Programmed but Arbitrary Control Minimization of Amplitude and phase for speckle Nulling (PACMAN)

We revive a cross-platform focal-plane wavefront sensing and control algorithm originally released in 1980 and show that it can provide significant contrast improvements over conventional control methods on coronagraphic instruments. Its simplicity makes it applicable to various coronagraph models and we demonstrate it on a classical Lyot coronagraph and a phase-apodized pupil Lyot coronagraph, both in simulation and in laboratory experiments. Surprisingly, it had been forgotten for decades, but we present its unbeatable advantages considering the increase in computational power in the last 40 years. We consider it a major game changer in the planning for future, space-based high-contrast imaging missions and recommend it be intensively revisited by all readers.Comment: Intended for public access on 1 April 202

Correction active des aberrations de bas ordre pour l'observation d'exo-Terres avec les futurs grands observatoires

Imaging exoplanets, especially exo-Earths, is a considerable technological challenge because of the high contrast, up to 10^-10, and the small angular separation, less than 50 milliarcseconds, between the star and the planet. Large telescopes represent an attractive option to achieve the resolution and sensitivity to access the light flux from the planet. These future observatories will use segmented primary mirrors, leading to a total diameter of more than 30 m on the ground and more than 6 m in space. In addition, combining these telescopes with a coronagraph that rejects starlight is an encouraging solution to make the contrast more favorable for exoplanet studies.Recent designs of coronagraphs coupled with active optics allow digging a dark hole in the image of an observed star, thus providing the required contrast levels. Nevertheless, many sources of instability, such as thermal variations, vibrations, or mechanical constraints, generate optical aberrations that prevent the maintenance of contrast during observations. The combined use of several wavefront sensors and deformable mirrors appears to be a critical means to control these aberrations in real time.By revisiting adaptive optics (AO) methods designed to correct for the effects of atmospheric turbulence on the image of a star, this thesis shows approaches for active stabilization of a coronagraph contrast. The proposed solutions cover various spatial or temporal frequency regimes of aberrations, in the case of large telescopes on the ground or in space. In the presented approach, a Zernike wavefront sensor (ZWFS) is implemented to take advantage of its excellent sensitivity and thus to optimally exploit the available photons.For future large, segmented ground-based telescopes, such as the Extremely Large Telescope or the Thirty Meter Telescope, the first solution presented in this thesis is to use a second stage AO inserted between the first stage and a coronagraph. This ZWFS-based strategy enables the measurement and the correction for several types of aberrations as close as possible to the coronagraph. These aberrations include non-common path aberrations that are invisible by the AO, uncorrected turbulence residuals, or cophasing errors of the primary mirror segments. Experimental tests on the MITHiC testbed have validated the control of these errors at the nanometric level.In the prospect of post-James Webb telescopes, a second approach is then proposed to stabilize a coronagraph designed for space applications. In this case, the ZWFS is installed in the light rejected by the coronagraph and containing information on the low-order aberrations. The sensor thus leaves the signal intact for observations. A first demonstration of a control loop is presented on the HiCAT testbed. The experiment validates the correction at the sub-nanometric level of aberrations introduced by testbed perturbations, as well as the blind stabilization of the contrast in the focal plane. In the perspective of a complete high-contrast imaging architecture, the correction