cementing process on the final surface figures of the doublets before and after positioning in their cells.

The segmented pupil experiment for exoplanet detection (SPEED) facility aims to improve knowledge and insight into various areas required for gearing up high-contrast imaging instruments adapted to the unprecedented high angular resolution and complexity of the forthcoming extremely large telescopes (ELTs). SPEED combines an ELT simulator, cophasing optics, wavefront control and shaping with a multi-deformable mirror (DM) system, and optimized small inner-working angle (IWA) coronagraphy. The fundamental objective of the SPEED setup is to demonstrate deep contrast into a dark hole optimized for small field of view and very small IWA, adapted to the hunt of exoplanets in the habitable zone around late-type stars. SPEED is designed to implement an optimized small IWA coronagraph: the phase-induced amplitude apodization complex mask coronagraph (PIAACMC). The PIAACMC consists in a multi-zone phase-shifting focal plane mask (FPM) and two apodization mirrors (PIAA-M1 and PIAA-M2), with strong manufacturing specifications. Recently, a first-generation prototype of a PIAACMC optimized for the SPEED facility has been designed and manufactured. The manufacturing components exhibit high optical quality that meets specifications. In this paper, we present how these components have been characterized by a metrological instrument, an interferential microscope, and then we show what is yielded from this characterization for the FPM and the mirrors. Eventually, we discuss the results and the perspectives of the implementation of the PIAACMC components on the SPEED setup.

In order to study distant galaxies and extra-solar planets, image slicers are now widely used in most integral field spectrographs (IFSes). Nonetheless, their multiple small tilted surfaces make them particularly difficult to manufacture, but equipment such as that of AOFI (Advanced Optical Fabrication Infrastructure) of University Laval eases this process. In this regard, the AOFI team has been tasked with the fabrication and characterization of the image slicer of the GIRMOS (Gemini Infra-Red Multi-Object Spectrograph) instrument. However, first attempts to produce small slicer prototypes have shown an issue with a monolithic diamond turning approach: manufacturing time. To address this matter and minimize tool wear while achieving good surface quality and optical performance, we have developed a unique manufacturing procedure, based on aluminum mechanical sub assemblies. This paper discusses this strategy and the metrology tests that were then applied to the image slicer of GIRMOS, including the analyses of surface roughness and other parameters such as tilts, curvature and active area.

JSC LZOS applies various mechanical and optical methods to test optics at all stages of figuring, from milling and grinding to polishing stage. Computer Generated Holograms (CGH) are successfully applied at the polishing stage for the following purposes: testing of large-size optical components, on- and off-axis surfaces; alignment with respect to interferometer; defining of an aspherical surface optical vertex position with respect to the geometrical center for optical systems alignment. All this allows manufacturing of large-size aspherical on- and off-axis astronomical and space mirrors with high accuracy.

A measurement platform including a Shack-Hartmann Wavefront Sensor (SHWFS) has been designed, integrated and tested at Imagine Optic, Orsay France and delivered to the Rubin Observatory (previously known as the LSST). This instrument will be used for the initial on-axis optical alignment and testing of the LSST telescope. The optical configuration of the Rubin Telescope without the presence of the main LSST camera requires advanced capabilities in term of quantities of aberrations to measure, linearity, accuracy and sensitivity for usage on a natural star. The HASO 128 GE 2, available from Imagine Optic, was identified as the most relevant WFS to meet all those requirements. In that paper, we provide details about the simulations that led to the choice of the HASO 128 GE 2. We also provide the specifics regarding the opto-mechanical design allowing the relay imaging of the pupil compatible with the fast aperture of the telescope, a reference source for the on-site calibration of the system and the optical output made available for a viewer camera.

