Deformable mirror-based pupil chopping for exoplanet imaging and adaptive optics

Due to turbulence in the atmosphere images taken from ground-based telescopes become distorted. With adaptive optics (AO) images can be given greater clarity allowing for better observations with existing telescopes and are essential for ground-based coronagraphic exoplanet imaging instruments. A disadvantage to many AO systems is that they use sensors that can not correct for non-common path aberrations. We have developed a new focal plane wavefront sensing technique to address this problem called deformable mirror (DM)-based pupil chopping. The process involves a coronagraphic or non-coronagraphic science image and a deformable mirror, which modulates the phase by applying a local tip/tilt every other frame which enables correcting for leftover aberrations in the wavefront after a conventional AO correction. We validate this technique with both simulations (for coronagraphic and non-coronagraphic images) and testing (for non-coronagraphic images) on UCSC's Santa Cruz Extreme AO Laboratory (SEAL) testbed. We demonstrate that with as low as 250 nm of DM stroke to apply the local tip/tilt this wavefront sensor is linear for low-order Zernike modes and enables real-time control, in principle up to kHz speeds to correct for residual atmospheric turbulence.Comment: Conference Proceeding for 2023 SPIE Optics & Photonics, Techniques and Instrumentation for Detection of Exoplanets X

Using the Gerchberg-Saxton algorithm to reconstruct non-modulated pyramid wavefront sensor measurements

Adaptive optics (AO) is a technique to improve the resolution of ground-based telescopes by correcting, in real-time, optical aberrations due to atmospheric turbulence and the telescope itself. With the rise of Giant Segmented Mirror Telescopes (GSMT), AO is needed more than ever to reach the full potential of these future observatories. One of the main performance drivers of an AO system is the wavefront sensing operation, consisting of measuring the shape of the above mentioned optical aberrations. Aims. The non-modulated pyramid wavefront sensor (nPWFS) is a wavefront sensor with high sensitivity, allowing the limits of AO systems to be pushed. The high sensitivity comes at the expense of its dynamic range, which makes it a highly non-linear sensor. We propose here a novel way to invert nPWFS signals by using the principle of reciprocity of light propagation and the Gerchberg-Saxton (GS) algorithm. We test the performance of this reconstructor in two steps: the technique is first implemented in simulations, where some of its basic properties are studied. Then, the GS reconstructor is tested on the Santa Cruz Extreme Adaptive optics Laboratory (SEAL) testbed located at the University of California Santa Cruz. This new way to invert the nPWFS measurements allows us to drastically increase the dynamic range of the reconstruction for the nPWFS, pushing the dynamics close to a modulated PWFS. The reconstructor is an iterative algorithm requiring heavy computational burden, which could be an issue for real-time purposes in its current implementation. However, this new reconstructor could still be helpful in the case of many wavefront control operations. This reconstruction technique has also been successfully tested on the Santa Cruz Extreme AO Laboratory (SEAL) bench where it is now used as the standard way to invert nPWFS signal

Adaptive optics with an infrared pyramid wavefront sensor at Keck

The study of cold or obscured, red astrophysical sources can significantly benefit from adaptive optics (AO) systems employing infrared (IR) wavefront sensors. One particular area is the study of exoplanets around M-dwarf stars and planet formation within protoplanetary disks in star-forming regions. Such objects are faint at visible wavelengths but bright enough in the IR to be used as a natural guide star for the AO system. Doing the wavefront sensing at IR wavelengths enables high-resolution AO correction for such science cases, with the potential to reach the contrasts required for direct imaging of exoplanets. To this end, a new near-infrared pyramid wavefront sensor (PyWFS) has been added to the Keck II AO system, extending the performance of the facility AO system for the study of faint red objects. We present the Keck II PyWFS, which represents a number of firsts, including the first PyWFS installed on a segmented telescope and the first use of an IR PyWFS on a 10-m class telescope. We discuss the scientific and technological advantages offered by IR wavefront sensing and present the design and commissioning of the Keck PyWFS. In particular, we report on the performance of the Selex Avalanche Photodiode for HgCdTe InfraRed Array detector used for the PyWFS and highlight the novelty of this wavefront sensor in terms of the performance for faint red objects and the improvement in contrast. The system has been commissioned for science with the vortex coronagraph in the NIRC2 IR science instrument and is being commissioned alongside a new fiber injection unit for NIRSPEC. We present the first science verification of the system—to facilitate the study of exoplanets around M-type stars

