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A prototype of the laser guide star wavefront sensor for the E-ELT Multi-Conjugate Adaptive Optics Module

Abstract 137

Submitted by Matteo LOMBINI


M. Lombini (1,2), L. Schreiber (2), I. Foppiani (2), G. Bregoli (1), G. Cosentino (2), E. Diolaiti (1), J.-M. Conan (3), E. Marchetti (4)


(1) INAF – Osservatorio Astronomico di Bologna; (2) Università di Bologna – Dipartimento di Astronomia; (3) ONERA; (4) ESO


The wavefront sensing system of MAORY, the Multi-Conjugate Adaptive Optics (MCAO) module for the future 42m E-ELT, is based on multiple sodium Laser Guide Stars (LGS). The use of LGSs on Extremely Large Telescopes is related to some peculiar problems. The finite distance and the thickness of the sodium layer are responsible for a perspective elongation effect: in the sub-apertures of a Shack-Hartmann WaveFront Sensor (WFS), which is the current baseline for the MCAO module, the LGS images appear more and more elongated as the sub-aperture distance from the laser launcher increases, causing an increase of the required laser power to achieve the necessary wavefront measurement accuracy. Moreover the sodium layer average height is affected by random fluctuations, translating into a defocus signal which has to be compensated in some way in the WFS. Also the density profile of the sodium layer changes in time, producing a temporal variation of the shape of the LGS image in the sub-apertures which may reduce the performance of the spot position measurement algorithms. In addition to the above critical aspects another important effect is linked to the LGS aberrations due to the MCAO post-focal relay, that is optimized for infinite conjugate imaging while the LGSs are at a finite distance from the telescope aperture. We present in this paper a LGS WFS laboratory prototype that simulates an elongated source and that can trace out the most critical LGS features such as the varying intensity profile, the mean height variation and the optics aberrations. The main scope of the prototype is to test the performance of various centroiding algorithms and to provide a basis for the design and specification of the LGS WFS of MAORY.