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The detection and characterization of Earth-like exoplanets around Sun-like stars for future flagship missions requires coronagraphs to achieve contrasts on the order of 1e-10 at close angular separations and over large spectral bandwidths (>=20%). We present our progress thus far on exploring the potential for scalar vortex coronagraphs (SVCs) in direct exoplanet imaging. SVCs are an attractive alternative to vector vortex coronagraphs (VVCs), which have recently demonstrated 6e-9 raw contrast in 20% broadband light but are polarization dependent. SVCs imprint the same phase ramp on the incoming light and do not require polarization splitting, but are inherently limited by their chromatic behavior. Several SVC designs have been proposed in recent years to solve this issue by modulating or wrapping the azimuthal phase function according to specific patterns. For one such design, the staircase SVC, we present our best experimental SVC results demonstrating raw contrast of 2e-7 in 10% broadband light. Since SVC broadband performance and aberration sensitivities are highly dependent on topology, we conducted a comparative study of several SVC designs to optimize for high contrast across a range of bandwidths. Furthermore, we present a new coronagraph optimization tool to predict performance in order to find an achromatic solution.

A key challenge of high contrast imaging (HCI) is to differentiate a speckle from an exoplanet signal. The sources of speckles are a combination of atmospheric residuals and aberrations in the non-common path. Those non-common path aberrations (NCPA) are particularly challenging to compensate for as they are not directly measured, and because they include static, quasi-static and dynamic components. The proposed method directly addresses the challenge of compensating the NCPA. The algorithm DrWHO - Direct Reinforcement Wavefront Heuristic Optimisation - is a quasi-real-time compensation of static and dynamic NCPA for boosting image contrast. It is an image-based lucky imaging approach, aimed at finding and continuously updating the ideal reference of the wavefront sensor (WFS) that includes the NCPA, and updating this new reference to the WFS. Doing so changes the point of convergence of the AO loop. We show here the first results of a post-coronagraphic application of DrWHO. DrWHO does not rely on any model nor requires accurate wavefront sensor calibration, and is applicable to non-linear wavefront sensing situations. We present on-sky performances using a pyramid WFS sensor with the Subaru coronagraph extreme AO (SCExAO) instrument.