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

Adaptive optics with an infrared pyramid wavefront sensor at Keck

The study of cold or obscured, red astrophysical sources can significantly benefit from adaptive optics (AO) systems employing infrared (IR) wavefront sensors. One particular area is the study of exoplanets around M-dwarf stars and planet formation within protoplanetary disks in star-forming regions. Such objects are faint at visible wavelengths but bright enough in the IR to be used as a natural guide star for the AO system. Doing the wavefront sensing at IR wavelengths enables high-resolution AO correction for such science cases, with the potential to reach the contrasts required for direct imaging of exoplanets. To this end, a new near-infrared pyramid wavefront sensor (PyWFS) has been added to the Keck II AO system, extending the performance of the facility AO system for the study of faint red objects. We present the Keck II PyWFS, which represents a number of firsts, including the first PyWFS installed on a segmented telescope and the first use of an IR PyWFS on a 10-m class telescope. We discuss the scientific and technological advantages offered by IR wavefront sensing and present the design and commissioning of the Keck PyWFS. In particular, we report on the performance of the Selex Avalanche Photodiode for HgCdTe InfraRed Array detector used for the PyWFS and highlight the novelty of this wavefront sensor in terms of the performance for faint red objects and the improvement in contrast. The system has been commissioned for science with the vortex coronagraph in the NIRC2 IR science instrument and is being commissioned alongside a new fiber injection unit for NIRSPEC. We present the first science verification of the system—to facilitate the study of exoplanets around M-type stars

GPI 2.0: Performance Evaluation of the Wavefront Sensor's EMCCD

The Gemini Planet Imager (GPI) is a high contrast imaging instrument that aims to detect and characterize extrasolar planets. GPI is being upgraded to GPI 2.0, with several subsystems receiving a re-design to improve the instrument's contrast. To enable observations on fainter targets and increase stability on brighter ones, one of the upgrades is to the adaptive optics system. The current Shack-Hartmann wavefront sensor (WFS) is being replaced by a pyramid WFS with an low-noise electron multiplying CCD (EMCCD). EMCCDs are detectors capable of counting single photon events at high speed and high sensitivity. In this work, we characterize the performance of the HN\"u 240 EMCCD from N\"uv\"u Cameras, which was custom-built for GPI 2.0. The HN\"u 240 EMCCD's characteristics make it well suited for extreme AO: it has low dark current ($<$ 0.01 e-/pix/fr), low readout noise (0.1 e-/pix/fr at a gain of 5000), high quantum efficiency ( 90% at wavelengths from 600-800 nm; 70% from 800-900 nm), and fast readout (up to 3000 fps full frame). Here we present test results on the EMCCD's noise contributors, such as the readout noise, pixel-to-pixel variability and CCD bias. We also tested the linearity and EM gain calibration of the detector. All camera tests were conducted before its integration into the GPI 2.0 PWFS system.Comment: 16 pages, 14 figures. Conference Proceedings for AO4ELT7, held in June 2023 in Avignon, Franc

Optimisation de l'analyse de surface d'onde par filtrage de Fourier pour les systèmes d'optique adaptative à hautes performances

