Maria Mitchell Leboulleux, Lucie
Photoniques,
10/2021
110
Journal Article
Recenzirano
Odprti dostop
Maria Mitchell was a pioneer in many aspects: first observer of a comet with a telescope, she received the Gold Medal from the King of Denmark and became the first female astronomer and astronomy ...professor in the United States of America. But she also got involved in feminism, participating in the foundation of the
Association for the Advancement of Women
in 1873 as well as promoting the access to higher education for women and their inclusion in science.
Caroline Émilie « Lili » Bleeker était une physicienne et auto-entrepreneuse néerlandaise. Après un doctorat sur les spectres des alcaloïdes, elle monte la première entreprise néerlandaise de ...production de matériel et d’instruments optiques. Elle signe notamment avec Frederik Zernike le brevet sur le microscope à contraste de phase, pour lequel le physicien obtient le prix Nobel de Physique en 1953. Elle se démarque également pour son engagement contre le nazisme lors de la Seconde Guerre Mondiale.
Context. Direct imaging and spectroscopy of Earth-like planets and young Jupiters require contrast values up to 106−1010 at angular separations of a few dozen milliarcseconds. To achieve this goal, ...one of the most promising approaches consists of using large segmented primary mirror telescopes with coronagraphic instruments. While they are able to reach high contrast at small angular separations, coronagraphs are highly sensitive to wavefront errors, however. The segmentation itself is responsible for phasing errors and segment-level vibrations that have to be controlled at a subnanometric accuracy. Aims. We propose an innovative method for a coronagraph design that allows a consequent relaxation of the segment phasing and stability constraints for low segment-count mirrors and generates an instrument that is more robust to segment-level wavefront errors. Methods. This method is based on an optimization of the coronagraph design that includes a segment-level apodization. This is repeated over the pupil to match the segmentation redundancy and improves the contrast stability beyond the minimum separation set by the single-segment diffraction limit. Results. We validate this method on a Giant Magellan Telescope (GMT)-like pupil (seven circular segments) for two coronagraph types: apodized pupil Lyot coronagraphs, and apodizing phase plate coronagraphs. For the apodized pupil Lyot coronagraphs, redundant apodization enables releasing the piston phasing constraints by a factor of 5-20 compared to classical designs. For the apodizing phase plate coronagraphs, the contrast remains almost constant up to 1 radian RMS of the phasing errors. We also show that redundant apodizations increase the robustness of the coronagraph to segment tip-tilt errors, as well as to missing segment errors. Conclusions. Redundant apodization enables reducing or even removing any constraints on the primary mirror segment phasing at the price of larger angular separations and lower throughputs. This method cannot be applied to higher-segment count mirrors such as the ELT or the TMT, but it is particularly suitable for low segment-count mirrors (fewer than ~20 segments) such as the GMT aperture. These mirrors aim for high-contrast imaging of debris disks or exoplanets down to 100 mas.
Context.
Direct imaging and spectroscopy of Earth-like planets and young Jupiters require contrast values up to 10
6
−10
10
at angular separations of a few dozen milliarcseconds. To achieve this ...goal, one of the most promising approaches consists of using large segmented primary mirror telescopes with coronagraphic instruments. While they are able to reach high contrast at small angular separations, coronagraphs are highly sensitive to wavefront errors, however. The segmentation itself is responsible for phasing errors and segment-level vibrations that have to be controlled at a subnanometric accuracy.
Aims.
We propose an innovative method for a coronagraph design that allows a consequent relaxation of the segment phasing and stability constraints for low segment-count mirrors and generates an instrument that is more robust to segment-level wavefront errors.
Methods.
This method is based on an optimization of the coronagraph design that includes a segment-level apodization. This is repeated over the pupil to match the segmentation redundancy and improves the contrast stability beyond the minimum separation set by the single-segment diffraction limit.
Results.
We validate this method on a Giant Magellan Telescope (GMT)-like pupil (seven circular segments) for two coronagraph types: apodized pupil Lyot coronagraphs, and apodizing phase plate coronagraphs. For the apodized pupil Lyot coronagraphs, redundant apodization enables releasing the piston phasing constraints by a factor of 5-20 compared to classical designs. For the apodizing phase plate coronagraphs, the contrast remains almost constant up to 1 radian RMS of the phasing errors. We also show that redundant apodizations increase the robustness of the coronagraph to segment tip-tilt errors, as well as to missing segment errors.