loop is then combined with the dark hole algorithm with common deformable mirrors, paving the way for parallel use of multiple wavefront control systems.These coronagraph stabilization results represent an essential step towards the realization of observatories capable of imaging and spectrally analyzing Earth-like planets.Former des images d'exoplanètes, et plus particulièrement des exo-Terres, représente un défi technologique considérable en raison du fort contraste, allant jusqu'à 10^-10, et de la faible séparation angulaire, moins de 50 milliarcsecondes, entre l'étoile et la planète. Les grands télescopes constituent une option attrayante pour obtenir la résolution et la sensibilité suffisantes afin d'accéder au flux lumineux issu de la planète. Ces futurs observatoires disposeront de miroirs primaires segmentés, débouchant sur un diamètre total de plus de 30 m au sol et au delà de 6 m dans l'espace. En outre, combiner ces télescopes avec un coronographe qui rejette la lumière stellaire est une solution encourageante pour rendre le contraste plus favorable à l'étude d'exoplanètes.Des designs récents de coronographes couplés à de l'optique active permettent de creuser un trou sombre dans l'image d'une étoile observée, afin d'atteindre les niveaux de contraste requis. Néanmoins, de nombreuses sources d'instabilité, telles que les variations thermiques, les vibrations ou les contraintes mécaniques, génèrent des aberrations optiques qui empêchent le maintien du contraste pendant les observations. L'utilisation combinée de plusieurs analyseurs de surface d'onde et miroirs déformables apparaît alors comme un moyen primordial pour maîtriser ces aberrations en temps réel.En revisitant les méthodes d'optique adaptative (OA) destinées à corriger les effets de la turbulence atmosphérique sur l'image d'une étoile, cette thèse propose des approches de stabilisation active de contraste d'un coronographe pour des aberrations à divers régimes de fréquences spatiales ou temporelles, dans le cas de grands télescopes au sol ou dans l'espace. Dans la démarche présentée, un analyseur de surface d'onde de Zernike (ZWFS) est mis en œuvre pour tirer profit de son excellente sensibilité et ainsi exploiter au mieux les photons disponibles.Pour les futurs grands télescopes segmentés au sol, tels que l'Extremely Large Telescope ou le Thirty Meter Telescope, la première solution présentée dans cette thèse est l'emploi d'un deuxième étage d'OA intercalé entre le premier étage et un coronographe. Cette stratégie basée sur un ZWFS rend possible la mesure et la correction de plusieurs types d'aberrations au plus près du coronographe, tels que les aberrations de chemin non-communes non visibles par l'OA, les résidus de turbulence non corrigés, ou les défauts de cophasage des segments du miroir primaire. Des tests expérimentaux sur le banc MITHiC ont permis de valider le contrôle de ces erreurs à un niveau nanométrique.Dans la perspective des télescopes post-James Webb, une seconde approche est ensuite proposée afin de stabiliser un coronographe conçu pour le spatial. Dans ce cas, le ZWFS est installé dans la lumière rejetée par le coronographe et contenant des informations sur les aberrations de bas ordre. L'analyseur laisse ainsi intact le signal pour les observations. Une première démonstration d'une boucle d'asservissement de ces erreurs est présentée sur le banc HiCAT. L'expérience valide la correction des aberrations introduites par des perturbations du banc au niveau sub-nanométrique, ainsi que la stabilisation du contraste en plan focal de manière aveugle. Dans l'optique d'un système complet d'imagerie à haut contraste, la boucle de correction est ensuite combinée à l'algorithme creusant les trous sombres en plan focal avec des miroirs déformables communs, ouvrant la voie à une utilisation en parallèle de multiples systèmes de contrôle de front d'onde.Ces résultats de stabilisation de coronographes constituent une étape essentielle vers la réalisation d'observatoires capables d'imager et d'analyser spectralement des planètes similaires à la Terre

