Academic literature on the topic 'Coronographe de Lyot'

Emission-line and K-coronal observations in the IR have the significant advantage of reduced sky brightness compared with the visible, while the effects of seeing are also reduced. Moreover, strong lines are available in the near-IR. Examples of the current capabilities of IR coronal observations using conventional Lyot coronagraphs are discussed briefly. Photometric measurements using the two IR lines of Fe XIII (10,747 Å and 10,798 Å), together with the Fe XIII 3,388 Å line, have provided a valuable electron-density diagnostic, but with low-angular-resolution. The 10,747 Å line has high intrinsic polarization. It has been used for extensive coronal magnetic field measurements, but only the direction of the field, and that with modest angular resolution, has been achieved due basically to flux limitations. Such studies suffer from the lack of high angular resolution and high photon flux. Moreover, the chromatic properties of a singlet objective lens preclude simultaneous observations at widely-differing wavelengths of the important inner coronal region. A coronagraph based on a mirror objective avoids such problems. Further, comparatively high-resolution and high-sensitivity arrays are now available with quantum efficiencies up to 90%. Reflecting coronagraphs with advanced arrays then provide the possibility of obtaining high-resolution images in the infrared to carry out a wide variety of studies crucial to many of the outstanding problems in coronal physics. A program for the development of reflecting coronagraphs is described briefly, with an emphasis on applications to IR coronal studies.

L’imagerie directe des exoplanètes reste difficile en raison du contraste élevé et de la faible séparation angulaire entre l'étoile et la planète. Cela nécessite de supprimer l'éblouissement dû à l'étoile et de s'assurer que la lumière faible de la planète n'est pas noyée au milieu de divers bruits. La détection dépend de la maturité des techniques et des algorithmes utilisés, tout en tenant compte des compromis importants sur le contraste brut, la résolution angulaire et la transmission. L'une de ses composantes clés est l'utilisation de coronagraphes - des instruments ayant pour seul but de bloquer/réduire la lumière provenant de l'étoile. Ce travail présente un nouveau type de coronographe de Lyot, inventé par le Dr Yves Rabbia, qui repose sur le principe de la réflexion interne totale frustrée (FTIR) pour supprimer la lumière de l'étoile. Ce coronographe est appelé Evanescent Wave Coronagraph (EvWaCo) en raison de sa nature : son masque au plan focal, comprenant une lentille et un prisme, réfléchit la source hors axe (planète) et transmet la source sur l'axe (étoile) à l’aide des ondes évanescentes. Cette thèse vise à fournir au lecteur les bases qui mettent en évidence les trois principaux avantages d'EvWaCo : i) le masque est intrinsèquement achromatique, ii) la taille du masque est ajustable en modifiant la pression entre la lentille et le prisme, et iii) à la fois la lumière stellaire et la lumière planétaire peuvent être collectées simultanément pour une détection de front d'onde de bas ordre et un centrage approprié de l’étoile. Les performances d'EvWaCo sont évaluées à travers des expériences en laboratoire, puis comparées à des simulations numériques. Les résultats expérimentaux montrent un contraste brut égal à quelques 10-4 à 3 ��/�� sur toute la bande I (��c = 800 nm, ∆��/�� ≈ 20%) et à 4 ��/�� sur toute la bande R (��c = 650 nm, ∆��/�� ≈ 23%). Les simulations confirment la capacité de rejet achromatique d'EvWaCo, montrant un contraste brut de 10-4 à la même distance radiale sur les deux bandes spectrales. Cette thèse se conclut sur un bilan de l’état du banc développé et les perspectives futures
Direct imaging of exoplanets remains challenging due to the high contrast and the small angular separation between the star and the planet. It requires suppressing the blinding glare from the star and ensuring that the planet's faint light is not buried deep in various noises. Successful detection depends on the technological readiness and maturity of techniques and algorithms employed while considering the significant trade-offs on raw contrast, inner working angle, and throughput. One of its key components is the use of coronagraphs – instruments with the sole purpose of blocking/reducing the light from the star. This work presents a new type of Lyot coronagraph, invented by Dr. Yves Rabbia, that relies on the frustrated total internal reflection (FTIR) principle to suppress the starlight. This coronagraph is aptly called the Evanescent Wave Coronagraph (EvWaCo) owing to its nature that its focal plane mask, comprising a lens and a prism, reflects the off-axis source (planet) and transmits the on-axis source (star) by capturing the evanescent waves. This thesis aims to provide the reader with the groundwork that highlights EvWaCo's three main advantages: i) the mask is inherently achromatic, ii) the size of the mask is adjustable by changing the pressure between the lens and the prism, and iii) both the stellar light and the planet light can be collected simultaneously for low-order wavefront sensing, and proper stellar light centering. The performance of EvWaCo is assessed through experiments in a laboratory and then compared to numerical simulations. The experimental results show a raw contrast equal to a few 10-4 at 3 ��/�� over the full I-band (��c = 800 nm, ∆��/�� ≈ 20%) and at 4 ��/�� over the full R-band (��c = 650 nm, ∆��/�� ≈ 23%). The simulations confirm the achromatic rejection capability of EvWaCo as it showed a raw contrast of 10-4 at the same radial distance over both bandpasses. This thesis concludes with the status of its testbed and future perspectives