The ASTEROID project is a H2020-COMPET EU project whose main goal is to provide Europe with the capability to manufacture high performance infrared focal plane arrays (FPA) devoted to scientific and astronomical space and ground telescope missions. The european consortium is composed by key research institutions (CEA-Leti, CEA-Saclay and IFAE) and industrial partners (Lynred, EVG and ADDL) being the resulting detector a SWIR hybridized MCT (mercury-cadmium-telluride) FPA of 2k x 2k pixels and 15 μm of pixel pitch. The project also looks for the validation of a thermo-mechanical model for large FPAs by reducing the stress build up in the ROIC and wafer bonded structures, both resulting in better detector reliability. On this framework, a cryo-vacuum system has been developed at the IFAE mechanical workshop with capabilities to perform low temperature thermal cycles in the MCT detector range and up to ~50K. The large volume cryostat (~120 liters) is cooled by a helium compressor and a single stage cold head, hosting the detector and its preamplification stage to provide a suitable light path and radiation shield for thermal background control. Most of the inner light path parts has been developed using off-the-shelf optomechanical components which allow us to modify the incoming light performance with standard 1 or 2-inches optical components. The cryostat design, configuration and the whole system performance will be reported on this paper with the aim to test the hybridized FPA to assess the reliability at operating temperature after several thermal cycles..

In the area of space astronomy, some electronic detectors should be cooled to liquid helium temperature level to improve their sensitivity and reduce the background noise. A hybrid J-T cooler has been developed for future space application by our laboratory. The J-T loop is precooled by two-stage thermally coupled pulse tube cooler. There is no moving part at low temperature in this system which features low vibration electromagnetic interference. The hybrid J-T cooler has been experimental tested and cooling capacity of about 34.5mW@4.32K is achieved when supply pressure is 1.97MPa. Besides, when sufficient precooling power is provided for the J-T loop cooling capacity of 102.3mW at 5.02K is achieved. In this experiment, three oil free linear compressors are used to drive the J-T loop and a GM cooler is used to provide the precooling power for the J-T cooler. This J-T cooler will be the potential cryocooler for the future space detectors requiring cooling power of 100mW at < 6K.

We undertake to build a new cross dispersed infrared spectrograph, whose design is a heritage of IGRINS, will be deployed at Gemini Telescope as facility instrument (IGRINS II). The Sunpower Cryotel GT cryocooler may be replaced with the current GM cooler in the future. The only drawback of using a stirling cryocooler is vibration and this may have a significant influence to astronomical instrument by decreasing the signal to noise ratio. On this test the induced vibration in all three axes were measured simultaneously on the external cyrostats bottom and internal cold stage linked to coldhead by a flexible thermal strap. The cryocooler was integrated with a compliant mount of the neoprene rubber or spring. The test was carried out with both the tuned vibration absorber (TVA) damper and active vibration cancellation (AVC) unit, when the temperature of cold stage went down to 70 K.

We present an update of the design of the Central Wheel Mechanism (CWM), a subsystem of the MICADO instrument for the Extremely Large Telescope (ELT)¹. The CWM consists of a Filter Wheel Mechanism (FWM) with two Filter wheels, the Pupil Wheel Mechanism (PWM) with one Pupil wheel and the Atmospheric Distortion Corrector (ADC). The CWM has a diameter of about 1.5 meter. Challenges in the mechanical design work are focused on fitting all the components in the available design volume and ensuring that the accuracy and repeatability requirements are met. The mechanisms should work in a cryogenic environment for an intended lifetime of 10 years and survive transport and seismic conditions. Additionally the system has to cool and warm-up properly. The most noticeable update since the PDR² is a self-contained ring assembly of a connected FWM and PWM that is suspended to the MICADO instrument using flexible supports. The arrangement of these supports creates a point of shrinkage at the optical position which is off-axis from the cryostat center. An alignment tool will guarantee a position accuracy within 50 μm for both the ADC as well as the Pupil masks that are mounted after the system is integrated into the instrument. The most sensitive bearings are protected from shock loads by a system that limits their load carrying capability by transferring the excess load to bumpers. These bumpers do not affect the repeatability of the system.

This paper presents recent advancements in the study of a new concept of space spectropolarimeter. The instrument, based on a triple prismatic structure from a birefringent material, avoids the need of rotating components and may cover the entire Stokes vector for a very broad wavelength band. The analysis of the concept in noise-free conditions has proved its consistency. Further simulations for different geometries of the modulator are presented. The results allow identifying critical values for the main parameters. Moreover, the previous analysis of the instrument in noisy conditions is completed with the impact study of the spectral resolution, dimension of pixels and signal to noise ratio..