Correction active des aberrations de bas ordre pour l'observation d'exo-Terres avec les futurs grands observatoires

Imaging exoplanets, especially exo-Earths, is a considerable technological challenge because of the high contrast, up to 10^-10, and the small angular separation, less than 50 milliarcseconds, between the star and the planet. Large telescopes represent an attractive option to achieve the resolution and sensitivity to access the light flux from the planet. These future observatories will use segmented primary mirrors, leading to a total diameter of more than 30 m on the ground and more than 6 m in space. In addition, combining these telescopes with a coronagraph that rejects starlight is an encouraging solution to make the contrast more favorable for exoplanet studies.Recent designs of coronagraphs coupled with active optics allow digging a dark hole in the image of an observed star, thus providing the required contrast levels. Nevertheless, many sources of instability, such as thermal variations, vibrations, or mechanical constraints, generate optical aberrations that prevent the maintenance of contrast during observations. The combined use of several wavefront sensors and deformable mirrors appears to be a critical means to control these aberrations in real time.By revisiting adaptive optics (AO) methods designed to correct for the effects of atmospheric turbulence on the image of a star, this thesis shows approaches for active stabilization of a coronagraph contrast. The proposed solutions cover various spatial or temporal frequency regimes of aberrations, in the case of large telescopes on the ground or in space. In the presented approach, a Zernike wavefront sensor (ZWFS) is implemented to take advantage of its excellent sensitivity and thus to optimally exploit the available photons.For future large, segmented ground-based telescopes, such as the Extremely Large Telescope or the Thirty Meter Telescope, the first solution presented in this thesis is to use a second stage AO inserted between the first stage and a coronagraph. This ZWFS-based strategy enables the measurement and the correction for several types of aberrations as close as possible to the coronagraph. These aberrations include non-common path aberrations that are invisible by the AO, uncorrected turbulence residuals, or cophasing errors of the primary mirror segments. Experimental tests on the MITHiC testbed have validated the control of these errors at the nanometric level.In the prospect of post-James Webb telescopes, a second approach is then proposed to stabilize a coronagraph designed for space applications. In this case, the ZWFS is installed in the light rejected by the coronagraph and containing information on the low-order aberrations. The sensor thus leaves the signal intact for observations. A first demonstration of a control loop is presented on the HiCAT testbed. The experiment validates the correction at the sub-nanometric level of aberrations introduced by testbed perturbations, as well as the blind stabilization of the contrast in the focal plane. In the perspective of a complete high-contrast imaging architecture, the correction loop is then combined with the dark hole algorithm with common deformable mirrors, paving the way for parallel use of multiple wavefront control systems.These coronagraph stabilization results represent an essential step towards the realization of observatories capable of imaging and spectrally analyzing Earth-like planets.Former des images d'exoplanètes, et plus particulièrement des exo-Terres, représente un défi technologique considérable en raison du fort contraste, allant jusqu'à 10^-10, et de la faible séparation angulaire, moins de 50 milliarcsecondes, entre l'étoile et la planète. Les grands télescopes constituent une option attrayante pour obtenir la résolution et la sensibilité suffisantes afin d'accéder au flux lumineux issu de la planète. Ces futurs observatoires disposeront de miroirs primaires segmentés, débouchant sur un diamètre total de plus de 30 m au sol et au delà de 6 m dans l'espace. En outre, combiner ces télescopes avec un coronographe qui rejette la lumière stellaire est une solution encourageante pour rendre le contraste plus favorable à l'étude d'exoplanètes.Des designs récents de coronographes couplés à de l'optique active permettent de creuser un trou sombre dans l'image d'une étoile observée, afin d'atteindre les niveaux de contraste requis. Néanmoins, de nombreuses sources d'instabilité, telles que les variations thermiques, les vibrations ou les contraintes mécaniques, génèrent des aberrations optiques qui empêchent le maintien du contraste pendant les observations. L'utilisation combinée de plusieurs analyseurs de surface d'onde et miroirs déformables apparaît alors comme un moyen primordial pour maîtriser ces aberrations en temps réel.En revisitant les méthodes d'optique adaptative (OA) destinées à corriger les effets de la turbulence atmosphérique sur l'image d'une étoile, cette thèse propose des approches de stabilisation active de contraste d'un coronographe pour des aberrations à divers régimes de fréquences spatiales ou temporelles, dans le cas de grands télescopes au sol ou dans l'espace. Dans la démarche présentée, un analyseur de surface d'onde de Zernike (ZWFS) est mis en œuvre pour tirer profit de son excellente sensibilité et ainsi exploiter au mieux les photons disponibles.Pour les futurs grands télescopes segmentés au sol, tels que l'Extremely Large Telescope ou le Thirty Meter Telescope, la première solution présentée dans cette thèse est l'emploi d'un deuxième étage d'OA intercalé entre le premier étage et un coronographe. Cette stratégie basée sur un ZWFS rend possible la mesure et la correction de plusieurs types d'aberrations au plus près du coronographe, tels que les aberrations de chemin non-communes non visibles par l'OA, les résidus de turbulence non corrigés, ou les défauts de cophasage des segments du miroir primaire. Des tests expérimentaux sur le banc MITHiC ont permis de valider le contrôle de ces erreurs à un niveau nanométrique.Dans la perspective des télescopes post-James Webb, une seconde approche est ensuite proposée afin de stabiliser un coronographe conçu pour le spatial. Dans ce cas, le ZWFS est installé dans la lumière rejetée par le coronographe et contenant des informations sur les aberrations de bas ordre. L'analyseur laisse ainsi intact le signal pour les observations. Une première démonstration d'une boucle d'asservissement de ces erreurs est présentée sur le banc HiCAT. L'expérience valide la correction des aberrations introduites par des perturbations du banc au niveau sub-nanométrique, ainsi que la stabilisation du contraste en plan focal de manière aveugle. Dans l'optique d'un système complet d'imagerie à haut contraste, la boucle de correction est ensuite combinée à l'algorithme creusant les trous sombres en plan focal avec des miroirs déformables communs, ouvrant la voie à une utilisation en parallèle de multiples systèmes de contrôle de front d'onde.Ces résultats de stabilisation de coronographes constituent une étape essentielle vers la réalisation d'observatoires capables d'imager et d'analyser spectralement des planètes similaires à la Terre

