534 research outputs found
High-contrast imaging in polychromatic light with the self-coherent camera
Context. In the context of direct imaging of exoplanets, coronagraphs are
commonly proposed to reach the required very high contrast levels. However,
wavefront aberrations induce speckles in their focal plane and limit their
performance. Aims. An active correction of these wavefront aberrations using a
deformable mirror upstream of the coronagraph is mandatory. These aberrations
need to be calibrated and focal-plane wavefront-sensing techniques in the
science channel are being developed. One of these, the self-coherent camera, of
which we present the latest laboratory results. Methods. We present here an
enhancement of the method: we directly minimized the complex amplitude of the
speckle field in the focal plane. Laboratory tests using a four-quadrant
phase-mask coronagraph and a 32x32 actuator deformable mirror were conducted in
monochromatic light and in polychromatic light for different bandwidths.
Results. We obtain contrast levels in the focal plane in monochromatic light
better than 3.10^-8 (RMS) in the 5 - 12 {\lambda}/D region for a correction of
both phase and amplitude aberrations. In narrow bands (10 nm) the contrast
level is 4.10^-8 (RMS) in the same region. Conclusions. The contrast level is
currently limited by the amplitude aberrations on the bench. We identified
several improvements that can be implemented to enhance the performance of our
optical bench in monochromatic as well as in polychromatic light.Comment: 4 pages, 3 figures, accepted in Astronomy & Astrophysics (02/2014
Mathematical study of the -plane model for rotating fluids in a thin layer
This article is concerned with an oceanographic model describing the
asymptotic behaviour of a rapidly rotating and incompressible fluid with an
inhomogeneous rotation vector; the motion takes place in a thin layer. We first
exhibit a stationary solution of the system which consists of an interior part
and a boundary layer part. The spatial variations of the rotation vector
generate strong singularities within the boundary layer, which have
repercussions on the interior part of the solution. The second part of the
article is devoted to the analysis of two-dimensional and three-dimensional
waves. It is shown that the thin layer effect modifies the propagation of
three-dimensional Poincar\'e waves by creating small scales. Using tools of
semi-classical analysis, we prove that the energy propagates at speeds of order
one, i.e. much slower than in traditional rotating fluid models.Comment: 46 page
Optique adaptative  : correction des effets de la turbulence atmosphérique sur les images astronomiques
Today, adaptive optics (AO) is installed on all very large astronomical telescopes. It allows to overcome the limitation in angular resolution imposed on large ground-based telescopes by the atmospheric turbulence perturbations. This review paper presents the state of the art of the field for astronomical applications. It gives the principles of AO and describes its main components which are the deformable mirrors, the wavefront sensors and the control algorithms. The paper also presents the main recent achievements and ongoing projects. First, for the direct imaging of extra-solar planets by extreme AO and coronography, taking as example the SPHERE instrument. Then to solve the problem of the very limited sky coverage by using laser guide star and to extend the AO correction beyond the isoplanatic patch. Thus, the various laser-assisted tomographic AO concepts are presented with examples of each realization as for the laser tomography AO, the multi-conjugate AO and the multi-object AO. Finally, the challenges of the new generation of giant telescopes are discussed at the end of the paper with a particular focus on the European Extremely Large Telescope (ELT) project.Lâoptique adaptative (OA), installĂ©e aujourdâhui sur tous les trĂšs grands tĂ©lescopes astronomiques, permet de sâaffranchir de la limitation en rĂ©solution angulaire imposĂ©e aux grands tĂ©lescopes au sol par les perturbations de la turbulence atmosphĂ©rique. Cet article de revue prĂ©sente lâĂ©tat de lâart du domaine pour les applications astronomiques. Il donne les principes de lâOA et dĂ©crit ses composants principaux que sont les miroirs dĂ©formables, les analyseurs de surface dâonde et les algorithmes de commande. Lâarticle prĂ©sente aussi les principales rĂ©alisations rĂ©centes et les projets en cours. Dâabord pour lâimagerie directe des planĂštes extrasolaires par OA extrĂȘme et coronographie, en prenant comme exemple lâinstrument SPHERE. Ensuite pour rĂ©soudre par Ă©toile guide laser la problĂ©matique de la couverture du ciel trop faible et Ă©tendre la correction de lâOA au-delĂ du domaine isoplanĂ©tique. Ainsi les systĂšmes dâOA tomographiques assistĂ©es par laser sont prĂ©sentĂ©s avec Ă chaque fois des exemples de rĂ©alisation comme pour lâOA Ă tomographie laser, lâOA multiconjuguĂ©e et lâOA multi-objet. Enfin les dĂ©fis des tĂ©lescopes gĂ©ants, dits extrĂȘmes, sont abordĂ©s en fin dâarticle avec un focus particulier sur le projet europĂ©en de lâExtremely Large Telescope (ELT)
Analytical model-based analysis of long-exposure images fromground-based telescopes
The search for Earth-like exoplanets requires high-contrast and high-angular
resolution instruments, which designs can be very complex: they need an
adaptive optics system to compensate for the effect of the atmospheric
turbulence on image quality and a coronagraph to reduce the starlight and
enable the companion imaging. During the instrument design phase and the error
budget process, studies of performance as a function of optical errors are
needed and require multiple end-to-end numerical simulations of wavefront
errors through the optical system.
In particular, the detailed analysis of long-exposure images enables to
evaluate the image quality (photon noise level, impact of optical aberrations
and of adaptive optics residuals, etc.). Nowadays simulating one long but
finite exposure image means drawing several thousands of random frozen phase
screens, simulating the image associated with each of them after propagation
through the imaging instrument, and averaging all the images. Such a process is
time consuming, demands a great deal of computer resources, and limits the
number of parametric optimization.
We propose an alternative and innovative method to directly express the
statistics of ground-based images for long but finite exposure times. It is
based on an analytical model, which only requires the statistical properties of
the atmospheric turbulence. Such a method can be applied to optimize the design
of future instruments such as SPHERE+ (VLT) or the planetary camera and
spectrograph (PCS - ELT) or any ground-based instrument.Comment: 7 pages, 0 figur
PSF reconstruction for NAOS-CONICA
Adaptive optics (AO) allows one to derive the point spread function (PSF)
simultaneously to the science image, which is a major advantage in
post-processing tasks such as astrometry/photometry or deconvolution. Based on
the algorithm of \citet{veran97}, PSF reconstruction has been developed for
four different AO systems so far: PUEO, ALFA, Lick-AO and Altair. A similar
effort is undertaken for NAOS/VLT in a collaboration between the group PHASE
(Onera and Observatoire de Paris/LESIA) and ESO. In this paper, we first
introduce two new algorithms that prevent the use of the so-called "
functions" to: (1) avoid the storage of a large amount of data (for both new
algorithms), (2) shorten the PSF reconstruction computation time (for one of
the two) and (3) provide an estimation of the PSF variability (for the other
one). We then identify and explain issues in the exploitation of real-time
Shack-Hartmann (SH) data for PSF reconstruction, emphasising the large impact
of thresholding in the accuracy of the phase residual estimation. Finally, we
present the data provided by the NAOS real-time computer (RTC) to reconstruct
PSF ({\em (1)} the data presently available, {\em (2)} two NAOS software
modifications that would provide new data to increase the accuracy of the PSF
reconstruction and {\em (3)} the tests of these modifications) and the PSF
reconstruction algorithms we are developing for NAOS on that basis.Comment: 12 pages & 13 figures. To be published in the proceedings of the SPIE
conference Advances in Adaptive Optics - Astronomical Telescopes &
Instrumentation, 24-31 May 2006, Orland
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