Over its more than thirty-year history, the Advanced Technologies and Instrumentation (ATI) program has provided grants to support technology development for ground-based astronomy. Research from this program has advanced adaptive optics, high resolution and multi-object spectroscopy, optical interferometry and synoptic surveys, to name just a few. Previous and ongoing scientific advances span the entire field of astronomy, from studies of the Sun to the distant universe. Through a combination of literature assessment and individual case studies, we present a survey of ATI funded research for optical-infrared astronomy. We find that technology development unfolds over a time period that is longer than an individual grant. A longitudinal perspective shows that substantial scientific gains have resulted from investments in technology.

This contribution is focused on the innovative aspects of the design of the Laser Guide Star (LGS) Facility for the Gran Telescopio Canarias (GTC) Adaptive Optics (GTCAO) System [6]. After a trade-off process considering different alternatives, a preliminary opto-mechanical design was defined, based on a “TOPTICA SodiumStar” laser to be launched on-axis. To maximize throughput, different novelties around the optical, and mechanical design of the Laser Launch System, including the Laser Head, the Beam Transfer Optics and the Launch Telescope are emphasized in this paper. In particular, all the elements of the Laser Launch System have been compacted to be placed at the backside envelope of the GTC M2 mechatronics. To fit in that envelope the thermal enclosure of the Laser Head had to be redefined to avoid mechanical interferences and science beam vignetting. An innovative closed-loop Laser Head cooling approach was defined to be also arranged at the backside of GTC M2. Performance simulations running in parallel to the on-axis LGS design could not determine any difference in performance between the on-axis and the off-axis launch. Hence, considering the higher packaging and maintenance complexity required by the on-axis launch, GTC decided to define the off-axis configuration as the new baseline approach. All the solutions already defined for the on-axis approach that were applicable to the new off-axis baseline were reused. To reduce the cost of future upgrades, the LGS design allows generating and launching several LGS with just one launch telescope splitting the light from the Laser Head. In parallel with keeping the volume of the facility to a minimum, an effort to keep its maintenance as simple as possible has been also made to avoid the impact on the telescope operational costs.

The power of the next generation of telescopes that will rely largely on the combination of light-collecting area with excellent (ideally: diffraction limited) image quality. Therefore, the focus will heavily lean on adaptive optics and the near infrared wavelength regime. A severe limiting factor is the presence and strength of atmospheric OH emission lines in the NIR. OH suppression techniques involving fiber Bragg gratings (FBG) have been proposed, however as yet not fully demonstrated on sky. We are involved in the first generation FBG prototype development with partners in Australia, including the GNOSIS and PRAXIS on-sky experiments. Since the supply of suitable multi-notch filters is no longer available from industry, we have made an effort at innoFSPEC Potsdam to build a specialized laboratory for the development and manufacture of 2nd generation FBGs for OH suppression. Suppression of the strong NIR OH emission lines requires a single grating that reflects multiple wavelengths, spaced at non-periodic intervals, with flat-top profile and high suppression ratio. It has been shown that aperiodic fiber Bragg gratings (AFBGs) can provide such functions. However, the fabrication technology requires accurate optical alignment of several degrees of freedom as well as complex control of modulated beams to form a varying interference pattern. In our work, an algorithm is developed from the index profile of a multi-notch AFBG to the design of a complex phase-mask that can generate a matching UV diffraction pattern, which will in turn inscribe an single-mode fiber into the chosen AFBG. With such a phase mask, the fabrication of the AFBGs will be reduced to a simple UV-exposure process, i.e., the complex alignment and control processes of the interference pattern from modulated beams are avoided altogether. The resulting reliable and reproducible fabrication process will dramatically reduce of the cost of such filters. Packaging aspects for a complete sky emission filter system will also be discussed.

WEAVE is a new wide-field multi-object spectroscopy (MOS) facility proposed for the prime focus of the 4.2m William Herschel Telescope (WHT), situated on the island of La Palma, Canary Islands, Spain. To allow for the compensation of the effects of temperature-induced and gravity-induced image degradation, the WEAVE prime focus assembly will be translated along the telescope optical axis. The assembly comprises the prime focus corrector (PFC), a central mount for the corrector known as FTS^[1], an instrument rotator and a twin-focal-plane fibre positioner. SENER, that manufactured and delivered the FTS, is also responsible for the final design, manufacturing, integration, alignment and testing of the PFC and its ancillary equipment. This manuscript describes the final design of the PFC along with the analyses and simulations performed and presents the procedures for the integration and alignment of the lenses in the corrector.

The Fast-Steering Secondary Mirror (FSM) of Giant Magellan Telescope (GMT) consists of seven 1.1m diameter segments with effective diameter of 3.2m. Each segment is held by three axial supports and a central lateral support with a vacuum system for pressure compensation. Both on-axis and off-axis mirror segments are optimized under various design considerations. Each FSM segment contains a tip-tilt capability for guiding to attenuate telescope wind shake and mount control jitter. The design of the FSM mirror and support system configuration was optimized using finite element analyses and optical performance analyses. The design of the mirror cell assembly will be performed including sub-assembly parts consisting of axial supports, lateral support, breakaway mechanism, seismic restraints, and pressure seal. . In this paper, the mechanical results and optical performance results are addressed for the optimized FSM mirror and mirror cell assembly, the design considerations are addressed, and performance prediction results are discussed in detail with respect to the specifications

With more rapid, affordable access to space and the emerging availability of large-volume fairings, owners and users of current and future space-based optical systems are desiring large-aperture or segmented-aperture primary mirrors for their missions. This demand is driving the need for new approaches to optical component fabrication to produce mirrors and mirror segments that are more cost-efficient with faster manufacturing lead times than traditional optical components. Harris Corporation is executing a mirror development strategy called Advanced Mirror Construction (AMC) to meet this need while still meeting the challenging requirements of space-based optics. A key component of this strategy is the utilization of replication to produce precision lightweight mirror components. We present the motivation and initial results for lightweight replicated, ultra-stable mirrors and mirror segments as well as other key elements of the AMC strategy.

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 the inception of the program, 47 tasks have been awarded under the auspices of the NASA Physics of the Cosmos (PCOS) Program in the areas of optics and detectors as well as lasers, electronics, and micro-thruster subsystems. In addition, 31 tasks have been awarded under the auspices of the NASA Cosmic Origins (COR) Program to develop optics, coatings, cooling subsystems, and detectors from the Far-IR to the Far-UV. We present the PCOS/COR portfolio distribution in terms of specific technology areas addressed and show an analysis of the rate and cost of TRL advancements. We present highlights of the infusion success stories that have emerged from the SAT maturation program as it relates to enabling future NASA astrophysics strategic missions. Finally, we present an outlook for future technology priorities for investment by the SAT Program.