La couronne solaire est la partie de l'atmosphère du Soleil qui s'étend de la photosphère (surface solaire d'où sont émis les photons) jusque dans le milieu interplanétaire. Sa compréhension relève d'un enjeu majeur car elle est à l'origine de phénomènes qui peuvent perturber les télécommunications, les êtres vivants et même le climat. L'instrument privilégié pour l'observer est le coronographe, système optique occultant le disque solaire au profit de la couronne, un million de fois moins intense. Ma thèse porte sur son étude, en particulier à travers les projets spatiaux :- SOLAR ORBITER, qui doit s'approcher du Soleil à 0.2 unité astronomique (distance Terre-Soleil), permettant ainsi une très haute résolution spatiale ;- SMESE, en coopération avec la Chine, qui étudiera la couronne dans l'infrarouge lointain ;- et ASPIICS, dont l'occulteur externe sera placé à 150 m de l'instrument imageur, permettant d'observer la couronne dans des conditions proches d'une éclipse solaire naturelle.Le premier aspect abordé est la réjection de la lumière parasite instrumentale, dont l'optimisation est une des problématiques majeures en coronographie. Le second concerne les modes d'observation par imagerie en lumière blanche, imagerie monochromatique, et interférométrie, en particulier le Fabry Perot. Le développement et l'amélioration de ces techniques permettra des avancées considérables en terme de résolution et l'accès à la couronne toujours plus proche de la surface du Soleil, lieu encore mal connu où l'activité solaire prend naissance
The solar corona is the part of the Sun's atmosphere that extends from the photosphere (solar surface where the photons are emitted) into the interplanetary medium. Its understanding is a major issue because it is the source of phenomena that can disrupt telecommunications, living beings and even climate. The most appropriate tool to observe it is the coronagraph, an optical system obscuring the solar disk in favor of the corona, a million times fainter. My thesis deals with its review, particularly through the spaceprojects :- Solar Orbiter, which will approach the Sun at 0.2 astronomical unit (distance between Earth and Sun), allowing a very high spatial resolution ;- SMESE, in cooperation with China, which should study the corona in the Lymanalpha (and far infrared) ;- and ASPIICS, which will observe the corona in conditions close to a natural solar eclipse, with its occulting disk located at 150 m from the imaging instrument.The first point tackled is the rejection of instrumental stray light, whose optimization is one of the major problems in coronagraphy. The second concerns the methods of observation and imaging in white light, monochromatic imaging, and interferometry, in particular the Fabry Perot. The development and improvement of these techniques will allow considerable progress in terms of resolution and access to the corona ever closer to the Sun's surface, the location yet little known where the solar activity originates

Detection of nonsolar planets is a challenging problem. Due to the planets' faintness relative to the star, it is necessary to suppress the stellar diffracted energy to very low levels. In addition, the planets' proximity to the star requires large diameter optics. This research examines various multiaperture and segmented telescopic systems incorporating Lyot coronagraphs to suppress the stellar diffracted energy and provide the required resolution so that the extrasolar planets can be directly imaged. Because of the phasing tolerances and required contrast ratios, the IR band of 10.5-13.5 µm was examined over a field of view of 0.2-2 arcsec. Various telescopic objectives with the Lyot coronagraph were examined for their ability to suppress the stellar diffracted energy levels. The designs examined were comprised of segmented circular, hexagonal, apodized hexagonal, and rectangular systems. The Lyot coronagraph consists of a Gaussian apodizer at the focal plane of the objective and a Lyot stop at the image plane of the objective.