Conclusions.
Redundant apodization enables reducing or even removing any constraints on the primary mirror segment phasing at the price of larger angular separations and lower throughputs. This method cannot be applied to higher-segment count mirrors such as the ELT or the TMT, but it is particularly suitable for low segment-count mirrors (fewer than ~20 segments) such as the GMT aperture. These mirrors aim for high-contrast imaging of debris disks or exoplanets down to 100 mas.
Future searches for bio-markers on habitable exoplanets will rely on telescope instruments that achieve extremely high contrast at small planet-to-star angular separations. Coronagraphy is a ...promising starlight suppression technique, providing excellent contrast and throughput for off-axis sources on clear apertures. However, the complexity of space- and ground-based telescope apertures goes on increasing over time, owing to the combination of primary mirror segmentation, the secondary mirror, and its support structures. These discontinuities in the telescope aperture limit the coronagraph performance. In this paper, we present ACAD-OSM, a novel active method to correct for the diffractive effects of aperture discontinuities in the final image plane of a coronagraph. Active methods use one or several deformable mirrors that are controlled with an interaction matrix to correct for the aberrations in the pupil. However, they are often limited by the amount of aberrations introduced by aperture discontinuities. This algorithm relies on the recalibration of the interaction matrix during the correction process to overcome this limitation. We first describe the ACAD-OSM technique and compare it to the previous active methods for the correction of aperture discontinuities. We then show its performance in terms of contrast and off-axis throughput for static aperture discontinuities (segmentation, struts) and for some aberrations evolving over the life of the instrument (residual phase aberrations, artifacts in the aperture, misalignments in the coronagraph design). This technique can now obtain the Earth-like planet detection threshold of contrast on any given aperture over at least a 10% spectral bandwidth, with several coronagraph designs.
Context.
Telescope pupil fragmentation from spiders generates specific aberrations that have been observed at various telescopes and are expected on the 30-meter class telescopes under construction. ...This is known as the island effect, and it induces differential pistons, tips, and tilts on the pupil petals, deforming the instrumental point spread function (PSF); it is one of the main limitations to the direct detection of exoplanets with high-contrast imaging. These petal-level aberrations can have different origins such as the low-wind effect or petaling errors in the adaptive optics reconstruction.
Aims.
In this paper, we propose a method for alleviating the impact of the aberrations induced by island effects on high-contrast imaging by adapting the coronagraph design in order to increase its robustness to petal-level aberrations.
Methods.
Following a method first developed and applied on robustness to errors due to primary mirror segmentation (e.g., segment phasing errors, missing segments), we developed and tested redundant apodized pupils (RAP): apodizers designed at the petal-scale, then duplicated and rotated to mimic the pupil petal geometry.
Results.
We applied this concept to the ELT architecture, made of six identical petals, to yield a 10
−6
contrast in a dark region from 8 to 40
λ
/
D
. Both amplitude and phase apodizers proposed in this paper are robust to differential pistons between petals, with minimal degradation to their coronagraphic PSFs and contrast levels. In addition, they are also more robust to petal-level tip-tilt errors than classical apodizers designed for the whole pupil, with which the limit of contrast of 10
−6
in the coronagraph dark zone is achieved for constraints up to 2 rad RMS of these petal-level modes.
Conclusions.
In this paper the RAP concept proves its robustness to island effects (low-wind effect and post-adaptive optics petaling), with an application to the ELT architecture. It can also be considered for other 8- to 30-m class ground-based units such as VLT/SPHERE, Subaru/SCExAO, GMT/GMagAO-X, and TMT/PSI.
The James Webb Space Telescope was not even launched yet when the Astro2020 Decadal Survey American report recommended the development of what is now called the Habitable World Observatory, also ...mentioned by the Voyage 2050 European report. This future space telescope, at 11 billions dollars and at least 6 m diameter, should allow, around 2040, the characterization of at least 25 exoplanets similar to Earth and orbiting around main sequence stars, with the hope of discovering one where life could have developed. This objective represents a technological challenge since it requires the design of spectro-imagers able to access very high contrasts (10^-8-10^-10) at low angular separations (smaller than 100 mas). This proceeding and the talk it is associated to address various obstacles that remain to be overcome in order to one day allow HabWorld to reach its ultimate performance.
Context.