Segmented mirror technology is essential for large telescopes. Actually, the world largest telescopes such as Keck I and II, and GTC employ segmented mirrors for the primary mirrors, and all the planning extremely large telescopes (TMT, E-ELT, and GMT) also plan to use the segmented mirrors although the size of one mirror is far from each other. Needless to say, the technology of segmented mirrors realizes not only telescopes with the large apertures, which cannot be fabricated with a single dish, but also lightens the total weight of the telescopes. In addition, it enables the manufacture of the mirrors with a relatively small equipment and can simplify the mechanical structure of the telescope, resulting in saving the total cost for development. However, there are native difficulties and issues for the segmented mirrors technology. First, the segmented mirrors must be aligned to each other in the positions and angles with an accuracy of several tens of nanometers and arcsec, and it must be stabile for a long time by the active control as for TMT, the very complicated technique and the human resources are needed for precise control of as many as 492 mirrors. Secondly, individual polishing process of segmented mirrors produces, unavoidable sagging at the outer circumference of the mirrors. Because these local surface deformation cannot be compensated by the alignment of mirrors, the PSF by the telescope is strongly affected by the remaining wave front error, which is the higher order. Finally, the ambient thermal radiation leaked t from the gaps between segment mirrors , which becomes a background noise source, significantly degrades the sensitivity of the instrument, especially near-IR or mid-IR instruments, compared with a single dish telescope. These problems arises from the fact that each segmented mirrors are independently supported with the gaps. Therefore, we have been developing Alignment-Free Gapless Segmented Mirror (AFGSM), which is mechanically fixed to each other without any gaps between mirrors, as if it were a monolithic type of mirror. Specifically, after fan-shaped or similar segment mirrors are manufactured individually, they are mechanically fixed to each other by fastening, fusing or bonding. Since the Orthogonality or parallelism of the mirror surface from its reference surface is finished with a machine precision of 1 um or less, the assembly can be mechanically carried out with a well accuracy. Polishing to obtain a highly precise surface for use as a telescope, can be to the extent of corrected (re-)polishing. ^ In order to realize such a segmented mirror, we selected ceramics cordierite CO720 as a material, which is a type of fine ceramic developed by Kyocera. The cordierite CO720 has a coefficient of thermal expansion (CTE): <0±0.02ppmK - 1 for 23 deg., which is comparable to that of low thermal expansion glass such as Clearceram, Zerodur, or ULE. Since it can be easily polished, it is suitable for the base material of a mirror for a telescope application. Indeed, as a material comparable to low thermal expansion glass such as Zerodur, cordierite CO720 is widely used in cutting-edge technology of semiconductor manufacturing equipment which requires extremely high accuracy. Cordierite has a stiffness (E / ρ) of about 1.5 times that of general low thermal expansion glass material. With AFGSM, it is necessary to fasten / bond / join segments to each other. Cordierite CO720 is more suitable for gapless mirrors because the mirror surface is less likely to be deformed compared to glass against the stress generated during assembly, thanks to its high stiffness characteristics. Also, the high flexibility of formability of cordierite is also important in order to realize a gapless segmented mirror. In order to minimize the alignment error, the two contacting surfaces must ideally be processed as perpendicularity angles to the mirror reference. In addition, if there is a gap on the contacting surface, deformation will occur due to the tightening force at the time of fastening, so flatness also becomes important. Further, its high flexibility of formability of cordierite is an advantage also for providing a honeycomb structure for weight reduction. In ordinary glass materials, a honeycomb structure is realized by grinding from the bulk state, so that much processing time is required. On the other hand, in the case of cordierite ceramics, machining can be carried out at the stage of a soft molded body such as chalk after raw material forming and before firing. Therefore, costly grinding can be performed quickly and easily, which contribute overall cost reduction. Finally, the long term dimensional stability of cordierite is also suitable for general telescope mirrors. Low thermal expansion glass is cooled for a very long time in order to release the internal stress in the material during molding process. Therefore, the molded material takes a long time to deform. For example, in the case of Zerodur, deformation value 129 nm/m/1 year is expected as according to our research with PTB. In case of cordierite, deformation after firing is extremely small (< 3.8nm/ m/ year) compared to the low thermal expansion glass and it is possible to maintain stable dimensions over a long period of time. In order to demonstrate the feasibility of Gapless mirrors, in this study we made a prototype of Dia.300mm parabolic mirror sample. As a result of the structural analysis for fixing the mirrors, the shape of the segmented mirror was a fan shape with an internal angle of 60 degrees and it was divided into six parts. Triangle parts (three parts) was one of the concepts for this trial, but distortion at mirror surface coming from assembling each individual mirrors tend to be created easily, and therefore, we rejected this three parts option. Regarding the honeycomb structure, there are many cases where honeycomb designs of the same shape are used over the entire surface such as triangle and hexagon in the case of a monolithic mirror. Segmented mirror used for this evaluation, however, has a relatively small diameter of 300 mm, and it was confirmed by simulation that sufficient rigidity of cordierite material can be obtained without providing a complicated honeycomb structure. For this reason, we prioritize the workability for bolting in the horizontal direction on the contacting surface of the mirror, and an assembling structure was selected that can easily perform the assembly process. In addition, the thickness of the rib is made thicker than the other parts so that the contacting surface does not deform even by tightening with a bolt. It is confirmed that this structure does not lose practicality even if the mirror up to 1 m in diameter is scaled up. Individual segmented mirrors are manufactured independently from forming, green machining, firing and grinding, which is a standard ceramic manufacturing process, and then six segment mirrors are fastened with bolts with precision. After aligning the height of all mirror surfaces by grinding process, the polishing process is performed at the end in order to remove any height gap at the mirror surface. As a result, it was confirmed that sagging hardly occurred at the joint area, and ideal mirror surface condition was achieved measured by interferometer. Furthermore, in order to quantitatively investigate the occurrence of deviation of each segment after assembly, a flat surface sample was also prepared. After checking the surface accuracy using the laser interferometer for the polished flat mirror, we confirmed that there was no significant segment mirror misalignment by thermal cycle test and vibration test. In this presentation, in addition to the design results of the prototype AFGSM, the optical performance of the parabolic mirror obtained as a result of the polishing process, and the results of the environmental test are reported. We also discuss applicability of ceramic segment mirrors for space telescopes and some other observation devices.