Avec les projets titanesques des "extremely large telescopes", l’astronomie mondiale va bientôt se doter d’outils à la puissance inégalée pour sonder l’univers. Ces télescopes suivent la trace de leurs compagnons de taille plus modeste, les télescopes de classe 8 m, dont les prouesses éclairent déjà le paysage scientifique depuis plus d’une vingtaine d’années. Ces télescopes au sol, présents ou futurs, se trouvent pourtant tous amputés d’une partie de leurs capacités dès leur mise en fonctionnement : la turbulence atmosphérique brouille les fronts d’onde de la lumière parvenant des astres, réduisant la résolution angulaire de ces géants à celle de simples télescopes amateurs. Pour lutter contre ce flou qui entache les images du cosmos, les scientifiques ont mis au point une technique appelée optique adaptative (OA). À l’aide d’optiques déformables, cette technique permet de compenser en temps réel les altérations de la lumière dues à l’atmosphère. Cette méthode équipe aujourd’hui tous les plus grands télescopes au sol, et est devenue indispensable pour un grand nombre d’applications astrophysiques. Motivé notamment par la chasse aux exoplanètes, des systèmes d’OA repoussant les limites de performances sont aujourd’hui mis au point. Les limites fondamentales de tels instruments reposent sur la qualité des mesures fournies par le dispositif optique au coeur de cette technique : l’analyseur de surface d’onde (ASO), dont l’objectif est d’estimer les formes imprégnées par la turbulence sur les fronts d’onde. Cette qualité des mesures est définie par deux grands aspects, la sensibilité et la dynamique.Cette thèse se concentre sur une classe très large d’ASO, appelée ASO à filtrage de Fourier. En s’appuyant sur un formalisme mathématique développé dans des travaux précédents, on y développe une meilleure compréhension de leur sensibilité grâce à l’étude de la propagation des différents bruits présents dans leurs mesures. Forts de cette interprétation, on mène une comparaison précise et inédite des différents éléments qui composent cette famille d’ASO. On en profite aussi pour proposer de nouveaux concepts de filtrage de Fourier permettant d’atteindre des sensibilités inégalées auparavant, avec une efficacité d’utilisation des photons très proche de la limite fondamentale possible pour l’analyse de front d’onde en général.Malheureusement, la sensibilité seule ne suffit pas pour définir les performances d’un ASO. La dynamique, mise à rude épreuve lors de la boucle d’OA, est tout autant cruciale pour assurer le bon fonctionnement des opérations. Pour sonder la dynamique des ASO étudiés, on utilise une approche bien particulière qui permet d’évaluer les non-linéarités tout en restant dans un formalisme matriciel : l’approche des systèmes à paramètres linéaires variants. Ainsi, on interprète les non-linéarités du système en considérant plus simplement leur effet sur les mesures comme une source de changement de régime de linéarité. Cette approche, déjà présentée dans des travaux précédents, permet de définir le concept essentiel des gains optiques. On pousse ici leur utilisation en proposant un suivi à haute cadence, à l’échelle de chaque mesure de l’ASO. On fournit aussi une façon pratique de réaliser ce suivi grâce au concept d’ASO à filtrage de Fourier assisté par imagerie plan focal et l’introduction de la Gains Scheduling Camera. Cette solution, consistant à fusionner les données d’une image plan focal et les signaux délivrés par l’ASO, semble s’imposer comme une solution pratique de la gestion des non-linéarités très prometteuse. Une partie plus expérimentale vient étayer tous ces travaux avec l’implémentation des nouveaux masques à filtrage de Fourier proposés sur le banc LOOPS au LAM d’un côté, et le développement du projet PAPYRUS visant à mettre sur ciel un analyseur pyramide assistée par imagerie en plan focal de l’autre.With the huge projects of "extremely large telescopes", world astronomy will soon acquire tools of unequaled power to probe the universe. These telescopes follow in the footsteps of their smaller companions, the 8 m class telescopes, whose feats have already illuminated the scientific landscape for more than twenty years. These ground-based telescopes, present or future, are however loosing a part of their capacities as soon as they are put into operation : atmospheric turbulence scrambles the light wavefronts coming from the stars, reducing the angular resolution of these giant instruments to the one of simple amateur telescopes. To combat this blur that taints images of the cosmos, scientists have developed a technique called Adaptive Optics (AO). Using deformable optics, this technique makes possible to compensate for alterations caused by the atmosphere in real time . This method equips all the largest ground-based telescopes today, and has become essential for a large number of astrophysical applications. Motivated in particular by the hunt for exoplanets, AO systems pushing the limits of performance are now being developed. The fundamental limits of such instruments are based on the quality of the measurements provided by the optical device at the heart of this technique : the wavefront sensor (WFS), whose objective is to estimate the shapes impregnated by the turbulence on wavefronts. This quality of measurements is defined by two main aspects, sensitivity and dynamics.This thesis focuses on a very large class of WFS, called Fourier-filtering WFS. By relying on a mathematical formalism developed in previous works, we develop a better understanding of their sensitivity thanks to the study of the propagation of the different noises present in their measurements. Armed with this interpretation, we conduct a precise and unprecedented comparison of the different elements that belong to this WFS family. We also take the opportunity to propose new concepts of Fourier filtering achieving previously unmatched sensitivities, with an photon efficiency very close to the fundamental limit possible for wavefront sensing in general. Unfortunately, sensitivity alone is not enough to define the performance of a WFS. The dynamics, strongly affected during the AO loop, are also crucial to ensure the smooth running of operations. To probe the dynamics of the studied WFS, we use a very specific approach which allows to evaluate the non-linearities while remaining in a matrix formalism : the linear varying parameters systems approach. Thus, the non-linearities of the system are interpreted by considering their effects on the measurements more simply as a source of change in the linearity regime. This approach, already presented in previous works, allows to define the essential concept of optical gains. Their use is pushed here by offering high-speed monitoring, on the scale of each WFS measurement. A practical way of performing this tracking is also provided through the concept of Fourier filtering WFS assisted by focal plane imaging and the introduction of the Gains Scheduling Camera. This solution, consisting of merging the data of a focal plane image and the signals delivered by the WFS, seems to impose itself as a very promising practical solution for the management of non-linearities.A more experimental part comes to support all this work with the implementation of the new Fourier filtering masks proposed on the LOOPS bench at LAM and the development of the PAPYRUS project aiming at testing on sky a focal plane assisted pyramid wavefront sensor

