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End-to-end model for the Polychromatic Laser Guide Star project (ELP-OA)

Abstract 132

Submitted by Nicolas MEILARD


N. Meilard, R. Foy, M. Tallon, É. Thiébaut


CRAL - Observatoire de Lyon


In order to optimize parameters for the ELP-OA project, we have developped an end to end model from the laser emission to the computing of the tip-tilt Strehl ratio.

ELP-OA relies on the 2-photon excitation of mesospheric sodium, achieved by two laser chains (at 589nm and 569nm) delivering 22W averaqge power each. The twin laser beams are projected through a three aperture interferometer, which produces an interferometric pattern in the mesosphere after the passage by a Kolmogorov phase screen. The backscatter flux is computed with an optical Bloch equations code (Bellanger & al., 2004). The result of this excitation is an emission at differents wavelenghts. In the project case, the wavelengths used are 330nm, 569nm and 589nm.

Then, we compute the image of this PLGS, as it will be observed at the 1.52m telescope at the Observatoire de Haute-Provence (OHP), through a Kolmogorov phase screen again. Then we simulate the adaptive optics device (adapted from ONERA’s BOA system) at the telescope except for tip-tilt. Air refraction index variations due to atmospheric turbulence create a dispersion between 330nm and 569nm, and the tip-tilt value is deduced from this dispersion.

We measure this dispersion between 330nm and 569nm using a correlation method and a sub-pixel interpolation. The calculation of the relative position between two images (330nm and 569nm) gives a position error, from which we deduce the Strehl ratio for tip-tilt.

At this time, we have got Strehl ratios up to approximatively 35% at 550nm, for tilt corrected r0 = 10cm and a projector baseline of 70cm.