Estimating low-order aberrations through a Lyot coronagraph with a Zernike wavefront sensor for exoplanet imaging

International audienceImaging exo-Earths is an exciting but challenging task because of the 10-10 contrast ratio between these planets and their host star at separations narrower than 100 mas. Large segmented aperture space telescopes enable the sensitivity needed to observe a large number of planets. Combined with coronagraphs with wavefront control, they present a promising avenue to generate a high-contrast region in the image of an observed star. Another key aspect is the required stability in telescope pointing, focusing, and co-phasing of the segments of the telescope primary mirror for long-exposure observations of rocky planets for several hours to a few days. These wavefront errors should be stable down to a few tens of picometers RMS, requiring a permanent active correction of these errors during the observing sequence. To calibrate these pointing errors and other critical low-order aberrations, we propose a wavefront sensing path based on Zernike phase-contrast methods to analyze the starlight that is filtered out by the coronagraph at the telescope focus. In this work we present the analytical retrieval of the incoming low order aberrations in the starlight beam that is filtered out by an Apodized Pupil Lyot Coronagraph, one of the leading coronagraph types for starlight suppression. We implement this approach numerically for the active control of these aberrations and present an application with our first experimental results on the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed, the STScI testbed for Earth-twin observations with future large space observatories, such as LUVOIR and HabEx, two NASA flagship mission concepts

Experimental validation of active control of low-order aberrations with a Zernike sensor through a Lyot coronagraph

International audienceFuture large segmented space telescopes and their coronagraphic instruments are expected to provide the resolution and sensitivity to observe Earth-like planets with a 1010 contrast ratio at less than 100 mas from their host star. Advanced coronagraphs and wavefront control methods will enable the generation of high-contrast dark holes in the image of an observed star. However, drifts in the optical path of the system will lead to pointing errors and other critical low-order aberrations that will prevent maintenance of this contrast. To measure and correct for these errors, we explore the use of a Zernike wavefront sensor (ZWFS) in the starlight rejected and filtered by the focal plane mask of a Lyot-type coronagraph. In our previous work, the analytical phase reconstruction formalism of the ZWFS was adapted for a filtered beam. We now explore strategies to actively compensate for these drifts in a segmented pupil setup on the High-contrast imager for Complex Aperture Telescopes (HiCAT). This contribution presents laboratory results from closed-loop compensation of bench internal turbulence as well as known introduced aberrations using phase conjugation and interaction matrix approaches. We also study the contrast recovery in the image plane dark hole when using a closed loop based on the ZWFS

Experimental validation of exoplanet centring strategies for high dispersion coronagraphy

The combination on large ground-based telescopes of extreme adaptive optics (ExAO), coronagraphy and high-dispersion spectroscopy is starting to emerge as a powerful technique for the direct characterisation of giant exoplanets. High spectral resolution not only brings a major gain in terms of accessible spectral features, but it also enables to better disentangle between the stellar and planetary signals thanks to the much higher spectral content. On-going projects such as KPIC for Keck, REACH for Subaru and HiRISE for the VLT base their observing strategy on the use of a few science fibres, one of which is dedicated to sampling the PSF of the planet, while the others sample the stellar residuals in the speckle field. The main challenge in this approach is to blindly centre the science fibre on the planet’s PSF, with typically a tolerance of less than one resolution element (0.1 λ/D). Several possible centring strategies can be adopted, either based on calibration fibres retro-injecting signal to mark the position of the science fibres or based on the use of focal-plane features introduced by the ExAO system. In this proceeding, we describe different possible approaches and we compare their centring accuracy using the MITHiC high-contrast imaging testbed. For this work, MITHiC has been upgraded to reproduce a setup close to the one that will be adopted in HiRISE, the coupling system that will soon be implemented between VLT/SPHERE and VLT/CRIRES+. Our results demonstrate that reaching a specification accuracy of 0.1 λ/D is extremely challenging regardless of the chosen centring strategy. It requires a high level of accuracy at every step of the centring procedure, which can be reached with very stable instruments. We studied the contributors to the centring error in the case of MITHiC and we quantified some of the most important terms