Optimisation of Fourier filtering wavefront sensing for high performance adaptive optics systems

Avec les projets titanesques des "extremely large telescopes", l'astronomie mondiale va bientôt se doter d'outils à la puissance inégalée pour sonder l'univers. Ces télescopes suivent la trace de leurs compagnons de taille plus modeste, les télescopes de classe 8 m, dont les prouesses éclairent déjà le paysage scientifique depuis plus d'une vingtaine d'années. Ces télescopes au sol, présents ou futurs, se trouvent pourtant tous amputés d'une partie de leurs capacités dès leur mise en fonctionnement: la turbulence atmosphérique brouille les fronts d'onde de la lumière parvenant des astres, réduisant la résolution angulaire de ces géants à celle de simples télescopes amateurs. Pour lutter contre ce flou qui entache les images du cosmos, les scientifiques ont mis au point une technique appelée optique adaptative. À l'aide d'optiques déformables, cette technique permet de compenser en temps réel les altérations de la lumière dues à l'atmosphère. Cette méthode équipe aujourd'hui tous les plus grands télescopes au sol, et est devenue indispensable pour un grand nombre d'applications astrophysiques. Motivé notamment par la chasse aux exoplanètes, des systèmes d'OA repoussant les limites de performances sont aujourd'hui mis au point. Les limites fondamentales de tels instruments reposent sur la qualité des mesures fournies par le dispositif optique au cœur de cette technique: l'analyseur de surface d'onde (ASO), dont l'objectif est d'estimer les formes imprégnées par la turbulence sur les fronts d'onde. Cette thèse s’intéresse à l’analyse de surface d’onde, et plus particulièrement l’analyse de surface d’onde par filtrage de FourierWith the huge projects of "extremely large telescopes", world astronomy will soon acquire tools of unequaled power to probe the universe. These telescopes follow in the footsteps of their smaller companions, the 8 m class telescopes, whose feats have already illuminated the scientific landscape for more than twenty years. These ground-based telescopes, present or future, are however losing a part of their capacities as soon as they are put into operation: atmospheric turbulence scrambles the light wavefronts coming from the stars, reducing the angular resolution of these giant instruments to the one of simple amateur telescopes. To combat this blur that taints images of the cosmos, scientists have developed a technique called Adaptive Optics (AO). Using deformable optics, this technique makes it possible to compensate for alterations caused by the atmosphere in real time. This method equips all the largest ground-based telescopes today, and has become essential for a large number of astrophysical applications. Motivated in particular by the hunt for exoplanets, AO systems pushing the limits of performance are now being developed. The fundamental limits of such instruments are based on the quality of the measurements provided by the optical device at the heart of this technique: the wavefront sensor (WFS), whose objective is to estimate the shapes impregnated by the turbulence on wavefronts. This thesis focus on wavefront sensing, and more particularly on Fourier filtering wavefront sensing

Including the pyramid optical gains into analytical models

International audienceFourier-filtering wavefront sensors (WFS), such as the pyramid of Zernike WFS, are shown to be highly sensitive.They are becoming the baseline for future adaptive optics (AO) systems for astronomy. The next generationExtremely Large Telescopes (ELTs) will be equipped with such sensitive WFS. However the main drawback ofthese sensors is a quick loss of linearity when subject to strong turbulence residuals.Two major methods can be identified to simulate the AO point-spread-function (PSF): the end-to-endsimulation and the analytical model. The first one propagates random samples of phase screens through a fullysimulated AO loop, it can thus reproduce fine spatial and temporal effects, inlcuding the WFS non linearities.The second method is based on analytical formulas that provide a quick simulation with a good understanding ofthe AO system (separation of the AO error terms) but require a linear response of the system.We develop here a method to include the non linearities of the WFS into analytical formulas. It consequentlyimproves the accuracy of the simulation and enables to describe with good accuracy Fourier-filtering WFS. We testour method against end-to-end simulations, and derive possible applications for AO system design or performanceestimation