The astronomical community is paving the way for extremely large telescopes, with notably the ELT, a European telescope equipped with a 40-meter diameter primary mirror. This telescope will enable the study of fainter astrophysical objects with a better resolution, provided that atmospheric turbulence which degrades imaging quality is properly removed. Adaptive Optics (AO) provides such a compensation in real time. Its principle is based on measuring the incoming wavefront distortions using a specific sensor, this information is then transmitted through a computer to a deformable mirror. The latter physically and almost instantaneously compensates for the distortions caused by the turbulence. In this context, the pyramid wavefront sensor and its high sensitivity was chosen to equip the AO systems of the ELT instruments. However, the implementation of the AO systems has to take into account the unique structure of the ELT and in particular, the fragmentation of its pupil into six petals by the spider which supports the secondary mirror. With tens of centimeters thickness, i.e. several times the turbulence coherence length, the spider arms break the continuity of the incoming wavefront. It then becomes necessary to measure the phase differences between each of the six petals, called the differential pistons, in order to reconstruct the full wavefront. Based on comprehensive numerical simulations of the ELT and deformable mirror parameters, this thesis studies the capacities and limitations of the pyramid sensor to perform differential piston measurements including different instrumental configurations, and extends these discussions to another sensor, the Zernike phase mask. The proposed conclusions can also be applied to thermal effects that occur within the dome of the telescope and which introduce phase discontinuities. Facing the poor reliability of the sensors to measure differential pistons in the presence of strong AO residuals relative to their sensing wavelength, two algorithmic reconstruction methods based on Kolmogorov's theory are developed: the use of an estimator minimizing the variance of the reconstructed phase error and a modal basis ensuring the continuity of the wavefront correction on either side of the spider arms. Finally, experiments were carried out in laboratory to validate the conclusions of the numerical studies. They were made on an AO bench which reproduces the relevant characteristics of the ELT. Thus, this thesis explores the problematics of differential pistons inside a same telescope pupil and introduces the need for a sensor specifically addressing this measurement., La communauté astronomique prépare l’arrivée des télescopes géants avec notamment l’ELT, télescope européen équipé d’un miroir primaire de près de 40 mètres de diamètre. Ce télescope rendra possible l’étude d’objets astrophysiques plus ténus avec une meilleure résolution, à la condition de se défaire correctement des effets de la turbulence atmosphérique qui altèrent la qualité d’imagerie. L’optique adaptative (OA) permet une telle compensation en temps réel. Son principe repose sur la mesure de la déformation du front d’onde incident à l’aide d’un analyseur, cette information est ensuite transmise par le biais d’un calculateur à un miroir déformable. Celui-ci compense alors physiquement et de manière quasi instantanée les distorsions causées par la turbulence. C’est dans ce cadre que l’analyseur de front d’onde pyramide et sa grande sensibilité a été retenu pour équiper l’OA des instruments de l’ELT.Dans sa mise en œuvre, l’OA va cependant devoir s’adapter à la structure inédite de l’ELT et notamment à la fragmentation de sa pupille en 6 pétales par l’araignée qui supporte son miroir secondaire. Avec plusieurs dizaines de centimètres d’épaisseur, soit plusieurs fois la longueur de cohérence de la turbulence, les bras de l’araignée rompent la continuité du front d’onde incident. Il devient alors nécessaire de mesurer les différences de phase entre chacun des 6 pétales, appelées pistons différentiels, afin de reconstruire pleinement le front d’onde.En s’appuyant sur des simulations numériques complètes des paramètres de l’ELT et de son miroir déformable, cette thèse étudie les capacités et limites de l’analyseur pyramide à effectuer ce type de mesure pour différentes configurations instrumentales et étend ces discussions à un autre analyseur, le masque de Zernike. Les conclusions proposées peuvent également s’appliquer aux effets thermiques qui s’exercent au sein même du dôme et introduisent des discontinuités de phase. Face à l’incapacité des analyseurs à mesurer les pistons différentiels en présence de forts résidus d’OA relativement à leur longueur d’onde d'utilisation, deux méthodes de reconstruction s’appuyant sur la théorie de Kolmogorov sont développées : l’utilisation d’un estimateur minimisant la variance de l’erreur de la phase reconstruite et la formulation d’une base modale permettant d’assurer la continuité du front d’onde de part et d’autre des bras de l’araignée. Enfin, des expérimentations ont été menées en laboratoire pour valider les conclusions des études numériques. Elles ont été faites sur un banc d’OA pour lequel les caractéristiques pertinentes de l’ELT sont reproduites. Cette thèse explore ainsi les problématiques de pistons différentiels à l’intérieur d’une même pupille de télescope et introduit enfin la nécessité de mettre en œuvre un analyseur spécifiquement dédié à leur mesure.

The presence of a six legged 50cm-wide spider supporting the secondary mirror of the Extremely Large Telescope (ELT) breaks the spatial continuity of the incoming wave-front. Atmospheric turbulence, low wind effect and thermo-mechanical drift of the deformable mirror are all potential contributors to discontinuities between the six segments of the ELT pupil. It is therefore necessary to measure these differential pistons in order to reconstruct the full wave-front. The pyramid wave-front sensor is currently the preferred design for adaptive optics systems. However, it was shown to be a poor differential piston sensor in the visible, under partial turbulence correction, leading to a severe degradation of the image quality. Using the COMPASS adaptive optics (AO) simulator, we first investigate strategies to ensure the spatial continuity of the correction applied on the deformable mirror. These methods present some limitations in strong seeing conditions, when the corrugated phase varies a lot below the spider legs, and lead to a significant degradation of the Strehl Ratio. To tackle this critical issue, we propose as a second step to couple the continuity hypothesis with a petalometer: a sensor specifically designed for sensing the differential piston. As candidates, we compare an unmodulated pyramid, a Zernike wavefront-sensor and a Zernike coupled with a field stop. We present results regarding their sensitivity and their reliability when working in operation, in presence of realistic AO residuals.

