The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in near-simultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope's active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small high-resolution maps, and used these to show the impact of the small features on images is acceptably small.

All future Extremely Large Telescopes (ELTs) will be segmented. However, as their size grows, they become increasingly sensitive to external disturbances, such as gravity, wind and temperature gradients and to internal vibration sources. Maintaining their optical quality will rely more and more on active control means. This thesis studies active optics of segmented primary mirrors, which aims at stabilizing the shape and ensuring the continuity of the surface formed by the segments in the face of external disturbances. The modelling and the control strategy for active optics of segmented mirrors are examined. The model has a moderate size due to the separation of the quasi-static behavior of the mirror (primary response) from the dynamic response (secondary, or residual response). The control strategy considers explicitly the primary response of the telescope through a singular value controller. The control-structure interaction is addressed with the general robustness theory of multivariable feedback systems, where the secondary response is considered as uncertainty. Scaling laws allowing the extrapolation of the results obtained with existing 10m telescopes to future ELTs and even future larger telescopes are addressed and the most relevant parameters are highlighted. The study is illustrated with a set of examples of increasing sizes, up to 200 segments. This numerical study confirms that scaling laws, originally developed with simple analytical models, can be used in confidence in the preliminary design of large segmented telescopes.

Segmented mirrors are to be used in the next generation of the ground-based optical telescopes to increase the size of the primary mirrors. A larger primary mirror enables the collection of more light, which results in higher image resolutions. The main reason behind the choice of segmented mirrors over monolithic mirrors is to reduce manufacturing, transportation, and maintenance costs of the overall system. However, segmented mirrors bring new challenges to the telescope design and control problem. The large number of inputs and outputs make the computations for centralized control schemes intractable. Centralized controllers also result in systems that are vulnerable to a complete system failure due to a malfunction of the controller. Distributed control is a viable alternative that requires the use of a network of simple individual segment controllers that can address two levels of coupling among segments and achieve the same performance objectives. Since segments share a common support structure, there exists a coupling among segments at the dynamics level. Any control action in one segment may excite the natural modes of the support structure and disturb other segments through this common support. In addition, the objective of maintaining a smooth mirror surface requires minimization of the relative displacements among neighbouring segment edges. This creates another level of coupling generally referred to as the objective coupling. This dissertation investigates the distributed H∞ control of the segmented next generation telescope primary mirrors in the presence of wind disturbances. Three distributed H∞ control techniques are proposed and tested on three segmented primary mirror models: the dynamically uncoupled model, the dynamically coupled model and the finite element model of Thirty Meter Telescope (TMT) project. It is shown that the distributed H∞ controllers are able to satisfy the stringent imaging performance requirements. / Graduate / 0548

Pour relever les défis de l'observation de la Terre et de l'astronomie, les futurs observatoires spatiaux vont requérir des télescopes de plusieurs mètres de diamètre. Les différentes contraintes liées à l'environnement spatial vont induire des déformations et des désalignements des miroirs du télescope dégradant ainsi la qualité optique des observations. L'intégration de l'optique active, utilisée depuis la fin des années 80 pour l'alignement et le maintien de la qualité optique des télescopes au sol, devient donc nécessaire. Un tel système est constitué de trois éléments : un dispositif de mesure, une fonction de correction et une boucle de contrôle qui fait le lien entre les deux éléments précédents.Les travaux de recherche présentés dans ce mémoire de doctorat s'attachent, d'une part, au développement d'un estimateur des perturbations minimisant le front d'onde résiduel dans le champ du télescope. L'analyse de cet estimateur et plus généralement des performances ultimes d'un système d'optique active spatial nous a permis de mettre en évidence l'impact des hautes fréquences spatiales sur la qualité optique. La présence de telles hautes fréquences dues à l'empreinte des supports du miroir primaire nous a poussés, d'autre part, à évaluer les possibilités de leur réduction dans le cas d'un miroir de grande dimension. A l'aide d'une modélisation éléments finis d'un tel miroir nous avons réalisé des modifications locales de la structure du miroir pour adapter ses déformations aux capacités de correction de l'optique active. / To meet the challenges of Earth observation and astronomy, future space observatories will require telescopes of several meters in diameter. The various space environment constraints will induce deformations and misalignments of the telescope mirrors, thus degrading the optical quality of observations. The integration of active optics, used since the late 80s for alignment and preservation of the optical quality of the ground telescopes, becomes mandatory. Such a system consists of three elements: a measuring device, a correction set-up and a control loop which links both previous elements.The research works presented in this PhD thesis focus, on one hand, on the development of a perturbations estimator minimizing the residual wave front in the telescope field of view. The analysis of this estimator and more generally of the ultimate performance of a space active optics system enabled us to highlight the impact of high spatial frequencies on optical quality. On the other hand, the presence of such high frequencies due to the primary mirror supports print through, urged us to evaluate the possibilities of their reduction in the case of large mirrors. Using a finite element model of such a mirror, we made local modifications of the mirror structure to adapt its deformation to the correcting capabilities of active optics.