Optical design of the wide field of view telescope is limited by obscuration of the secondary mirror in onaxis system and by serious linear astigmatism in conventional common off-axis system. We have developed an innovative off-axis reflective system in which the optical design is based on the confocal off-axis to completely compensate the linear astigmatism. The main objective of this paper is to compare alignment sensitivity of the confocal system to those of on-axis and common off-axis systems. All three optical designs are based on the classical Cassegrain reflecting telescope and have identical entrance pupil diameter of 1000 mm and F/8 focal ratio. Tilt and decenter of each optical component, and despace which indicates inter-mirror distance are considered as tolerancing parameters that are explored within fabrication tolerance ranges. Encircled energy diameter is evaluated as a criterion of the analysis while tilts of the secondary mirror and the focal position are set to the compensator. The statistical tolerancing method based on Monte-Carlo simulation is also performed to analyze system tolerances. From sensitivity analysis and Monte-Carlo simulation, we concluded that the confocal off-axis system is more sensitive than on-axis and common off-axis systems but it is a feasible system in terms of fabrication and alignment errors.

Direct detection of faint exoplanets is challenging due to a high-contrast ratio between a primary star and a planet. A high-contrast imaging system has an important role in directly detecting exoplanets. The system consists of coronagraph and speckle reduction technique. A common-path visible nulling coronagraph (VNC) is one of the attracting methods for high-contrast observation because of a simple optical configuration and achromatic stellar elimination. We introduced a spatial light modulator (SLM) into the common-path VNC to suppress residual speckles caused by wavefront aberrations. The SLM can potentially generate a huge dark hole thanks to its large pixel format. As a focal plane wavefront sensor, we utilized the self-coherent camera (SCC) method for the common-path VNC. We carried out the laboratory demonstration of the speckle reduction technique for the common-path VNC combined with the SCC methods. The experimental results show an initial contrast of 2.2 × 10^-5 and a final contrast of 1.3 × 10^-6 in monochromatic light at a wavelength of 633 nm. We discuss the limiting factors of the contrast for improvement of our demonstration to achieve the higher contrast.

Volume Phase Holographic Grating (VPHG) technology provides efficient dispersion solutions for astronomical spectroscopy, but its fabrication and parameter control remain in the realm of nonlinearity. More accurate and repeatable control can be achieved using micro-lithographic patterning processes and reactive-ion plasma etching (RIE), to fabricate surface relief gratings, free from surface defects and completely embedded into the substrate material. Such gratings promise potential for low/medium resolution spectroscopy applications not only in usual ground-based spectrographs, but in those subject to much harsher conditions (e.g. space-based, cryogenic, high-power laser instruments). When combined with nano-textured anti-reflective surfaces using the same fabrication technology, the relief gratings can serve astronomical spectroscopy needs that may be sub-optimally met or not feasible with VPHGs. We detail prototype micro-lithographic grating fabrication results in this paper.

Lithium fluoride (LiF) is difficult to work with because of its hygroscopic nature (it pulls water out of air). The stability limits of LiF thin films and the nature of their failure when exposed to humid air are poorly understood. We show that LiF films undergo irreversible changes in optical properties and microstructure as determined by ellipsometry and SEM when exposed to dew points greater than 6 C. On the other hand, samples stored at a dew point of -22 C (4% RH at room temperature), showed only small changes in ellipsometric parameters. The ones stored at intermediate humidity 6 C (21% RH at room temperature) showed larger changes in ellipsometric parameters. SEM shows that deliquescence as well as efflorescence is important in LiF thin films. In situ spectroscopic ellipsometric measurements using a controlled variable humidity environment illuminates the changes in LiF thin films moving from moisture absorption to complete deliquescence.

The H2020 project PULSAR (Prototype for an Ultra Large Structure Assembly Robot) development objective is to create three demonstrators that that will pave the way for the construction of large structures in orbit. The study case considered in PULSAR is the assembly of a segmented primary mirror for next generation 35m space-based telescope. In the frame of this project, CSEM is developing Single Mirror Tile demonstrators (SMT) that host a positioning mechanism capable of adjusting the position of the hexagonal mirror in order to compensate for inaccuracies generated by the robotic assembly. This mechanism has a tripod architecture. It is composed of three linear actuators and of transmission stages, each made of a flexible pivot and of a gimbal. This mechanism allows for controlling the mirror position along three degrees of freedom (piston translation and tip and tilt rotations). The piston stroke is required to be ±3 mm with a resolution of 1 μm and a repeatability better than 5 μm while the tip/tilt strokes are ±1° with 4 μrad of resolution and 20 μrad of repeatability. The design of the tiles benefits from CSEM extensive experience in compliant mechanisms and additive manufacturing applied to the domain of scientific instrumentations for space applications.