Optimization techniques are powerful tools for producing lightweight structures with the maximum structural stiffness. They allow an optimized design to be produced directly for a given structure and, in this way, save considerable time in the design phase of a structure by avoiding multiple iterations between the definition of the design under computer-aided design (CAD) and the verification of the performance under finite-element (FE) analysis. There are three classes of optimization: size optimization, shape optimization and topology optimization. The topology optimization technique aims to find an optimal distribution of material given boundary conditions, i.e. the fixing points and the external loads. It starts from an initial volume representing a blank of the structure and removes the most unused material to meet the objective of mass reduction. In optomechanical engineering, this technique is met in the design of lightweight mirrors and especially in the design of their core-cell shapes. To provide reliable and useable results, this technique requires a fine and regular mesh of the mirror as well as a postprocessing of the results by the mechanical engineers. These constraints, combined with the necessity of using 3-D models, contribute an increase in the computation time and complicate the meshing. We propose here an innovative approach to this design problem by using topography optimization instead of topology optimization. Topography optimization, also named bead optimization, is a branch of the shape optimization and consists in introducing beads to a surface in order to increase its structural stiffness. The main advantage of this technique is that shell models can be used instead of solid models, easing the meshing operation and decreasing the number of degrees of freedom in the FE model, and thereby reducing computation cost. This paper presents an example of the application of this technique to the design of the primary mirror panel of the GCT (Gamma-ray Cherenkov Telescope), a dual-mirror 4-meter telescope proposed for the future Cherenkov Telescope Array. FE models and optimizations are made with MD.Patran and MD.Nastran respectively.

Large optic fabrication is a delicate and time consuming process. Obtaining a large prime optic is often in the critical path of a project and poses a serious risk to both the schedule and budget. The Optical Engineering and Fabrication Facility (OEFF) at the College of Optical Sciences, the University of Arizona, has developed a new way of optimizing its large optic fabrication process for maximum efficiency in convergence. The new process optimization takes the amount of stock material removal, tool characteristics, metrology uncertainty, optic prescription, optic material properties, and resource availability as input parameters and provides an optimized process along with an achievable convergence. This paper presents technical details of the new process optimization and demonstrates performance on 6.5m mirror fabrication at the University of Arizona. Two case studies for an 8.4m GMT off-axis primary mirror segment and a 3.1m TMT convex secondary mirror fabrication are also presented.

A manufacturing technology of mirrors for free-space optical communication is presented: a thin layer of Nickel is deposited on a master and bonded on a light-weighted structure by adhesive. After separation, the master is ready for another cycle. The process is cost-effective because only the master needs to be of optical quality. The structure is machined by traditional tooling, with figure errors compensated by the adhesive. Its curing time defines the process throughput to one mirror per day per master. Several 200 mm-aperture Ritchey-Chrétien telescopes have been manufactured and tested.

We present two methods for generating novel hyper-crossing tool paths for use with CNC and robotic sub-aperture polishing techniques. One generation method utilizes an optimization based on a Voronoi diagram. The second method is seeded by a unicursal pseudo-random pattern to generate a hyper-crossing tool path which features many self-crossings over the entire part surface. Each instance of generation for a given surface for either algorithm will result in a new hyper-crossing pattern. Tool paths can be generated for any surface area including those with interior holes. We also present results of an experiment using a hyper-crossing tool path to remove diamond turning marks.

Systems like TESS require specialized components that challenge all involved. These systems consist of many sub-components, but we are focused on the refractive and reflective optical components. One purpose of this talk is to introduce optical system designers to application specific manufacturing processes. We, as manufacturers, need to tailor our processes for the optic’s specific operational environment. In addition, we want to introduce some of our more unique manufacturing capabilities to allow system designers to widen their design space. It is now feasible to manufacture a wide range of sizes, shapes, and materials for many different applications.

We present a manufacturing process based on the deterministic polishing and figuring of thin glass substrates (shells), with thickness ranging from 0.5 up to a few millimeters, in “stand-alone” configuration or as part of a composite sandwich structure with low-density core. The methods will be well suited for a broad range of applications in astronomy like, e.g., the production of thin substrates for adaptive optics, the realization of sandwiched lightweight segments for the mirrors of future ground-based and space telescopes, and the realization of low-cost mirrors for amateur astronomy telescopes. The method foresees the pre-shaping of thin glass substrates via hot slumping technology followed by highprecision form correction of the optical surfaces via computer-controlled bonnet and ion beam figuring technologies. During the phase of bonnet polishing of a shell in stand-alone configuration, a removable holder stiffens it temporarily. This paper describes the main steps of the process under study and reports on the realization of the first prototypes.

Additive manufacturing (AM), more commonly known as 3D printing, is a commercially established technology for rapid prototyping and fabrication of bespoke intricate parts. To date, research quality mirror prototypes are being trialled using additive manufacturing, where a high quality reflective surface is created in a post-processing step. One advantage of additive manufacturing for mirror fabrication is the ease to lightweight the structure: the design is no longer confined by traditional machining (mill, drill and lathe) and optimised/innovative structures can be used. The end applications of lightweight AM mirrors are broad; the motivation behind this research is low mass mirrors for space-based astronomical or Earth Observation imaging. An example of a potential application could be within nano-satellites, where volume and mass limits are critical. The research presented in this paper highlights the early stage experimental development in AM mirrors and the future innovative designs which could be applied using AM.

The surface roughness on a diamond-turned AM aluminium (AlSi₁₀Mg) mirror is presented which demonstrates the ability to achieve an average roughness of ~3.6nm root mean square (RMS) measured over a 3 x 3 grid. A Fourier transform of the roughness data is shown which deconvolves the roughness into contributions from the diamond-turning tooling and the AM build layers. In addition, two nickel phosphorus (NiP) coated AlSi₁₀Mg AM mirrors are compared in terms of surface form error; one mirror has a generic sandwich lightweight design at 44% the mass of a solid equivalent, prior to coating and the second mirror was lightweighted further using the finite element analysis tool topology optimisation. The surface form error indicates an improvement in peak-to-valley (PV) from 323nm to 204nm and in RMS from 83nm to 31nm for the generic and optimised lightweighting respectively while demonstrating a weight reduction between the samples of 18%. The paper concludes with a discussion of the breadth of AM design that could be applied to mirror lightweighting in the future, in particular, topology optimisation, tessellating polyhedrons and Voronoi cells are presented.

WEAVE is the new wide field multi-object and integral field survey facility for the prime focus of the 4.2 m William Herschel Telescope. It is located at the Observatorio Roque de los Muchachos, in La Palma, Canary Islands, Spain. WEAVE fiber-fed spectrograph offers two resolutions, R ~ 5000 and 20,000. It has a collimator mirror and two cameras optimized for the wavelength intervals of 366 - 959 nm and 579 - 959 nm, respectively. One of the responsibilities of INAOE within the WEAVE collaboration is the polishing of the collimator mirror, made of OHARA CLEARCERAM®- Z HS. The collimator has a diameter of 660 mm, a central thickness of 44.7 mm and a weight of 56.8 kg. The main specifications are: 2 fringes irregularity in a clear aperture of 624 mm diameter and a radius of curvature of 1224.65 mm +/- 0.15. In this work, we present the polishing process and final results for the collimator. In particular, we describe the tools developed for its manufacturing, the modifications carried out to the conventional polishing machine to support the glass. Additionally, the interferometric optical irregularity measurements are presented. The collimator polishing process is finished fulfilling all the optical specifications.

