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Proceedings Paper

ELPOA: toward the tilt measurement from a polychromatic laser guide star
Author(s): Renaud Foy; Jean-Paul Pique; Alain Denis Petit; Patrick Chevrou; Vincent Michau; Gilbert Grynberg; Arnold Migus; Nancy Ageorges; Veronique Bellanger; Francois Biraben; Ruy Deron; Hayden Fews; Francoise-Claude Foy; Claudia Karin Hoegemann; Markus Laubscher; Daniel Mueller; Celine d'Orgeville; Olivier Peillet; Mike Redfern; Matthias Schoeck; Patricia Segonds; Richard A. J. Soden; Michel Tallon; Eric Thiebaut; Andrei A. Tokovinin; Jerome Vaillant; Jean-Marc Weulersse
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Paper Abstract

Adaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source, which is located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength of the observation, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. Several papers have addressed the problem of the sky coverage as a function of these parameters (see e.g.: Le Louarn et al). It turns out that the sky coverage is disastrously low in particular in the short (visible) wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (which is not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return- of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because approximately equals 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial or total sky coverage for the tilt, such as the dual adaptive optics concept, the elongation perspective method, or the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play.

Paper Details

Date Published: 7 July 2000
PDF: 12 pages
Proc. SPIE 4007, Adaptive Optical Systems Technology, (7 July 2000); doi: 10.1117/12.390401
Show Author Affiliations
Renaud Foy, Ctr. de Recherches Astronomique de Lyon (France)
Jean-Paul Pique, Univ. Joseph Fourier (France)
Alain Denis Petit, Commissariat a l'Energie Atomique (France)
Patrick Chevrou, Commissariat a l'Energie Atomique (France)
Vincent Michau, Office National d'Etudes et de Realisations Aerospatiales (France)
Gilbert Grynberg, Ecole Normale Superieure (France)
Arnold Migus, Ecole Polytechnique (France)
Nancy Ageorges, National Univ. of Ireland/Galway (Chile)
Veronique Bellanger, Commissariat a l'Energie Atomique (France)
Francois Biraben, Ecole Normal Superieure (France)
Ruy Deron, Offices National d'Etudes et de Realisations Aerospatiales (France)
Hayden Fews, National Univ. of Ireland/Galway (Ireland)
Francoise-Claude Foy, Ctr. de Recherche Astronomique de Lyon (France)
Claudia Karin Hoegemann, Ctr. de Recherche Astronomique de Lyon (Spain)
Markus Laubscher, Univ. Joseph Fourier (Switzerland)
Daniel Mueller, Univ. Joseph Fourier (France)
Celine d'Orgeville, Univ. Joseph Fourier (France)
Olivier Peillet, Commissariat a l'Energie Atomique (France)
Mike Redfern, National Univ. of Ireland/Galway (United Kingdom)
Matthias Schoeck, Ctr. de Recherches Astronomique de Lyon (United States)
Patricia Segonds, Univ. Joseph Fourier (France)
Richard A. J. Soden, Univ. Joseph Fourier (France) and Centre de Recherches Astronomique de Lyon (Switzerland)
Michel Tallon, Ctr. de Recherches Astronomique de Lyon (France)
Eric Thiebaut, Ctr. de Recherches Astronomique de Lyon (France)
Andrei A. Tokovinin, Ctr. de Recherches Astronomique de Lyon (Chile)
Jerome Vaillant, Ctr. de Recherches Astronomique de Lyon (France)
Jean-Marc Weulersse, Commissariat a l'Energie Atomique (France)

Published in SPIE Proceedings Vol. 4007:
Adaptive Optical Systems Technology
Peter L. Wizinowich, Editor(s)

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