We provide an update on our ongoing technology advancement and new mission development for on-orbit deposition of optical coatings. This technology leverages NASA’s Wake Shield Facility heritage (STS 60, 69 and 80, achieving a final TRL of 8) to create coatings technology that can be directly applied to substrates to achieve the required performance for the next generation of space telescopes. The USAF has recently funded this technology to fabricate optical coatings on-orbit for its future systems. We will discuss this funded effort including progress in ongoing ground testing and the on-orbit demonstration mission scheduled for 2023-2024.

High contrast imaging at small inner working angles can be achieved using a vector vortex coronagraph in the focal plane of the telescope providing a helical phase ramp with a singularity at its center. The form birefringence of full-diamond subwavelength gratings has proven to be well suited to manufacture such vortex phase masks for coronagraphic applications (Subwavelength Grating Vortex Coronagraph, SGVC). In the past years our group has developed and manufactured SGVCs of topological charge 2 (Annular Groove Phase Mask, AGPM) made of a concentric diamond subwavelength grating. For future applications including ELT-class telescopes in the near- to mid-infrared that will partly resolve nearby stars, it is however useful to increase the topological charge of the vortex. After shortly reviewing our previous attempts at optimizing the grating structure for SGVC of charge 4, we present the first laboratory results obtained with such devices. We then introduce and discuss more realistic simulations compared to prior studies using finite-difference time-domain methods. The quality of the simulation results obtained with the open source software MEEP for an AGPM is shown to be appropriate for developing and assessing the performance of future vortex phase masks. We therefore perform updated simulations for SGVC of charge 4 including various designs with straight and curved grating lines. We conclude with a perspective on the potential of metasurfaces and their applications to design novel vortex coronagraphs based on subwavelength structures.

The crossed-sine wavefront sensor (WFS), which is based on a gradient transmission filter (GTF) and a 2´2 minilens array (MLA), enables simultaneous achromatic imaging at a high spatial resolution. The GTF is formed by four circular sub-apertures with the crossed-sine pattern for spatial filtering, producing four initial images. The 2´2 MLA, which is placed on the image plane of the optical system, is adjusted to transfer the image of the exit pupil onto the detector array. In this paper, we present the design of the GTF and MLA, and some preliminary experimental tests of the crossed-sine WFS system. For the first prototype, a spatial light modulator (SLM) is utilized to simulate the GTF functions, and an achromat is applied to transfers the pupil images instead of the MLA. The pupil image and the wavefront error have been obtained. This crossed-sine WFS imaging system is user-friendly and will serve a large range of high throughput applications in astronomy, biomedicine and metrology.

With the great development of solar telescope, astronomers are always trying to achieve higher guiding precision, which is one of the most important technologies for the solar observation. At present, the most popular solar guiding depends on the imaging mapping, correlation procession and capture the solar image by CCD directly. In this paper, a new full disk solar guiding method based on post-focus lens obstruction are proposed. A simple structural modification has been used to improve our solar guiding measurement and even for other extended sources. It has been proved that our new method is correct and feasible after building a simulated experiment system and running lots of experiment tests.

Aluminum is the best choice of material for broadband mirrors. However, once an oxide layer forms on the surface of the mirrors the reflectance in the far ultraviolet range decreases. The study of Al mirrors is difficult because they oxidize so quickly in the air. This makes reproducibility and joint work between laboratories difficult because the mirrors will oxidize and make successive measurements inaccurate. We have found that storing aluminum thin-film mirrors in low oxygen environments (such as liquid nitrogen, dry ice, and hexane) retards mirror oxidation. We examined the retardation of the growth of aluminum oxide during storage in these environments. This oxidation retardation was most pronounced when mirrors were stored in liquid nitrogen. In comparing the growth rate of oxide out of storage to that while it was in storage, we found that the apparent growth of aluminum oxide, is 1/500 in liquid nitrogen, 1/200 in hexane and 1/40 in dry ice.

This paper provides a description of the mechanical design for the single panel of the MAORY thermal enclosure. The thermal enclosure ensures the internal environment temperature stability for improved optical performance. The panel is composed of layers of different materials, with a cooling coil inside of it. The materials and their thicknesses are chosen so that the internal environment is insulated from the outside, while the inner surface of the panel shows a uniform temperature distribution, for improved thermal stability. The paper focuses on the mechanical structure design and analyses of the panel. Particular attention is given to the interface with the electrical harness that needs to reach the inside of the enclosure.