The GMT is an aplanatic Gregorian telescope consisting of 7 primary and secondary mirror segments that must be phased to within a fraction of an imaging wavelength to allow the 25.4 meter telescope to reach its diffraction limit. When operating in Laser Tomographic Adaptive Optics (LTAO) mode, on-axis guide stars will not be available for segment phasing. In this mode, the GMT's Acquisition, Guiding, and Wavefront Sensing system (AGWS) will deploy four pickoff probes to acquire natural guide stars in a 6-10 arcmin annular FOV for guiding, active optics, and segment phasing. The phasing sensor will be able to measure piston phase differences between the seven primary/secondary pairs of up to 50 microns with an accuracy of 50 nm using a J-band dispersed fringe sensor. To test the dispersed fringe sensor design and validate the performance models, SAO has built and commissioned a prototype phasing sensor on the Magellan Clay 6.5 meter telescope. This prototype uses an aperture mask to overlay 6 GMT-sized segment gap patterns on the Magellan 6.5 meter primary mirror reimaged pupil. The six diffraction patterns created by these subaperture pairs are then imaged with a lenslet array and dispersed with a grism. An on-board phase shifter has the ability to simulate an arbitrary phase shift within subaperture pairs. The prototype operates both on-axis and 6 arcmin off-axis either with AO correction from the Magellan adaptive secondary MagAO system on or off in order to replicate as closely as possible the conditions expected at the GMT.

The development of compact imaging systems capable of transmitting high-resolution images in real-time while covering a wide field-of-view (FOV) is critical in a variety of military and civilian applications: surveillance, threat detection, target acquisition, tracking, remote operation of unmanned vehicles, etc. Recently, optical foveated imaging using liquid crystal (LC) spatial light modulators (SLM) has received considerable attention as a potential approach to reducing size and complexity in fast wide-angle lenses. The fundamental concept behind optical foveated imaging is reducing the number of elements in a fast wide-angle lens by placing a phase SLM at the pupil stop to dynamically compensate aberrations left uncorrected by the optical design. In the recent years, considerable research and development has been conducted in the field of optical foveated imaging based on the LC SLM technology, and several foveated optical systems (FOS) prototypes have been built. However, most research has been focused so far on the experimental demonstration of the basic concept using off the shelf components, without much concern for the practicality or the optical performance of the systems. Published results quantify only the aberration correction capabilities of the FOS, often claiming diffraction limited performance at the region of interest (ROI). However, these results have continually overlooked diffraction effects on the zero-order efficiency and the image quality. The research work presented in this dissertation covers the methods and results of a detailed theoretical research study on the diffraction analysis, image quality, design, and optimization of fast wide-angle FOSs based on the current transmissive LC SLM technology. The amplitude and phase diffraction effects caused by the pixelated aperture of the SLM are explained and quantified, revealing fundamental limitations imposed by the current transmissive LC SLM technology. As a part of this study, five different fast wide-angle lens designs that can be used to build practical FOSs were developed, revealing additional challenges specific to the optical design of fast wide-angle systems, such as controlling the relative illumination, distortion, and distribution of aberrations across a wide FOV. One of the lens design examples was chosen as a study case to demonstrate the design, analysis, and optimization of a practical wide-angle FOS based on the current state-of-the-art transmissive LC SLM technology. The effects of fabrication and assembly tolerances on the image quality of fast wide-angle FOSs were also investigated, revealing the sensitivity of these fast well-corrected optical systems to manufacturing errors. The theoretical study presented in this dissertation sets fundamental analysis, design, and optimization guidelines for future developments in fast wide-angle FOSs based on transmissive SLM devices.

