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suivant: Session Enseignement monter: sf2a_boa précédent: Poster contributions

Session ASHRA-Jeunes_Chercheurs

E2E simulator for AO for the Extremely Large Telescope ELT
Manal Chebbo Brice Le Roux
The control of AO systems dedicated to ELT is a difficult problem related to the large number of degrees of freedom. The standard and most used adaptive optics AO control starting from the integrator to the LQG are not useful in such a case. In fact, for future Extremely Large Telescope (ELT's) the number of degrees of freedom is very large related to the large diameter of the ELT's and the emergence of new architectures for the AO systems. So that the necessary computational power for real time control RTC on such systems is currently unattainable when using these control methods. Thus, more efficient algorithms are required. We present simulation results of a tomographic AO system in the configuration of EAGLE instrument (multi-object adaptive optics).

Space active optics: in-situ compensation of lightweight primary mirrors' deformations
M. Laslandes, M. Ferrari, E. Hugot
(Laboratoire d'Astrophysique de Marseille)
The need for both high quality images and light structures is a constant concern in the conception of space telescopes. The goal here is to determine how an active optics system could be embarked on a satellite in order to correct the wave front deformations of the optical train. The optical aberrations appearing in a space environment are due to mirrors' deformations, with three main origins: the thermal variations, the weightlessness conditions and the use of large weightlighted primary mirrors. We are developing a model of deformable mirror as minimalist as possible, especially in term of number of actuators, which is able to correct the first Zernike polynomials in a specified range of amplitude and precision. Flight constraints as weight, volume and power consumption are considered. Firstly, such a system is designed according to the equations from the elasticity theory: we determine the geometrical and mechanical characteristics of the mirror, the location of the forces to be applied and the way to apply them. Then the concept is validated with a Finite Element Analysis, allowing to optimize the system by taking into account parameters absent from the theory. At the end, the mirror will be realized and characterized in a representative optical configuration.

Visible Spectroscopy of Terrestrial Exoplanets with SEE-COAST
A.-L. Maire, R. Galicher, A. Boccaletti et al.
(Observatoire de Paris/LESIA)
While more than 450 exoplanets have been discovered, mid-infrared photometry and near-infrared (NIR) low-resolution spectroscopy ($\sim$40) were obtained for a few transiting gazeous planets. Nevertheless, transit photometry and spectroscopy are limited to close-in planets ($\sim$0.05 AU). In order to perform photometry and spectroscopy of wide-separated planets (more than 1 AU), direct imaging is requested. To date, 12 planet candidates were detected by this method. In a near future (2011-2014), dedicated ground-based instruments (SPHERE, GPI) and the JWST will provide the first NIR photometry and spectroscopy measurements of gazeous planets around young stars and M-dwarf stars in the solar neighborhood. However, the detection and the characterization of terrestrial planets require extremely good and stable conditions that are met in space. Our team propose the SEE-COAST mission, a 1.5-m space telescope, that aims at visible spectroscopy and polarimetry of mature giant and massive terrestrial planets. My PhD work deals with the simulation of the SEE-COAST instrumental concept and image analysis techniques to assess its ability to retrieve the spectrum of these planets in conditions as realistic as possible.

Experimental advances in phase mask coronagraphy
Mamadou N'Diaye Kjetil Dohlen Salvador Cuevas
(Laboratoire d'Astrophysique de Marseille)
Stellar coronagraphy is a key technology for current and future planet search and characterization instruments, both on the ground and in space. We pursue the research on coronagraphs based on cylindrical phase-masks and report in this paper on recent advances in terms of the trade between spectral bandwidth and achievable contrast. We also consider some interesting options concerning focal-plane wavefront sensing in such coronagraph systems.

Exoplanet Characterization with Angular and Spectral Differential Imaging
A. Vigan, C. Moutou, M. Langlois, F. Allard, A. Boccaletti, M. Carbillet, D. Mouillet and I. Smith
(Laboratoire d'Astrophysique de Marseille)
In the near future, new high-contrast imaging instruments dedicated to the direct detection of exoplanets at large orbital separations will be installed at the focus of large ground-based telescopes. Data obtained with these instruments optimized for very high contrast are strongly limited by the speckle noise. Specific observing strategies and data analysis methods, such as angular and spectral differential imaging, are required to attenuate the noise level and possibly detect the faint planet flux. Even though these methods are very efficient at suppressing the speckles, the photometry of the faint planets is dominated by the speckle residuals and it has a direct impact on the determination of the physical parameters of the detected planets. We present here the simulations that have been performed in the context of IRDIS, the dual-band imager of VLT-SPHERE, to estimate the influence of the photometric error on exoplanet characterization. In particular we show that the expected photometric performances will allow the detection and characterization of exoplanets down to the Jupiter mass at angular separations of 1.0arcsec and 0.2arcsec respectively around high mass and low mass stars with 2 observations in different filter pairs. We also show that the determination of the planets physical parameters from photometric measurements in different filter pairs is essentialy limited by the error on the determination of the surface gravity.

Approximate analytical expression for AO-corrected coronagraphic imaging in preparation for the characterisation of exoplanets by IFS.
Marie Ygouf - Laurent Mugnier - Jean-François Sauvage - Thierry Fusco - David Mouillet - Jean-Luc Beuzit
The next step in the field of extrasolar planets is on the verge of being reached with instruments such as SPHERE (Spectro Polarimetric High contrast Exoplanet REsearch), which will be capable of performing at the same time direct detection and spectral characterization. Integral Field Spectrographs(IFS's) produce multispectral images made up of two spatial and one spectral dimensions.

In these multispectral images, the star is not completely cancelled by the AO-corrected coronagraphic system because of residual aberrations of the latter. In particular, the star image comprises quasi-static speckles that must be disentangled from the planet signal in order to get the sought information: is there a planet, where is it and what is its spectrum?

To this aim, we are developing a specific image post-processing method using a Bayesian inverse problem approach. The essential required building block of such a method is a data model (often called "direct model") with a minimal number of unknown parameters. In the framework of the SPHERE project for the VLT, we propose an approximate analytical model of a long-exposure star image for an AO-corrected coronagraphic imaging system and we present some preliminary numerical simulations to validate this model.

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suivant: Session Enseignement monter: sf2a_boa précédent: Poster contributions
Samuel Boissier 2010-06-16