The project "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 overview and initial development results from our Adaptive Optics-photonic test bed, Ultrafast Laser Inscribed waveguides for interferometric beam combination and 3D printing structures for astronomical instrumentation. The project is expected to lead to important technological breakthroughs facilitating uniquely functionality and technical solutions for the next generation of instrumentation.

COLIBRI is one of the two robotic ground follow-up telescopes for the SVOM (Space Variable Object Monitor) mission dedicated to the study of gamma-ray bursts, allowing determination of precise celestial coordinates of the detected bursts. COLIBRI telescope is a two-mirror Ritchey-Chrétien telescope whose concave primary and convex secondary mirrors have diameters of 1325mm and 485mm respectively. The mirrors are currently manufactured at LAM (Laboratoire d’Astrophysique de Marseille). In this article, the advancement of the work is presented. We also give a global overview and status of the COLIBRI project.

High-precision aspheric components are widely used in modern systems with the capability of high image quality, but an high-slope convex aspheric aspheric surface is more challenging to fabricate because of its more complex shape and measuring difficulty compared with other surfaces. As the traditional aspheric surface manufacturing will cause many problems like the tools cannot conform to the local varying curvatures of high slope convex aspheric optics. In this paper, a high efficient approach include CNC generating, MRF fine polishing and multi-wavelength interferometer measurement to get a high-precision convex aspheric surface is presented. A kind of flexible tools was designed to polish the surface and correct MHSF errors on the aspheric surface which can always conform to the surfaceduring the process. And the pressure needed during the polishing process is simulated by COMSOL software. A high-precision convex aspheric surface is successfully obtained and the final surface RMS is better than λ/30.

Cordierite ceramics offer several interesting advantages over traditional glass materials for ultra-lightweight and thermally-stable space optics. The authors have conducted a research and development activity to establish a technology for manufacturing lightweight cordierite mirrors for next-generation space telescopes.

In this activity, a 0.7-m spherical cordierite mirror was manufactured as a mock-up. The mirror substrate was formed to a lightweight rib structure using a near net shape forming process. The rib pattern was designed assuming a hexagonalshaped mirror segment, and the original outer shape was circular for an easy polishing process before outline machining. The weight was 21.3 kg with the original circular shape, but will be reduced to 12.3 kg (at areal density of 38.5 kg/m²) by processing it into the hexagonal outline. The surface figure accuracy was 154 nm RMS (root mean square). As the outer shape will then be processed to become hexagonal, there was no need to further improve the accuracy.

The prototype of a 0.3-m aspherical cordierite mirror was also manufactured with more difficulty. This mirror was parabolic in shape, assuming the primary mirror of a Cassegrain telescope, and its substrate was further reduced in weight. Thus, a weight of 1.91 kg was achieved (at areal density of 24.6 kg/m²), and the surface figure accuracy reached 35 nm RMS.

As a result of these prototyping efforts, cordierite mirrors are expected to be applied in space optics that require extremely high observation performance, combined with large-size, ultra-lightweight, and high-precision features.

ZERODUR is utilized in many ultra-precision applications such as IC and LCD Lithography, High-end Metrology, Aviation and space born or ground based Astronomy. Glass ceramics exhibit an extremely low coefficient of thermal expansion (CTE) together with an excellent homogeneity of CTE. Currently, the material demand of those ZERODUR applications is continuously growing. In parallel, CNC machined parts are getting more and more complex and require more advanced machining capabilities. This paper reports on the actions SCHOTT is taking to fulfill the increasing demand on high precision material and components. SCHOTT increased capacity along the entire value stream of ZERODUR production, starting from melting up through CNC machining. New CNC machines, together with process improvements, establish a new set of capabilities for up to four-meter-class substrates. Results are reported, demonstrating tighter tolerance on mirror surface figure together with reduced sub surface damage for accelerated polishing. The new equipment will enable SCHOTT to lightweight 4 m class mirror substrates for future space optics as well as serial production of smaller segments for larger diameter telescopes.

Most materials naturally expand when heated and contract when cooled; this is known as thermal expansion and is typically characterized by a Coefficient of Thermal Expansion (CTE). Competition between different materials, each with their own CTE, can push optic systems out of focus when the system’s temperature changes. The use of low CTE materials like Invar, carbon fiber composites, and silicon carbide help reduce these temperature effects. Unfortunately, they each have drawbacks such as the high density and low corrosion resistance of Invar, the polymer outgassing of carbon fiber composites, and the low fracture toughness of silicon carbide. In comparison, ALLVAR alloys shrink when heated, known as a negative thermal expansion (NTE), and have low density, high corrosion resistance, high ductility, and do not outgas. This NTE behavior, down to -16 x 10^-6 °C-1 at room temperature, offers a new way to athermalize optic systems by combining NTE alloys with positive CTE materials. The NTE alloy can compensate for a positive CTE material to achieve a desired CTE. Here we evaluate ALLVAR alloys as a potential material for optics applications.

The Liverpool Telescope group have been designing astronomical instruments for the LT and other 2-4m class telescopes for a number of years. This paper covers a variety of issues which need to be addressed in order to benefit from the unique advantages of 3D printing. In particular we discuss our experience of designing, building and testing a simple prototype structure that is analagous to a simple reflecting grating spectrograph.

The current quest for higher resolution and sensitivity for Astronomy and Earth Observation space missions is leading to larger entrance apertures for future optical payloads, resulting in new technological challenges in terms of optomechanical manufacturing, integration and testing. Ensuring feasibility and minimizing schedule impact of tight manufacturing and integration constraints or mitigating adverse in-orbit effects, Active Optics encompasses a range of enabling technologies for future large optical space instruments.

We present here an updated status overview of our current R and D activities in Active Optics, ranging from deformable space-compatible components to full correction chains. Finally we will share our perspectives on the way-forward to reaching technological maturity and ensuring implementation within future large optical missions.

The Optical Engineering and Fabrication Facility (OEFF) at the College of Optical Sciences, University of Arizona has successfully fabricated a 6.5m primary mirror and conducted integration and testing (I & T) of the primary mirror and telescope cell assembly for the University of Tokyo Atacama Observatory. The mirror has been fabricated using a new facility and advanced technologies developed by the group, including fabrication process optimization, a super-stable optical metrology support, infrared and visible metrology with high accuracy, and interferometry. Fabrication process optimization has simulated the entire fabrication process from the initial grinding to final polishing to specification, and optimized the tooling, frequency of metrology, and estimated convergence. Through the project the new process optimization successfully demonstrated the performance and guided the process in a deterministic way. The 6.5m primary mirror is made of E6 bolo-silicate glass with a honeycomb structure. Due to the scale it requires a mechanically and thermally stable metrology support to ensure highly accurate metrology. The new metrology support has demonstrated its extremely stable force stability and thermal stability over the project period. Metrology of large optics is always a challenge and the OEFF group has made outstanding advancement in IR and visible metrology and applied them seamlessly during the mirror fabrication with extremely low uncertainty. By applying the newly developed facility and technologies, the OEFF group successfully fabricated the 6.5m primary mirror in less than 7months, which is about 3 times faster than past 6.5m mirror fabrication at UA. After fabrication the mirror was integrated to the telescope cell and the primary mirror cell assembly has been tested using optical metrology to identify the static and dynamic behavior of the system. The testing includes support actuator influence functions, the nominal support force set, bending mode correction, and assessment of deliverable image quality. This paper presents the technical aspects of the mirror fabrication, advancement of metrology and I & T of the 6.5m primary mirror assembly.