Imageur à haut contraste pour les télescopes à ouverture complexe (HICAT) : 8. Démonstration de Dark Zone avec boucles fermées simultanées à bas ordres

International audienceWe present recent laboratory results demonstrating high-contrast coronagraphy for the future space-based large IR/Optical/Ultraviolet telescope recommended by the Decadal Survey. The High-contrast Imager for Complex Aperture Telescopes (HiCAT) testbed aims to implement a system-level hardware demonstration for segmented aperture coronagraphs with wavefront control. The telescope hardware simulator employs a segmented deformable mirror with 37 hexagonal segments that can be controlled in piston, tip, and tilt. In addition, two continuous deformable mirrors are used for high-order wavefront sensing and control. The low-order sensing subsystem includes a dedicated tip-tilt stage, a coronagraphic target acquisition camera, and a Zernike wavefront sensor that is used to measure and correct low-order aberration drifts. We explore the performance of a segmented aperture coronagraph both in "static" operations (limited by natural drifts and instabilities) and in "dynamic" operations (in the presence of artificial wavefront drifts added to the deformable mirrors), and discuss the estimation and control strategies used to reach and maintain the dark-zone contrast using our low-order wavefront sensing and control. We summarize experimental results that quantify the performance of the testbed in terms of contrast, inner/outer working angle and bandpass, and analyze limiting factors

Imageur à haut contraste pour les télescopes à ouverture complexe (HICAT) : 8. Démonstration de Dark Zone avec boucles fermées simultanées à bas ordres

International audienceWe present recent laboratory results demonstrating high-contrast coronagraphy for the future space-based large IR/Optical/Ultraviolet telescope recommended by the Decadal Survey. The High-contrast Imager for Complex Aperture Telescopes (HiCAT) testbed aims to implement a system-level hardware demonstration for segmented aperture coronagraphs with wavefront control. The telescope hardware simulator employs a segmented deformable mirror with 37 hexagonal segments that can be controlled in piston, tip, and tilt. In addition, two continuous deformable mirrors are used for high-order wavefront sensing and control. The low-order sensing subsystem includes a dedicated tip-tilt stage, a coronagraphic target acquisition camera, and a Zernike wavefront sensor that is used to measure and correct low-order aberration drifts. We explore the performance of a segmented aperture coronagraph both in "static" operations (limited by natural drifts and instabilities) and in "dynamic" operations (in the presence of artificial wavefront drifts added to the deformable mirrors), and discuss the estimation and control strategies used to reach and maintain the dark-zone contrast using our low-order wavefront sensing and control. We summarize experimental results that quantify the performance of the testbed in terms of contrast, inner/outer working angle and bandpass, and analyze limiting factors

Connecting SPHERE and CRIRES+ for the characterisation of young exoplanets at high spectral resolution: status update of VLT/HiRISE

International audienceNew generation exoplanet imagers on large ground-based telescopes are highly optimised for the detection of young giant exoplanets in the near-infrared, but they are intrinsically limited for their characterisation by the low spectral resolution of their integral field spectrographs (R < 100). High-dispersion spectroscopy at R 10 4 would be a powerful tool for the characterisation of these planets, but there is currently no high-resolution spectrograph with extreme adaptive optics and coronagraphy that would enable such characterisation. With project HiRISE we propose to use fiber coupling to combine the capabilities of two flagship instruments at the Very Large Telescope in Chile: the exoplanet imager SPHERE and the high-resolution spectrograph CRIRES+. The coupling will be implemented at the telescope in early 2023. We provide a general overview of the implementation of HiRISE, of its assembly, integration and testing (AIT) phase in Europe, and a brief assessment of its expected performance based on the final hardware