ELPOA:: Toward the tilt measurement from a polychromatic laser guide star.

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.(1) Measuring the phase errors requires a bright source, which is located within the isoplanatic patch of the programme 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(2)). 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(3) have proposed to overcome this difficulty by creating an articial 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 thereafter 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(4) 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.(5-7) Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because approximate to 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.(8) Hence current adaptive optics devices coupled with a LGS face low sky coverage.(9,2) Several methods have been proposed to get a partial or total sky coverage for the tilt, such as the dual adaptive optics concept,(8) the elongation perspective method,(10,11) or the polychromatic LGS(12,13) thereafter referred to as PLGS). We present here a progress report of the R&D programme 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.