The Richard F. Caris Mirror Lab at the University of Arizona continues the 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 identical off-axis segments, plus an additional off-axis segment to allow continuous operation as segments are removed for coating. Production highlights of the last two years include the spin-casting of Segment 5, preliminary polishing of Segment 2, and completion of the rear surface processing for Segments 3 and 4. We completed a preliminary design of the significant modifications of the test systems required for Segment 4, the center segment. We finished an upgrade of the 8.4 m polishing machine; both the upgrade and experience gained with Segment 1 have contributed to much faster polishing convergence for Segment 2. Prior to polishing Segment 2, we verified the stability and accuracy of the measurement systems by re-measuring Segment 1, achieving good agreement among multiple independent tests as well as good agreement with the original acceptance tests of Segment 1.

Green light for the construction of the 39-m aperture Extremely Large Telescope (ELT) was given by the Council of the European Southern Observatory (ESO) on Dec 4th, 2014. Procurement of the key elements, especially the optics, the dome and structure and the glass substrates, was soon after initiated by ESO team. Safran Reosc is proud to have been awarded all the key optical polishing and testing contracts with:

2015-07: the contract for thin glass petals of the Adaptive Optics M4 mirror unit,

2016-07: the contract for polishing the 4-m secondary convex mirror M2,

2017-02: the contract for polishing of the 4-m tertiary mirror M3,

2017-05: the contract for polishing and integration of the 931 1.45-m hexagonal segments constituting the giant 39-m primary mirror assembly M1.

This paper reports Safran Reosc’s work progresses along these four contracts with their various challenges and more specifically those related to the mass production of the M1 segments.

The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 square degrees will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs.

We describe the DESI corrector optics, a series of six fused silica and borosilicate lenses. The lens diameters range from 0.8 to 1.1 meters, and their weights 84 to 237 kg. Most lens surfaces are spherical, and two are challenging 10^th-order polynomial aspheres. The lenses have been successfully polished and treated with an antireflection coating at multiple subcontractors, and are now being integrated into the DESI corrector barrel assembly at University College London.

We describe the final performance of the lenses in terms of their various parameters, including surface figure, homogeneity, and others, and compare their final performance against the demanding DESI corrector requirements. Also we describe the reoptimization of the lens spacing in their corrector barrel after their final measurements are known. Finally we assess the performance of the corrector as a whole, compared to early budgeted estimates.

The European Extremely Large Telescope (E-ELT), with its primary mirror diameter of 39 meters, will be the largest optical/near-infrared telescope in the world and will allow scientists to investigate important unsolved issues of the universe.

The dome and telescope (DMS) are designed to ensure high performance in one of the most seismic active areas in the world, Cerro Armazones in Chile.

The Dome diameter is 86 m and sits on the top of a stiff concrete pier, which has been designed with horizontal seismic devices to reduce the seismic accelerations on the structures. The isolation system consists of a combination of High Damping Rubber Bearings (HDRBs) and lubricated spherical bearings.

The Telescope structure has been designed to be adaptive, during operation it is extremely stiff with low damping to guarantee the pointing and tracking (fixed condition) and when subjected to strong earthquakes it is flexible with high damping (isolated condition) to reduce the accelerations on the mirrors and instruments.

To fulfill these requirements, a 3D adaptive seismic isolation system has been designed with unique features. The telescope natural frequencies and damping change suddenly when the telescope is subjected to a 1-year return period seismic event, which is the maximum threshold acceleration acceptable without isolation. In the isolated configuration, the telescope frequencies range between 0.3 Hz (isolation frequency) and 30 Hz (highest frequency of interest), while in the fixed configurations the frequencies range between 2.6 Hz and 30Hz.

The vertical and horizontal acceleration reduction is obtained with special devices designed for this type of applications.

This paper presents the design and shows the results of the sophisticated nonlinear time history analyses performed on the DMS. The large finite element models consist of about 75000 nodes and over 110000 elements and include nonlinear spring damper elements calibrated experimentally to model the vertical and horizontal behavior of the seismic devices.

The new deployable tertiary mirror for the Keck I telescope (K1DM3) at the W. M. Keck Observatory has been assembled, tested and shipped to the telescope site, and is currently being installed. The mirror is capable of reflecting the beam to one of six positions around the telescope elevation ring or to retract out of the way to allow the use of Cassegrain instruments. This new functionality is intended to allow rapid instrument changes for transient event observations and improve telescope operations. This paper presents the final as-built design. Additionally, this paper presents detailed information about our alignment approach in the attempt to duplicate the instrument pointing orientation of the existing M3.

The demanding science goals of future astrophysics missions currently under study for the 2020 Decadal Survey impose significant technological requirements on their associated telescopes. These concepts currently call for apertures as large as 15 m (LUVOIR), and operational temperatures as low as 4 Kelvin (OST). Advanced mirror technologies, such as those implementing a high degree of actuation at the primary, can help to overcome the challenges associated with these missions by providing in-situ wavefront correction capabilities. Active mirrors can also greatly reduce the cost/complexity associated with mirror fabrication as well as system I and T as on-orbit performance specifications can be achieved under a variety of test conditions (i.e. room/cryogenic temperatures, 0g/1g). JPL has significant experience in this area for visible/near-infrared applications, however future mission requirements create a new set of challenges for this technology. This paper presents design, analysis, and test results for lightweight silicon-carbide mirrors with integrated actuation capabilities. In particular, studies have been performed to test the performance of these mirrors at cryogenic temperatures.

For the development of the Focus Mechanism of the CLUPI instrument of the ExoMars 2020 mission, the CSEM implemented a design based on flexible structure technology and the use of Commercial Off-The-Shelf (COTS) components. This choice was essentially motivated by the availability of the miniature sensor and actuator components. Such approach presents many challenges such as:

the qualification of the OTS voice-coil motor and Linear Variable Differential Transformer (LVDT),
the implementation of a reliable launch locking system,
the design of a flexible structure mechanism compatible with harsh mechanical environment.

The present article describes each of the above problematics and the investigation carried by CSEM to find solutions meeting the mission needs and constrains.

