Three-sided pyramid wavefront sensor. II. Preliminary demonstration on the new CACTI testbed

The next generation of giant ground and space telescopes will have the light-collecting power to detect and characterize potentially habitable terrestrial exoplanets using high-contrast imaging for the first time. This will only be achievable if the performance of Giant Segmented Mirror Telescopes (GSMTs) extreme adaptive optics (ExAO) systems are optimized to their full potential. A key component of an ExAO system is the wavefront sensor (WFS), which measures aberrations from atmospheric turbulence. A common choice in current and next-generation instruments is the pyramid wavefront sensor (PWFS). ExAO systems require high spatial and temporal sampling of wavefronts to optimize performance, and as a result, require large detectors for the WFS. We present a closed-loop testbed demonstration of a three-sided pyramid wavefront sensor (3PWFS) as an alternative to the conventional four-sided pyramid wavefront (4PWFS) sensor for GSMT-ExAO applications on the new Comprehensive Adaptive Optics and Coronagraph Test Instrument (CACTI). The 3PWFS is less sensitive to read noise than the 4PWFS because it uses fewer detector pixels. The 3PWFS has further benefits: a high-quality three-sided pyramid optic is easier to manufacture than a four-sided pyramid. We detail the design of the two components of the CACTI system, the adaptive optics simulator and the PWFS testbed that includes both a 3PWFS and 4PWFS. A preliminary experiment was performed on CACTI to study the performance of the 3PWFS to the 4PWFS in varying strengths of turbulence using both the Raw Intensity and Slopes Map signal processing methods. This experiment was repeated for a modulation radius of 1.6 lambda/D and 3.25 lambda/D. We found that the performance of the two wavefront sensors is comparable if modal loop gains are tuned.Comment: 28 Pages, 15 Figures, and 4 Table

Overview of AO calibration strategies in the ELT context

The scientific potential of the ELT will rely on the performance of its AO systems that will require to be perfectly calibrated before and during the operations. The actual design of the ELT will provide a constraining environment for the calibration and new strategies have to be developed to overcome these constraints. This will be particularly true concerning the Interaction Matrix of the system with no calibration source upward M4 and moving elements in the telescope. After a brief presentation of the ELT specificities for the calibration, this communication focuses on the different strategies that have already been developed to get/measure the Interaction Matrix of the system, either based on synthetic models or using on-sky measurements. First tests of these methods have been done using numerical simulations for a simple AO system and a proposition for a calibration strategy of the ELT will be presented