With the upcoming giant class of telescopes, Adaptive Optics (AO) has become more essential than ever before to get access to the full potential offered by those telescopes. The complexity of such AO systems is reaching extreme heights, and disruptive developments will have to be made in order to build them. One of the critical component of a AO system is the Real Time Controller (RTC) which will have to compute the slopes and the Deformable Mirror (DM) commands at high frequency, in a range of 0.5 to several kHz. Since the complexity of the computations involved in the RTC is increasing with the size of the telescope, fulfilling RTC requirements for Extremely Large Telescope (ELT) class is a challenge. As an example, the MICADO SCAO (Single Conjugate Adaptive Optics) system requires around 1 TMAC/s for the RTC to get sufficient performance. This complexity brings the need for High Performance Computing (HPC) techniques and standards, such as the use of hardware accelerator like GPU. On top of that, building a RTC is often project-dependent as the components and the interfaces change from one instrument to an other. The COSMIC platforms aims at developing a common AO RTC platform which is meant to be powerful, modular and available to the AO community. This development is a joint effort between Observatoire de Paris and the Australian National University (ANU) in collaboration with the Subaru Telescope. We focus here on the current status of the core hard real-time component of this platform. The H-RTC pipeline is composed of Business Units (BU): each BU is an independent process in charge of one particular operation, such as Matrix Vector Multiply (MVM) or centroid computation, that can be made on CPU or on GPU. BUs read and write data on Shared Memory (SHM) handled by the CACAO framework. Synchronization between each BU can then be made either by using semaphore or by busy waiting on the GPU to ensure very low jitter. The RTC pipeline can then be controlled through a Python interface. One of the key point of this architecture is that the interfaces of a BU with the various SHM is abstracted, so adding a new BU in the collection of available ones is straight forward. This approach allows a high performance, scalable, modular and configurable RTC pipeline that could fit the needs of any AO system configuration. Performance has been measured on a MICADO SCAO scale RTC pipeline with around 25,000 slopes by 5,000 actuators on a DGX-1 system equipped with 8 Tesla V100 GPUs. The considered pipeline is composed of two BUs : the first one takes an input the raw pyramid WFS image (produced by simulator), applies on it dark and flat references, and then extract the useful pixel from the image. The second BU performs the MVM and the integration of the commands following a classical integrator command law. Synchronization between the BU is made through GPU busy waiting on the BU inputs. Performance obtained shows a mean latency up to 235 μs using 4 GPUs, with a jitter of 4.4 μs rms and a maximum jitter of 30 μs

Context. The pyramid was proven to be a highly sensitive and versatile wave-front sensor (WFS) and has been selected to be installed on the single conjugate adaptive optics (AO) systems of the extremely large telescope (ELT). The pupil of the ELT is fragmented by the secondary support spider arms, which are larger than the spatial coherence length of the atmospheric turbulence. This causes a rupture of the incoming wavefront continuity, which means that we need to be able to measure the differential pistons across the spider arms to achieve full wavefront reconstruction. Aims. We investigate the reaction of the modulated pyramid WFS to discontinuous aberrations in presence of phase residuals after AO compensation for a range of expected observing conditions at the location of the ELT. We then explore some parameters of the sensor in order to improve its sensitivity to the wavefront discontinuities, including optical gain compensation, specific sensor modulation paths, and the number of faces of the pyramidal prism. Methods. We derived sensitivity loss and modal cross-talk strength coefficients around static post-AO residual phases using the COMPASS end-to-end AO simulation software. Moreover, extensive closed-loop AO simulations let us derive two wavefront error criteria that are appropriate for a fragmented pupil geometry. We used these to assess the wavefront reconstruction performance. Results. We show that on the ELT, the pyramid experiences a drastic loss in sensitivity and also non-linear modal cross-talks. Added to the limited capture range, this makes it poorly suited for phase discontinuity measurements at visible wavelengths. The strategies we studied to increase the sensitivity and reduce the modal cross-talk provide an improvement of the reconstruction for low D/r0(λWFS) values, that is, for a K-band pyramid. In presence of a large residual wavefront variance, however, a similar sensor in visible light fails to provide the necessary trade-off to measure both the continuous modes and the wavefront discontinuities. Conclusions. The ELT instruments, designed with only visible-band pyramid WFSs, will not be able to perform a direct measurement of the wavefront discontinuities. They will have to rely on Kolmogorov statistics to restore the continuity of the atmospheric corrugated wavefront. If any other source of discontinuities arises on the ELT, instruments will need an additional, dedicated WFS.