The construction of the next generation of 40 m-class astronomical telescopes poses an enormous challenge for the design of their instruments and the manufacture of their optics. Optical elements typically increase in both size and number, placing ever more demands on the system manufacturing and alignment tolerances. This challenge can be met by using the wider design space offered by freeform optics, by for instance allowing highly aspherical surfaces. Optical designs incorporating freeform optics can achieve a better performance with fewer components. This also leads to savings in volume and mass and, potentially, cost.

This paper describes the characterization of the FAME system (freeform active mirror experiment). The system consists of a thin hydroformed face sheet that is produced to be close to the required surface shape, a highly controllable active array that provides support and the ability to set local curvature of the optical surface and the actuator layout with control electronics that drives the active array.

A detailed characterisation of the fully-assembled freeform mirror was carried out with the physical and optical properties determined by coordinate measurements (CMM), laser scanning, spherometry and Fizeau interferometry. The numerical model of the mirror was refined to match the as-built features and to predict the performance more accurately.

Each of the 18 actuators was tested individually and the results allow the generation of look-up tables providing the force on the mirror for each actuator setting. The actuators were modelled with finite element analysis and compared to the detailed measurements to develop a closed-loop system simulation. After assembling the actuators in an array, the mirror surface was measured again using interferometry. The influence functions and Eigen-modes were also determined by interferometry and compared to the FEA results.

The European Extremely Large Telescope (ELT) is a 39-m Class telescope with active and adaptive capability included into the telescope being developed by the European Southern Observatory (ESO).

The Telescope Secondary (M2) and Tertiary (M3) Mirrors are 4-metre class Zerodur mirrors close to 3.2 Tons that are passively supported by Cells with an 18 points axial whiffletree and a warping harness system that allows to correct low order deformations of the Mirrors. Laterally the Mirrors are supported on 12+2 points by Lateral Supports. In addition, the Cells have alignment capabilities by means of a high precision hexapod.

SENER has been contracted by ESO for the design, construction, validation and delivery of the ELT Secondary Mirror (M2) and Tertiary Mirror (M3) Cells.

The Cells’ mechanisms guarantee the alignment of the Telescope during observation while correcting optics deformations. In this process, a high precision hexapod will be responsible for aligning and tracking the mirrors and an active structure will be used to compensate errors in the mirrors’ surface. These are large-size critical elements of the Telescope that require extremely high precision levels to give the Telescope optimal image quality.

This manuscript describes the preliminary design and key aspects of the ELT M2 and M3 Mirrors Cells mechanisms, in particular the Mirror Suppor.

TNO is developing Deformable Mirror (DM) technology, targeted for aberration correction in high-end Adaptive Optics (AO) applications in the field of lithography, astronomy, space and laser communication. The heart of this deformable mirror technology is a unique actuator technology based on the variable reluctance principle. The main advantages of this technology are the inherent high reliability, linearity (>99%), and high efficiency in terms of force per volume and unit power. Based on this actuator technology TNO built and tested a prototype DM, with 57 actuators, and a mirror diameter of Ø160mm. The test results show a highly linear actuator response, with less than 1% hysteresis over a stroke of 40μm. Atmospheric aberration correction has been shown with these DM’s in a free space laser-communication bread board. The same actuator technology is also used in the application of a highly compact Fine Steering Mirror (FSM), with an overall volume of Ø27x30mm, with a Ø20mm mirror. This FSM is targeted for satellite-based laser-communication terminals. Furthermore, a design study has been carried out to show the scalability of this technology towards large (~Ø1m to ~Ø3m) adaptive (secondary) mirrors with several hundreds, up to thousands of actuators. In this paper these different DM and FSM’s are discussed, and the latest test results obtained with the DM prototypes are presented.

MATISSE is the mid-infrared interferometric spectrograph and imager for ESO’s Very Large Telescope Interferometer (VLTI). A core mechanism inside the Cold Optical bench is the photometric slider, that enables the choice of observation with or without photometric beams which is achieved by sliding in 4 mirrors or 4 beam splitters into the four telescope beams. To achieve the stringent requirements on beam precision -which asks for mounting pad differences of order of 1 micrometer- all optical components were mounted on a single body and one lapped surface. Test results on the final instrument showed behavior significantly outside specification suggesting contact point height differences up to 6 micrometer. Also repeatability was non-compliant. We will present the cause analysis, the suspected culprit, unsuspected side effects and the implementation of the final solution which lead to a photometric slider well within specification.

The CEA Cryo-Mechanism (CM) was created to actuate infrared instruments wheels equipped with filters, coronographs or diffractive optics. Based on an optimized integration of basic industrial components, the CM operates with a high positioning repeatability (down to ±13 arcsec) in infrared astrophysical environment (very low temperatures and under vacuum). In 2004, for the first light of the mid-infrared imager/spectrometer VISIR, 12 CM units were produced. Among them, 10 units are operating once every hour since 13 years with a very high reliability.

From 2010, the CM was improved with the goal of space missions. Today, the CM reaches the status of a flight model mechanism already delivered for the EUCLID space mission (launch expected by 2021). It is also derived into a cost optimized actuator so called ICAR that will be manufactured in approximately 20 units for the ELT-METIS ground based instrument.

This paper gives an overview of the CM design and its different configurations. The paper will describe more in details the different tests that were carried out on the Euclid-CM, covering performances, vibrations, electromagnetism, thermal cycles, exported torques and life-time tests.

We present the design and performance of a cryogenic, angle-scanned Fabry-Pérot interferometer for far infrared astronomical spectroscopy. Novel features of the design are discussed, and the spectral response of the instrument is modeled. Experimental methods being developed to validate the spectral response are presented.

The European Southern Observatory (ESO) is building the Extremely Large Telescope (ELT), a 40-m class telescope to be installed on top of the 3046 m high mountain Cerro Armazones in the central part of Chile’s Atacama Desert. Once operational the ELT will be the largest optical/near-infrared telescope in the world. Powerful facility instruments that can deliver the science cases for the ELT are under development. The instrument roadmap lists more than six scientific instruments, each of them in the 15-35 tons range. While the telescope optics operate at ambient temperature, the instrument optics structure and in particular the detectors will be cooled to cryogenic temperatures down to as low as 4 Kelvin. ESO is aiming to implement proven technologies and commercial off-the-shelf components to build the cryogenic infrastructure for the ELT instruments. A combination of open loop Liquid Nitrogen cooling and low-vibration mechanical cryo-coolers will be installed to provide the required temperature levels and cooling capacities. ESO’s vacuum and cryogenic standards required major updates in order to match with the needs and challenges of this new class of huge instruments, each of them coming with up to 50 m³ vessel volume and more than 5 tons cold mass.

The paper outlines the instruments vacuum and cryogenic requirements, gives a brief overview of the ESO vacuum and cryogenic standards, and of the ELT cryogenic infrastructure baseline concept. The current testing approach for selected standard components such as low-vibration cryo-coolers and vibration damping systems will be presented.

