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Proceedings Paper

The laser guide stars wavefront sensor prototype for the E-ELT: test results
Author(s): Laura Schreiber; Matteo Lombini; Emiliano Diolaiti; Clelia Robert; Italo Foppiani; Giuseppe Cosentino; Giovanni Bregoli; Enrico Marchetti
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Paper Abstract

The current baseline for the Multi-Conjugate adaptive optics module (MAORY) for the European Extremely Large Telescope is based on the Sodium Laser Guide Stars (LGS) approach, in order to ensure correction uniformity and sky coverage. However, since the Sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture, with elongation depending on the actual telescope diameter, on the Sodium layer properties and on the laser launcher position. We studied numerically, by means of simulations, the performance of three different algorithms for the instantaneous LGS image position measurement in presence of elongated spots: the Weighted Center of Gravity, the Correlation and the Quad-cell. Since all the three considered algorithms require some sort of 'reference', the problem of the temporal variation of the Sodium Layer properties has to be addressed. Another important aspect that has be addressed in the analysis is the impact of the low-order aberrations introduced by the MAORY re-imaging optics, by the Sodium Density profile shape and by its projection in the LGS image plane. An LGS wavefront sensor laboratory prototype was designed to reproduce the relevant aspects of a Shack-Hartmann wavefront Sensor for the European Extremely Large Telescope and, through laboratory tests, to evaluate the performance of different centroid algorithms in presence of elongated spots, with realistic Sodium profiles, as previously investigated numerically and analytically, analyzing the impact of the low-order aberrations and of relevant WFS parameters, like signal-to-noise ratio, sampling and subaperture size. This paper shows the results of the tests performed with this prototype.

Paper Details

Date Published: 27 July 2010
PDF: 12 pages
Proc. SPIE 7736, Adaptive Optics Systems II, 77365B (27 July 2010); doi: 10.1117/12.857307
Show Author Affiliations
Laura Schreiber, Univ. degli Studi di Bologna (Italy)
Matteo Lombini, INAF, Osservatorio Astronomico di Bologna (Italy)
Emiliano Diolaiti, INAF, Osservatorio Astronomico di Bologna (Italy)
Clelia Robert, ONERA (France)
Italo Foppiani, INAF, Osservatorio Astronomico di Bologna (Italy)
Giuseppe Cosentino, Univ. degli Studi di Bologna (Italy)
Giovanni Bregoli, INAF, Osservatorio Astronomico di Bologna (Italy)
Enrico Marchetti, European Organisation for Astronomical Research in the Southern Hemisphere (Germany)

Published in SPIE Proceedings Vol. 7736:
Adaptive Optics Systems II
Brent L. Ellerbroek; Michael Hart; Norbert Hubin; Peter L. Wizinowich, Editor(s)

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