International audience; The pyramid wavefront sensor (PWFS), due to its extremely high sensitivity in comparison to the Shack-Hartmann WFS, is the design choice of most single-conjugate adaptive optics (AO) instruments currently being developed for extremely large telescopes (ELTs). This sensitivity benefit is served, however, with several technical drawbacks to overcome, one of which is the intrinsic non-linearity of the sensor. Even in modulated operation, the gradient measurement saturation non-linearity is strongly exacerbated by in-loop phase residuals for typical on-sky regimes, with high spatial frequencies inducing a dramatic sensitivity reduction for the controlled modes. This phenomenon has been dubbed "optical gain", and it was demonstrated that a modal gain compensation on an appropriate control basis provides an adequate mitigation of the sensitivity reduction, and improves the end-to-end performance on ELT SCAO systems across all relevant guide star magnitudes and seeing conditions. Several techniques have been proposed to achieve this nominal performance recovery, however most require offline computation of interaction matrices; or some alterations to the usual AO loop operation. In this paper, we present the CLOSE (Correlation-Locked Optimization StratEgy) algorithm, which achieves determination of the modal gains, in a real-time fashion, through the sole use of the modal decomposition of the WFS measurements. Real-time estimators are implemented to implicitly measure resonance levels of the integrator transfer function, which is used as a proxy to control the modal gains through a second-layer servo loop. Using this method, we achieve compensation of the optical gain in the command law, and a fully automatic optimization of the modal integrator depending on the signal-to-noise level. Simulated end-to-end results are presented, for stationary or quickly varying seeing conditions, all at the scale of the MICADO SCAO design on the ELT. We also discuss the applicability of the CLOSE scheme on other known AO problems.

International audience; MICADO is the European ELT first-light imager, working in the NIR at the telescope diffraction limit. In addition to a wide-field MCAO correction provided by MAORY, the ELT first-light adaptive optics module, MICADO will also benefit from a SCAO correction developed under MICADO's responsibility and jointly by MICADO and MAORY. In a phased approach of the AO integration at the ELT, SCAO will be the first AO mode to be tested with MICADO at the telescope and will be ultimately offered as a MAORY mode. The SCAO module is made of a pyramid wave-front sensor, a real-time computer, a dedicated calibration unit, a dichroic plate and a specific mechanical structure, all of them being associated to their corresponding electronics and (either real-time or non real-time) software. MICADO and its SCAO module have successfully passed their preliminary design review last November (2018) at ESO. In this contribution, we first present the design of the SCAO module and its different subsystems as they have been validated by the PDR board. We also present the development plan of the SCAO module, emphasizing the key milestones with their specific tests during the AIT phase. These tests comprise an on-sky experiment with the SCAO pyramid wave-front sensor and RTC prototypes, using the CANARY demonstrator at the William Herschel Telescope, as well as successive validations in lab in France, in Germany and finally in Chile. We finally make a focus on the calibrations we plan to perform until and during the operations at the telescope.