Optimisation des analyseurs de front d'onde à filtrage optique de Fourier

Europe is currently preparing the largest telescope of the world: the E-ELT (European Extremely Large Telescope). Planned by 2026, this huge telescope (diameter: 39 m) will allow to answer fundamental questions of contemporary astrophysics by imaging exoplanets or studying large scales of universe. However, images of astrophysical objects done by ground based telescopes suffer from the distortion caused by the atmospheric turbulence which reduces the capacity of instruments to distinguish objects too close to each other.The Adaptive Optics (AO) is a technique which allows to restore this loss of angular resolution by correcting the effects of atmospheric turbulence. In operation on several astronomical observatories for almost 20 years, it is now applied to other domains where imaging suffers from inhomogeneous media as cellular microscopy, ophthalmology, functional imaging, etc. This technology is based on a deformable mirror which corrects in real time the incoming wave front by using information coming from a sensor which measures the turbulent phase called "WaveFront Sensor" (WFS).Until very recently, the great majority of AO systems had used the Shack-Hartmann WFS. New concepts based on Fourier filtering have however just been put in operation in several professional observatories and their results seem to outperform the Shack-Hartmann. We mention in particular the Pyramid WFS which provides images with an astonishing quality at the Large Binocular Telescope or the Zernike WFS which allows to considerably improve the performance of the exoplanets imager at the Very Large Telescope.In spite of these instrumental achievements, these Fourier based WFSs are still lacking of maturity. Since they would potentially be chosen for the AO systems of the future Extremely Large Telescopes, this thesis aims to consolidate their theoretical understanding and to optimize these Fourier based WFSs.We firstly developed a mathematical framework where the optical theories of diffraction are applied to the Fourier based wave front sensing. Resulting in general equations, it allows to describe all the WFSs based on this concept (as Pyramid and Zernike WFSs). They are, for the first time, unified under the same formalism. Secondly we developed a common performance criteria following the usual requirements of astronomical adaptive optics in order to rigorously compare these sensors.Another consequence of this theoretical framework's development is then introduced to test new WFSs which derive from the pre-existent designs. It allows us to introduce the notion of "WFSs class" which consists in a generalization of the existing sensors with original parameters which become flexible: from two Zernike and Pyramid WFSs we get an infinity of sensors in the Zernike's and Pyramid's WFSs classes.We then explored these two classes using both numerical simulations and analytical approach. The objective was to identify the influence of class' parameters on performance criteria in order to optimize optical designs with regard to the instrumental requirements. New configurations of the Pyramid class, that we called "Flattened pyramids", show promising behaviors and are studied in details.The thesis ends with the presentation of an optical bench able to generate almost all of the Fourier based WFSs which would allow to validate experimentally the predicted theoretical results.L'Europe prépare actuellement le plus grand télescope du monde : l'E-ELT (European Extremely Large Telescope). Prévu vers 2026, ce télescope géant (diamètre: 39 m) permettra de répondre à des questions fondamentales de l'astrophysique contemporaine, depuis l'imagerie d'exoplanètes jusqu'à l'étude des grandes échelles de l'univers. Cependant, l'imagerie d'objets astrophysiques depuis des télescopes au sol est fortement perturbée par l'effet des mouvements de masses d'air dans l'atmosphère réduisant ainsi la capacité des instruments au sol à distinguer deux objets proches.L'Optique Adaptative (OA) permet de restaurer cette résolution angulaire en corrigeant les effets de la turbulence atmosphérique. Cette technique, en plein essor en astronomie, s'exporte d'ailleurs actuellement aux nombreux autres domaines où l'imagerie pâtit d'un milieu inhomogène spatialement et temporellement comme la microscopie cellulaire, l'ophtalmologie, l'imagerie fonctionnelle, etc. Cette technologie s'appuie sur un (ou plusieurs) miroir(s) déformable(s) qui corrige(nt) en temps réel le front d'onde incident en utilisant les données provenant d'un (ou plusieurs) instrument(s) de mesure de la phase turbulente appelé "Analyseur de Surface d'Onde" (ASO). Jusqu'à très récemment, la grande majorité des systèmes d'OA utilisaient des ASO de type Shack-Hartmann. Des concepts concurrents basés sur le filtrage optique de Fourier viennent cependant d'être mis en fonctionnement dans plusieurs observatoires professionnels et leurs résultats semblent surpasser les performances du Shack-Hartmann. On mentionne notamment le senseur Pyramide qui a fourni des images d'une qualité sans précédent depuis le sol au Large Binocular Telescope ou l'analyseur Zernike qui a permis d'améliorer considérablement les performances de l'imageur d'exoplanètes du Very Large Telescope. En dépit de ces prouesses instrumentales, ces senseurs manquent encore de maturité et le retour sur expérience des systèmes opérationnels reste très faible. En vue de leur potentielle utilisation sur les Extremely Large Telescopes, cette thèse vise à consolider leur compréhension théorique ainsi qu'à optimiser ces ASO basés sur le filtrage de Fourier. Est développé dans un premier temps un cadre mathématique où les théories de diffraction optiques sont appliquées à l'analyse de front d'onde par filtrage de Fourier. Les équations très générales qui en émergent permettent de décrire tous les ASO basés sur ce principe (tels les ASO Pyramide et Zernike) qui se trouvent ainsi pour la première fois rassemblés sous un unique formalisme. Dans un second temps sont développés des critères de performance communs inspirés des besoins de l'optique adaptative astronomique en vue d'une comparaison rigoureuse de ces senseurs.Une conséquence logique de l'effort théorique précédent est ensuite présentée : puisqu'un cadre de comparaison existe pourquoi ne pas en profiter pour tester de nouveaux ASO qui seraient par exemple générés à partir des designs préexistants ? De ce questionnement apparaît ainsi naturellement la notion de "classe d'analyseurs de front d'onde" qui consiste en une généralisation des senseurs existants pour lesquels des grandeurs à l'origine fixées deviennent des degrés de liberté : des deux ASO Zernike et Pyramide, se dégage une infinité de dispositifs en les classes d'ASO de Zernike et de la Pyramide.Celles-ci sont ensuite explorées analytiquement et via des simulations numériques. L'objectif est d'identifier quel paramètre de classe joue sur quel critère de performance afin d'envisager une optimisation des designs optiques au regard des attentes instrumentales. Des configurations inédites de la classe Pyramide, ASO que l'on appelle "Pyramides aplaties", s'avèrent notamment très prometteuses et font l'objet d'une étude plus poussée.L'exposé se conclut par la présentation d'un banc optique capable de simuler la quasi-totalité des ASO à filtrage de Fourier ce qui permettrait de valider expérimentalement les résultats théoriques prédits

