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Proceedings Paper

Atlas Experiment To Test The Laser Probe Technique For Wavefront Measurements
Author(s): Renaud Foy; Michel Tallon; Marc Sechaud; Norbert Hubin
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Paper Abstract

We discuss that one needs to create an artificial reference source to operate adaptive optics devices at visible wavelengths, since the probability is too low to find a star bright enough in the isoplanatic patch surrounding a given direction in the sky. That artificial source could be the light backscattered from a laser beam emitted through the telescope aperture, e.g. by Sodium atoms in the mesosphere. We describe the ATLAS experiment which is being built to check the validity of the concept of the laser probe technique; we use the 300mJ doubled Nd:YAG laser of the CERGA Lunar Laser Ranging Station and specially developped Shack-Hartmann sensors. The goal is to compare the wavefronts restored from almost simultaneous observations of a real bright star and of a laser spot. The finite distance of the laser source compared to that of the remote star may induce systematic errors in the phase corrections determined from the laser spot and applied to the stellar wavefront. The magnitude of that "cone effect" depends on the telescope diameter, the isoplanatic patch and the wavelength. We propose to overcome it by measuring the 3D map of the ray deflexions in the turbulent layers in the atmosphere. From SCIDAR measurements, the refractive index structure function shows few well peaked features, so that the layers are assumed discrete. The correction of the cone effect requires to observe a small number of laser spots, typically 4 with an 8m telescope. It is a definite advantage to spread the spots outside the projection of the pupil onto the sky, so that the wavefront can be restored for directions largely beyond the isoplanatic patch. This method to correct the cone effect and to widen the isoplanatic domain could be tested with ATLAS and with real multiple stars.

Paper Details

Date Published: 20 September 1989
PDF: 10 pages
Proc. SPIE 1114, Active Telescope Systems, (20 September 1989); doi: 10.1117/12.960822
Show Author Affiliations
Renaud Foy, Observatoire de Paris-Meudon (France)
Michel Tallon, Observatoire de Paris-Meudon (France)
Marc Sechaud, Office National d'Etudes et de Recherches Spatiales (ONERA) (France)
Norbert Hubin, Office National d'Etudes et de Recherches Spatiales (ONERA) (France)

Published in SPIE Proceedings Vol. 1114:
Active Telescope Systems
Francois J. Roddier, Editor(s)

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