A conventional large aperture telescope required for binary star research is typically cost prohibitive. A prototype active optics system was created and fitted to a telescope frame using relatively low cost components. The active optics system was capable of tipping, tilting, and elevating the mirrors to align reflected star light. The low cost mirror position actuators have a resolution of 31 nm, repeatable to within 16 nm. This is accurate enough to perform speckle analysis for the visible light spectrum. The mirrors used in testing were not supported with a whiffletree and produced trefoil-like aberrations which made phasing two mirrors difficult. The active optics system was able to successfully focus and align the mirrors through manual adjustment. Interference patterns could not be found due to having no method of measuring the mirror surfaces, preventing proper mirror alignment and phasing. Interference from air turbulence and trefoil-like aberrations further complicated this task. With some future project additions, this system has the potential to be completely automated. The success of the active optics actuators makes for a significant step towards a fully automated sparse aperture telescope.

La prochaine décennie instrumentale en astronomie se veut extrême. Elle s’ouvre avec l'arrivée des ELTs (Extremely Large Telescopes). Leur miroir primaire géant permettra d'augmenter considérablement la quantité de flux collectée et d'améliorer la résolution angulaire, paramètres clés pour l'observation et l'imagerie de sources astrophysiques. Des conséquences directes sont l'augmentation de la complexité, de l'envergure et de la masse des instruments placés aux foyers de ces télescopes. Une solution passe par l'utilisation de miroirs de formes libres. Or aujourd’hui, obtenir ces formes exotiques via les méthodes traditionnelles de fabrication optique n’est pas possible et un appel à de nouvelles ruptures technologiques s'avère nécessaire. Cette thèse présente un travail de recherche et développement amont portant sur un procédé de fabrication innovant permettant de fournir des miroirs de formes libres, avec les performances optiques requises en observations visibles et infrarouges. Ce procédé est une évolution des techniques d'Optique Active et exploite la déformation plastique des matériaux métalliques. Cependant, le domaine plastique reste un domaine de comportements non-linéaires analytiquement complexes. Il est alors d'intérêt de comparer des modèles par éléments finis avec des essais réels. Ces derniers ont nécessité la mise en place de la gamme complète de fabrication des substrats et des moyens d’essais. Les premiers miroirs obtenus pourront mettre en évidence les paramètres principaux à prendre en compte ainsi que leur niveau de sensibilité, pour ensuite converger vers des modèles éléments finis fiables et une solution de fabrication optique maîtrisée. / The next instrumental decade in astronomy aims to be extreme. It opens with the arrival of ELTs (Extremely Large Telescopes). Their giant primary mirrors will increase the light collecting power and the angular resolution, key parameters for observing and imaging of celestial bodies. However, this also leads to an increase in the complexity, size and weight of their focal-plane instruments, to minimize flux lost and to correct for the aberrations introduced. A solution would be to implement freeform mirrors inside the optical systems of these instruments. Today, it is not possible to obtain these exotic mirror shapes using the current optical fabrication techniques and new technological breakthroughs in this domain are essential. This PhD thesis present research and development work, in upstream phase, of an innovative manufacturing process to supply freeform mirrors, which should meet required optical performances in Visible and Infrared wavelength astronomical observations. This method is an evolution of Active Optics techniques and based on the ability of metallic materials to plasticize. However, the plasticity of metallic materials remains a field of non-linear behaviours and analytically complex. It is important to compare modeling from finite element analysis and real tests. For these tests, the complete manufacturing steps of the metallic substrates were put in place. The first mirrors obtained will highlight the main working parameters and their sensibility levels, and then converge toward reliable finite elements models and a mastered solution of optical freeform mirrors fabrication.