Optimization of Fourier based wavefont sensors

L'Europe prépare actuellement le plus grand télescope du monde : l'European Extremely Large Telescope (E-ELT). Prévu vers 2026, ce télescope géant permettra de répondre à des questions fondamentales de l'astrophysique contemporaine. L'imagerie d'objets astrophysiques depuis des télescopes au sol est cependant perturbée par l'atmosphère qui réduit la capacité des instruments au sol à distinguer les objets proches. L'Optique Adaptative (OA) permet de restaurer cette résolution angulaire en corrigeant en temps réel (via un miroir déformable) le front d'onde perturbé par l'atmosphère (mesuré par l'Analyseur de Surface d'Onde (ASO)). Jusqu'à récemment, la majorité des systèmes d'OA utilisaient des ASO Shack-Hartmann (SH). Des concepts concurrents basés sur le filtrage optique de Fourier (le senseur Pyramide ou l'analyseur Zernike) viennent cependant d'être mis en fonctionnement et leurs résultats semblent surpasser les performances du SH. En vue de leur potentielle utilisation sur les ELTs, cette thèse vise à consolider leur compréhension théorique ainsi qu'à optimiser ces ASO basés sur le filtrage de Fourier. Cette thèse développe un cadre mathématique qui décrit sous un unique formalisme ces ASO. Il permet de généraliser les designs préexistants -passant ainsi d'ASO uniques à des "classes d'ASO"- en transformant leurs grandeurs caractéristiques à l'origine fixées en degrés de liberté. Les classes Pyramide et Zernike sont donc explorées dans le but d'optimiser ces ASO au regard des attentes expérimentales. Des configurations inédites de la classe Pyramide -ASO que l'on appelle Pyramides aplaties- s'avèrent notamment prometteuses et font l'objet d'une étude plus poussée.Europe is currently preparing the largest telescope of the world: the European Extremely Large Telescope (E-ELT). Planned by 2026, this huge telescope will allow to answer fundamental questions of contemporary astrophysics. However, images of astrophysical objects done by ground based telescopes suffer from the atmospheric turbulence which reduces the capacity of instruments to distinguish objects too close to each other. The Adaptive Optics (AO) allows to restore this loss of angular resolution by correcting (thanks to a deformable mirror) in real time the perturbed wave front (measured by the WaveFront Sensor (WFS)).Until very recently, the majority of AO systems had used the Shack-Hartmann (SH) WFS. New concepts based on Fourier filtering (the Pyramid or the Zernike WFSs) have however just been put in operation in several professional observatories and their results seem to outperform the SH. Since they would potentially be chosen for the AO systems of the future ELTs, this thesis aims to consolidate their theoretical understanding and to optimize these Fourier based WFSs.We firstly develop a mathematical framework which describes all these WFSs under an unique formalism. It allows to generalize the pre-existent designs -a WFS thus becomes a "WFS class"- by considering their optical parameters as flexible quantities. We then explored the two Pyramid and Zernike classes to identify the influence of class' parameters on performance criteria in order to optimize optical designs with regard to the instrumental requirements. New configurations of the Pyramid class -that we called Flattened pyramids- show promising behaviors and are studied in details