One of the major challenges of optical fabrication is measurement of the surface when first polished out, before its figure is within the capture range of interferometry. A high dynamic range instrument with good accuracy is needed to efficiently guide the processing. Our approach is to use the Software Configurable Optical Test System (SCOTS), a deflectometry technique that uses a camera and liquid crystal display to measure the surface slope. We describe the use of SCOTS in the fabrication of a 6.5 m on-axis mirror and an 8.4 m off-axis mirror segment (Giant Magellan Telescope primary). SCOTS has the dynamic range to measure high slope errors early in the mirror figuring process, with sufficient accuracy to corroborate later interferometric measurements. Accurately measuring low order figure errors with SCOTS requires careful calibration of the system geometry. Details of the data collection and processing, and comparison to interferometry measurements are presented.

We measured the influence functions of a 9-actuator warping harness of an ELT primary mirror segment prototype using a fiber interferometer. The compact setup consists of a stiff, lightweighted aluminium plate with a similar diameter as the segment (1.25 m), holding 24 fiber-fed collimators arranged in three concentric rings. We measure simultaneously the 24 absolute distances to the mirror surface with a nominal precision of 0.5 ppm. The recorded noise level in a quiet environment was below 50 nm rms. We found good agreement between the measured influence functions and the finiteelement model by the segment support manufacturer. These measurements were performed at ESO Garching in June 2017, with the “Absolute Multiline” metrology system.

Telescope design advancements are leading to the need for larger convex secondary elements, making the use of traditional refractive test geometries impractical. In response to requests for larger convex components, Harris has developed the Plano Holographic Aspheric Stitching Technique (PHAST)^[1], a novel metrology approach that offers versatility as well as improved performance for large convex components. This approach was conceived initially for the in-process testing of the Large Synoptic Survey Telescope (LSST) M2^[2], a 3.4-meter diameter convex asphere and has since been expanded to a versatile design that can be quickly modified to test multiple prescriptions with minimal cost and schedule impacts. The metrology system has facilitated the production of the largest convex optic that Harris has processed and tested.

The metrology approach is a sub-aperture stitching technique that uses a diffractive nulling element. This leverages the rapid production times of the lithography industry to reduce the lead time for test set assembly. For the most common convex component geometries, this test can be ready for use in as little as six months from receipt of specifications.

We will present the development and design of this test methodology. Existing PHAST systems are providing high resolution and accurate data while demonstrating the stability of the overall approach. In addition, the approach is capable of rapid reconfiguration to accommodate testing of multiple convex optics over a range of sizes and specifications.

Break through large astronomical telescopes of the near future (ELT, TMT, MSE, LOT, etc.) will be comprised of extremely large optical elements, requiring segmented primary mirrors. Optical performance of the primary mirror requires an optical shape with an accuracy of a fraction of a wavelength over the whole radius. This can be practically achieved by composing the largest primary mirrors of scores to several hundreds of individually actuated hexagonal mirrors. Multi-layered control of these actuators is performed over sensors observing the position of non-surface parts of these mirror elements. Alignment calibration or co-phasing to the front-surface is required. This procedure needs to be performed at the initial assembly and updated after each (scheduled) daily mirror replacement and to compensate for residual drift. Capital investment costs of these telescopes create the necessity to minimize the duration of maintenance during the valuable night time hours. TNO has developed an affordable instrument methodology, capable of delivering coordinates of each mirror element with respect to its neighbors during day time. This instrument combines nanometer accuracy/precision with low latency and meets the strict requirements of co-phasing the large multi-segmented telescopes of the future. This article describes the requirements, design and specifications of this instrument. TNO benefits from a rich heritage of designing world class optical instruments for science and industry. This metrology system takes advantage of the development of fast, full-field, time-domain, white-light interferometry, which has been demonstrated previously for in-line quality inspection at harsh industrial machining workshops.

Spectrographs in high-multiplex fiber integral-field spectroscopy tend to have fast large field-of-view camera systems in order to meet the desire of packing as many spatial elements and wide a spectral bandpass as possible, thereby maximizing the use of detector real estate. This, very often, leads to a camera design with a field flattening lens very close to the detector active area with tight tolerances in the relative alignment between those two, on the order of a few hundredths of a degree and millimeter. This requires dedicated optical metrology process, particularly in cases where a large number of spectrographs, on the order of 100 or more, need to be accurately and reliably aligned and integrated. We have developed such a metrology process for the 156 camera systems in the VIRUS instrument. We detail the working principle of this metrology process, the implementation for the VIRUS camera systems, and how it can be applicable to multi-lens camera system alignment and integration.

The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We will describe the methods and results for the commissioning instrument metrology program. The primary goals of this program are to calculate the transformations and further develop the systems that will place fibers within 5μm RMS of the target positions. We will use the commissioning instrument metrology program to measure the absolute three axis Cartesian coordinates of the five CCDs and 22 illuminated fiducials on the commissioning instrument.

High-resolution spectropolarimetry is a useful astronomical technique, in particular to study stellar magnetic fields. It has been extensively used in the past but mostly in the visible range. Space missions equipped with high-resolution spectropolarimeters working in the ultra-violet (UV) are now being studied. We propose a concept of a polarimeter working with temporal modulation and allowing to perform Stokes IQUV measurements over the full UV + Visible range. The purpose of this article is to describe the polarimeter concept, two prototypes and the bench developed to perform on ground testing to establish the performances of this new polarimeter.

One of the highest scientific priorities for the ELT is to image and characterise Earth-like planets around Sun-like stars. This will be achieved with the dedicated planetary camera and spectrograph, ELT-PCS. As ELT-PCS will push current contrast limits for high-contrast imaging, R and D needs to be undertaken to ensure the top-level requirements for the instrument will be met. In this paper we discuss plans to progress the required R and D for the integral field spectrograph technology, with the aim of qualitatively determining the best contrast achievable with both a lenslet-array and an imageslicer based spectrograph. More specifically, we present progression of the design for a new bench spectrograph capable of accepting either of the two competing technologies as its input.

Astrophysical calibration sources which can be used for high-precision radial-velocity spectroscopy require a calibration with even higher precision and accuracy. Calibration of these sources can be achieved with a high-resolution Fourier-Transform-Spectrograph (FTS). The precision (~ 20 m/s) of the FTS is mainly driven by its reference, often a stabilized HeNe-laser. To reach an acceptable precision, either averaging over a large number of measurements or a better reference is needed.

We developed a setup including a Laser-Frequency-Comb (LFC) for referencing a high-resolution FTS. Due to the pulsed source specific evaluation methods have to be used to retrieve the spectrum properly. We extend the sub-nominal method used in absorption spectroscopy by showing an algorithm to determine the interferogram cut points from the interferogram itself, rather than calculating them from the repetition rate and reference laser wavelength. Furthermore, we show that line position errors measured from comb spectra are associated with amplitude variability and phase noise. We give an estimate of the measured line position stability for different evaluation methods (truncation, shifting, apodization, zerofilling) on scales not dominated by these errors.