Substrate material is important while manufacturing of large-dimensioned optical mirrors for ground and space applications. The properties of Astrositall, its dimensional stability, design features are considered. Various types of supporting cells and optical components bases are used in the process of surfaces figuring. Applying of membranepneumatic support to base the mirror in the course of working surface testing is considered. However, in some cases it is necessary to support the mirror basing it on glued interface elements. In such case, surface figure difference between standard and manufacturing support cell, which is taken into account during figuring, is estimated via modeling.

The ELT, Europe’s Extremely Large Telescope, with its 39m main mirror will be the largest optical/infrared telescope in the world, able to work at the diffraction limit. METIS is one of its first light instruments with powerful imaging and spectroscopic capabilities in the thermal wavelengths. It contains several high contrast imaging (HCI) modes, which allow it to detect and characterize exoplanets amongst others. The HCI performance is highly dependent on pupil stabilization mechanisms and a closed loop compensation of non-common path aberrations degrading the wavefront error of the instrument. The Talbot effect is a near-field effect on collimated light, where spatial frequencies of the wavefront are re-imaged periodically along the optical path. The periodicity is known as the Talbot length, which is a function of the wavelength and the wavefront’s spatial frequencies with the latter being a result of the wavefront errors caused by the surface form errors of optical elements. The aberrations oscillate from amplitude to phase, in the spatial scale of one Talbot length, which can have an impact on the performance of the HCI modes. We evaluate the impact of the Talbot effect with respect to the METIS phase aberration budget by assuming representative power spectral density profile for the surface form error of each optical surface. We propagate the errors to the subsequent pupil plane and finally investigate the resulting point spread function profile. Simulations are fed back into the HCI error budget and if necessary, the specifications regarding instrument surface form are adjusted.

An afocal meniscus lens is proposed for reducing intrinsic astigmatism in an RC telescope. In comparison to existing solutions using thin aspheric plates near focus, the proposed solution does not disrupt the aplanatic correction of the initial RC design but reduces astigmatism and eliminates field curvature. Despite some residual lateral color introduced by the lens corrector, the field of view in the original RC design can be extended by a factor of 2 while maintaining the diffraction-limited image quality in the system.

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 will have a center segment surrounded by six off-axis segments, plus an additional off-axis segment to allow continuous operation as segments are removed for coating. The second off-axis segment was completed and accepted in 2019. We are currently polishing the optical surface of off-axis Segment 3. We have completed work on the rear surfaces of Segment 4, the center segment, and offaxis Segment 5. We are assembling the mold for off-axis Segment 6 with the spin-casting scheduled for March 2021.

ELT-HIRES (High Resolution Spectrograph for the Extremely Large Telescope) is a multifold fiber-fed spectrograph planned for the Nasmyth B focal station on ELT, covering a wavelength range between 0.4 and 1.8 μ. One of the most relevant science cases is represented by the detection of life signature in the extrasolar atmospheres for Earth-like planets. The polarimetric unit, feeding the UBVRI and the zYJH spectrograph modules, will consist of two subassemblies: the main one, aimed to be installed in the intermediate focus, will host the polarizer (a double Wollaston calcite prism) and the retarder plates splitting the optical beam into the four Stokes vector components; the other will be located in one of the four dedicated arms of the Front End on the Nasmyth platform, providing the atmospheric dispersion correction, field stabilization and selection of the operating modes before the fiber injection. At the conclusion of Phase A, we have presented this as the optimal design solution fulfilling the top level requirement of reaching a sensitivity of 10-5 S/I (S being equivalent to one of the Stokes vectors), a condition achievable only if the polarizers are installed in a rotationally symmetric focus. The current work, which can be considered a continuation of two preceding papers, illustrates new simulation results from the development of the polarization-ray-tracing tool. We have included also the crystal anisotropic properties of the polarizing components, analyzing the transmission loss that the light beam encounters propagating through the optical train before entering the fiber bundles in the different operating modes, bearing in mind the polarimetric aberrations induced by the active compensation of the primary mirror and the loads contributing as Zernike polynomials of different orders, namely the telescope cross-talk.