Direct imaging of Earth-like planets from space requires dedicated observatories, combining large segmented apertures with instruments and techniques such as coronagraphs, wavefront sensors, ...and wavefront control in order to reach the high contrast of 10
10
that is required. The complexity of these systems would be increased by the segmentation of the primary mirror, which allows for the larger diameters necessary to image Earth-like planets but also introduces specific patterns in the image due to the pupil shape and segmentation and making high-contrast imaging more challenging. Among these defects, the phasing errors of the primary mirror are a strong limitation to the performance.
Aims.
In this paper, we focus on the wavefront sensing of segment phasing errors for a high-contrast system, using the COronagraphic Focal plane wave-Front Estimation for Exoplanet detection (COFFEE) technique.
Methods.
We implemented and tested COFFEE on the High-contrast imaging for Complex Aperture Telescopes (HiCAT) testbed, in a configuration without any coronagraph and with a classical Lyot coronagraph, to reconstruct errors applied on a 37 segment mirror. We analysed the quality and limitations of the reconstructions.
Results.
We demonstrate that COFFEE is able to estimate correctly the phasing errors of a segmented telescope for piston, tip, and tilt aberrations of typically 100 nm RMS. We also identified the limitations of COFFEE for the reconstruction of low-order wavefront modes, which are highly filtered by the coronagraph. This is illustrated using two focal plane mask sizes on HiCAT. We discuss possible solutions, both in the hardware system and in the COFFEE optimizer, to mitigate these issues.
Imager une exo-terre à proximité d’une étoile est une tâche complexe : le signal de la planète est noyé dans le flux immense de l’étoile, très proche. Doivent donc être combinés :- de grands ...télescopes spatiaux segmentés. La segmentation du miroir primaire facilite le transport mais crée des erreurs liées à l’alignement.- un coronographe, permettant d’éteindre la lumière stellaire. - enfin, toute aberration optique crée un résidu lumineux nuisible dans l’image. La mesure et le contrôle des aberrations d’un système coronographique, notamment celles liées à la segmentation du télescope, sont donc primordiaux et constituent le sujet de ma thèse.Tout d’abord, j’ai développé PASTIS, un modèle simplifié du contraste d’un coronographe en présence d’une pupille segmentée, permettant d’analyser facilement les performances pour contraindre les aberrations optiques lors du design de l’instrument. PASTIS prend en compte les spécificités des instruments : structure de la pupille, aberrations optiques dues à la segmentation, coronographe. Je l’ai appliqué au télescope LUVOIR afin d’analyser les modes limitant le contraste et ainsi mieux répartir les contraintes sur les segments. Par la suite, j’ai travaillé sur l’analyse de front d’onde coronographique en présence d’un télescope segmenté sur le banc expérimental HiCAT avec une première démonstration de l’analyseur COFFEE permettant de reconstruire les erreurs de phasage avec une grande précision. Enfin, j’ai mené une analyse comparative des multiples méthodes de contrôle de front d’onde existantes et validé l’une d’elles (Dark Hole Non Linéaire) expérimentalement dans un cadre simplifié sur le banc MITHIC du LAM
Direct imaging of exo-Earths is extremely complex: the star is by far brighter and very close to the planet. Several tools have to be combined:- a giant primary mirror. For manufacturing and transportation reasons, we tend to use segmented mirrors, ie. mirrors made of smaller mirrors but that have to be well-aligned and stabilised.- a coronagraph, enabling to remove the starlight.- the smallest residual wavefront aberration into residual light that decreases the image quality. The measurement and control of the aberrations, including the ones due to the telescope segmentation, are crucial and consist in the topic of my thesis.First, I developed PASTIS, a model of the contrast of a coronagraphic system in presence of a segmented pupil, enabling to analyze the performance to set up constraints on the optical aberrations during the instrument design. PASTIS takes into account the specificities of high-contrast instruments: pupil structure, optical aberrations due to the segmentation, coronagraph. I applied it to the LUVOIR telescope to analyze the main modes limiting the contrast and therefore optimizing the repartition of the constraints on the segments. In parallel, I worked on the analysis of the coronagraphic wavefront in presence of a segmented telescope on the experimental testbed called HiCAT, with a first demonstration of the COFFEE sensor enabling to reconstruct phasing errors with a high precision.Eventually, I ran a comparative analysis of existing methods of wavefront control and experimentally validated one of them (Non Linear Dark Hole) in a simplified case on the MITHIC testbed at LAM
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