Polychromatic guide star: feasibility study

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

doi:10.1117/12.407359

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16 August 2000 Polychromatic guide star: feasibility study

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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Proceedings Volume 4065, High-Power Laser Ablation III; (2000) https://doi.org/10.1117/12.407359
Event: High-Power Laser Ablation, 2000, Santa Fe, NM, United States

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 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 required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the 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 (hereinafter 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 (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 $APEX 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 sky coverage for the tilt. The only one providing us with a full sky coverage is 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. We finally shortly described the effort in Europe to develop the LGS.

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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, and Jean-Marc Weulersse "Polychromatic guide star: feasibility study", Proc. SPIE 4065, High-Power Laser Ablation III, (16 August 2000); https://doi.org/10.1117/12.407359

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