Variation around the Pyramid theme: optical recombination and optimal use of photons

We propose a new type of Wave Front Sensor (WFS) derived from the Pyramid WFS (PWFS). This new WFS,called the Flattened Pyramid-WFS (FPWFS), has a reduced Pyramid angle in order to optically overlap the fourpupil images into an unique intensity. This map is then used to derive the phase information. In this paper thisnew WFS is compared to two existing WFSs, namely the PWFS and the Modulated PWFS (MPWFS) followingtests about sensitivity, linearity range and low photon flux behavior. The FPWFS turns out to be more linearthan a modulated pyramid for the high-spatial order aberrations but it provides an improved sensitivity comparedto the non-modulated pyramid. Furthermore, the pixel arrangement being more efficient than for the PWFS,the FPWFS seems particularly well suited for high-contrast applications. A first quick study of the influenceof the angle of the pyramid shows that, it is possible, with a quite constant linear range, to adjust the spatialfrequencies range where the sensitivity is the best. Finally, we also show that replacing the pyramid with aflattened cone also leads to very promising noise propagation properties

Variation around the Pyramid theme: optical recombination and optimal use of photons

We propose a new type of Wave Front Sensor (WFS) derived from the Pyramid WFS (PWFS). This new WFS,called the Flattened Pyramid-WFS (FPWFS), has a reduced Pyramid angle in order to optically overlap the fourpupil images into an unique intensity. This map is then used to derive the phase information. In this paper thisnew WFS is compared to two existing WFSs, namely the PWFS and the Modulated PWFS (MPWFS) followingtests about sensitivity, linearity range and low photon flux behavior. The FPWFS turns out to be more linearthan a modulated pyramid for the high-spatial order aberrations but it provides an improved sensitivity comparedto the non-modulated pyramid. Furthermore, the pixel arrangement being more efficient than for the PWFS,the FPWFS seems particularly well suited for high-contrast applications. A first quick study of the influenceof the angle of the pyramid shows that, it is possible, with a quite constant linear range, to adjust the spatialfrequencies range where the sensitivity is the best. Finally, we also show that replacing the pyramid with aflattened cone also leads to very promising noise propagation properties

General formalism for Fourier-based wave front sensing

International audienceWe introduce in this paper a general formalism for Fourier-based wave front sensing. To do so, we consider the filtering mask as a free parameter. Such an approach allows us to unify sensors like the pyramid wave front sensor (PWFS) and the Zernike wave front sensor (ZWFS). In particular, we take the opportunity to generalize these two sensors in terms of sensors’ class, where optical quantities such as the apex angle for the PWFS or the depth of the Zernike mask for the ZWFS become free parameters. In order to compare all the generated sensors of these two classes thanks to common performance criteria, we first define a general phase-linear quantity that we call meta-intensity. Analytical developments allow us to then split the perfectly phase-linear behavior of a WFS from the nonlinear contributions, making robust and analytic definitions of the sensitivity and the linearity range possible. Moreover, we define a new quantity called the SD factor, which characterizes the trade-off between these two antagonistic quantities. These developments are generalized for a modulation device and polychromatic light. A nonexhaustive study is finally conducted on the two classes, allowing us to retrieve the usual results and also make explicit the influence of the optical parameters introduced above