Le besoin tant en haute qualité d'imagerie qu'en structures légères est l'un des principaux moteurs pour la conception des télescopes spatiaux. Un contrôle efficace du front d'onde va donc devenir indispensable dans les futurs grands observatoires spatiaux, assurant une bonne performance optique tout en relâchant les contraintes sur la stabilité globale du système. L'optique active consiste à contrôler la déformation des miroirs, cette technique peut être utilisée afin de compenser la déformation des grands miroirs primaires, afin de permettre l'utilisation d'instrument reconfigurable ou afin de fabriquer des miroirs asphériques avec le polissage sous contraintes. Dans ce manuscrit, la conception de miroirs actifs dédiés à l'instrumentation spatiale est présentée. Premièrement, un système compensant la déformation d'un grand miroir allégé dans l'espace est conçu et ses performances sont démontrées expérimentalement. Avec 24 actionneurs, le miroir MADRAS (Miroir Actif Déformable et Régulé pour Applications Spatiales) effectuera une correction efficace du front d'onde dans un relais de pupille du télescope. Deuxièmement, un harnais de déformation pour le polissage sous contraintes des segments du télescope géant européen de 39 m (E-ELT) est présenté. La performance du procédé est prédite et optimisée avec des analyses éléments finis et la production en masse des segments est considérée. Troisièmement, deux concepts originaux de miroirs déformables avec un nombre minimal d'actionneurs ont été développés. VOALA (Variable Off-Axis parabola) est un système à trois actionneurs et COMSA (Correcting Optimized Mirror with a Single Actuator) est un système à un actionneur. / The need for both high quality images and light structures is one of the main driver in the conception of space telescopes. An efficient wave-front control will then become mandatory in the future large observatories, ensuring the optical performance while relaxing the specifications on the global system stability. Consisting in controlling the mirror deformation, active optics techniques can be used to compensate for primary mirror deformation, to allow the use of reconfigurable instruments or to manufacture aspherical mirror with stress polishing. In this manuscript, the conception of active mirrors dedicated to space instrumentation is presented. Firstly, a system compensating for large lightweight mirror deformation in space, is designed and its performance are experimentally demonstrated. With 24 actuators, the MADRAS mirror (Mirror Actively Deformed and Regulated for Applications in Space) will perform an efficient wave-front correction in the telescope's pupil relay. Secondly, a warping harness for the stress polishing of the 39 m European Extremely Large Telescope segments is presented. The performance of the process is predicted and optimized with Finite Element Analysis and the segments mass production is considered. Thirdly, two original concepts of deformable mirrors with a minimum number of actuators have been developed. The Variable Off-Axis parabola (VOALA) is a 3-actuators system and the Correcting Optimized Mirror with a Single Actuator (COMSA) is a 1-actuator system.

Space-based telescopes are limit by the payload requirements of existing launch vehicles. Thus, despite distinct advantages the resolution of terrestrial telescopes exceeds space-based telescopes due to larger size and powerful adaptive optics. To overcome payload limitations, a primary mirror technology that is lighter in weight, but no less effective, is required. As this will result in new structural conditions, new approaches to maintaining the optical shape (figure) of the mirror will also be required. This thesis culminates work at the Georgia Institute of Technology in modelling a hexagonal thin-shell deformation mirror manipulated by an adaptive truss. This research specifically examines the feasibility of a surface parallel actuated (SPA) thin-shell CuZr deformable mirror (DM) as an alternative to a typical surface normal actuated (SNA) based mirror. It is believed that by using a thin-shell mirror (100 m or less in thickness) with a light weight substrate, such as a truss, that a significant weight-savings will occur, thus enabling larger space based telescopes. This thesis advances the SPA DM concept by 1) creating a representative model, 2) developing design evaluation methods, 3) evaluating the FEA simulated response of the deformable mirror over Zernike error modes, 4) evaluating the FEA simulated response to select thermal loads, and 5) evaluating the ability of the DM to remove thermal error, and the forces required to do so. Finally, it is concluded that overall the SPA DM concept is feasible.