In the frame of the test of NISP instrument for ESA Euclid mission, the question was raised to perform a metrology measurement of different components during the thermal vacuum test of NISP instrument. NISP will be tested at Laboratoire d’Astrophysique de Marseille (LAM) in ERIOS chamber under vacuum and thermal conditions in order to qualify the instrument in its operating environment and to perform the final acceptance test before delivery to the payload. One of the main objectives of the test campaign will be the measurement of the focus position of NISP image plane with respect to the EUCLID object plane. To simulate the EUCLID object plane, a telescope simulator with a very well know focal distance will be installed in front of NISP into ERIOS chamber. We need to measure at cold and vacuum the position of reflectors installed on NISP instrument and the telescope simulator. From these measurements, we will provide at operational temperature the measurement of references frames set on the telescope simulator and NISP, the knowledge of the coordinates of the object point source provided by the telescope simulator and the measurement of the angle between the telescope simulator optical axis and NISP optical axis. In this context, we have developed a metrology method based on the use of a laser tracker to measure the position of the reflectors inside ERIOS. The laser tracker is installed outside the vacuum chamber and measure through a curved window reflectors put inside the chamber either at ambient pressure or vacuum pressure. Several tests campaigns have been done at LAM to demonstrate the measurement performance with this configuration. Using a well know reflectors configuration, we show that it is possible to correct the laser tracker measurement from the window disturbances and from the vacuum impact. A corrective term is applied to the data and allows retrieving the real coordinates of the reflectors with a bias lower than 30μm, which is lower than the laser tracker measurement uncertainties estimated at 60μm. No additional error term of the laser tracker measurement is observed when using the laser tracker with the curved window and in vacuum, comparing with a classical use of the laser tracker. With these test campaign, we have been able to demonstrate the possibility to use a laser tracker to measure in real time during a vacuum thermal test the position of different mechanical parts into a vacuum chamber with an accuracy better than 60μm.

The James Webb Space Telescope is a large, deployable telescope that will operate at cryogenic temperatures at the Earth-Sun Lagrange 2 point. The Webb Optical Telescope Element (OTE) consists of 18 actively controlled Primary Mirror Segment Assemblies (PMSAs), an actively controlled Secondary Mirror Assembly (SMA), and an Aft-Optics Subsystem (AOS) that contains a fixed Tertiary Mirror and a Fine Steering Mirror. The OTE is combined with the Integrated Science Instrument Module (ISIM) to create the full optical train called OTIS (OTE and ISIM).

OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.

The first large Fabry-Perot etalon (Ø35 cm) of the VTF instrument was coated successfully using IBS technique. The High Reflective (HR) coatings need to meet the reflectivity specifications (95 +/- 1%) over the entire wavelength range 520-870 nm and the entire aperture (Ø25 cm) and also preserve the plate's flatness and airgap uniformity between the two platesto be better than 3 nm RMS. The change of the figure error of the individual faces after HR coating was exceptionally small: For plate 1 (upper) it changed from 1.7nm RMS before coating to 2.12 nm after coating, no change at all for plate 2 (lower).

The Dark Energy Spectroscopic Instrument (DESI), currently under construction, will be used to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers, in turn, feed ten broad-band spectrographs. We will describe the broadband AR coating (360 nm to 980nm) that was applied to the lenses of the camera system for DESI using ion assisted deposition techniques in a 3 m coating chamber. The camera has 6 lenses ranging in diameter from 0.8 m to 1.14 m, weighing from 84 kg to 237 kg and made from fused silica or BK7. The size and shape of the surfaces provided challenges in design, uniformity control, handling, tooling and process control. Single surface average transmission and minimum transmission met requirements. The varied optical surfaces and angle of incidence considerations meant the uniformity of the coating was of prime concern. The surface radius of curvature (ROC) for the 12 surfaces ranged from nearly flat to a ROC of 611 mm and a sag of 140 mm. One lens surface has an angle of incidence variation from normal incidence to 40°. Creating a design with a larger than required bandwidth to compensate for the non-uniformity and angle variation created the ability to reduce the required coating uniformity across the lens and a single design to be used for all common substrate surfaces. While a perfectly uniform coating is often the goal it is usually not practicable or cost effective for highly curved surfaces. The coating chamber geometry allowed multiple radial positions of the deposition sources as well as substrate height variability. Using these two variables we were able to avoid using any masking to achieve the uniformity required to meet radial and angle performance goals. Very broadband AR coatings usually have several very thin and optically important layers. The DESI coating design has layers approaching 3 nm in thickness. Having sensitive thin layers in the design meant controlling layer thickness and azimuthal variation were critical to manufacturing repeatability. Through use of strategically placed quartz crystal monitors combined with stable deposition plumes, the manufacturing variability was reduced to acceptable levels. Low deposition rates and higher rotation rates also provided some stability to azimuthal variation.

The reflectivity of telescope primary mirror is one of the fundamental parameters that shows the telescope performance. However, it has been difficult to obtain absolute value, especially the wide range spectroscopic performance measured in-situ on the primary mirror due to the lack of suitable measuring instrument. To overcome this challenge, we developed a portable spectrophotometer to measure the absolute spectroscopic reflectivity of telescope primary mirror. Its small dimension and light weight enable in-situ measurement on the primary mirror. This spectrophotometer covers the spectral range from 380 nm to 1000 nm with 2 nm resolution. The incident angle to the measuring surface is 12 degrees. The measurement beam size is about 12 mm in diameter. To obtain the absolute value, we adopted the principle of V-N method for the spectrophotometer. A sequential measurement also enables us to cancel the instability of the instrument.

The Subaru Telescope primary mirror was recoated with Aluminum on October 20, 2017. It was the eighth coating work from its arrival at Maunakea, Hawaii in 1998 and was about four years from the previous recoating. Before the recoating work, the reflectivity measured with the spectrophotometer was 70~76 % (@400 nm), 75~80 % (@600 nm), and 73~78 % (@800 nm). The large dispersion of the reflectivity is from non-uniform contamination of the surface, especially from the accumulation of dust particles on the mirror.

After the fresh coating of Aluminum, the values returned to 92.1 % (@400 nm), 90.5 % (@600 nm), and 85.8 % (@800 nm) with standard deviation less than 0.6 %. There were the data taken at the outside of the vacuum chamber right after the recoating.

The great advantage of our spectrophotometer is its capability of getting absolute spectroscopic reflectivity of the primary mirror in-situ. We can continue to monitor the reflectivity of the primary mirror in-situ using this spectrophotometer, even after the primary mirror is mounted on the telescope. This helps us better understanding of the long-term reflectivity degradation.