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Shack-Hartmann tomographic wavefront reconstruction using laser guide star: analysis of spot elongation and fratricide effect.

Abstract 151

Submitted by Clélia ROBERT


C. Robert et al


ONERA Département d’optique théorique et appliquée- unité de haute résolution angulaire 29, avenue de la division Leclerc 92322 Châtillon Cedex


Present projects of Extremely Large Telescopes rely on wide-field-of-view adaptive optics systems such as multi-conjugate AO, multi-object AO, ground-layer AO and Laser Tomography AO. All these systems will use atmospheric tomography to reconstruct the turbulent phase volume using different constellations of natural (NGS) and laser guide stars (LGS). A laser guide star is not only at a finite distance leading to cone effect, but also extended over the whole height of the sodium layer. A Shack-Hartmann WFS looking at a LGS is confronted to spot elongation leading to specific slope noise with possibly anisoplanatism, and differential cone effect. Systems using multiple LGS beams with CW lasers are also affected by fratricide effect. A Shack-Hartmann WFS looking at a LGS may collect the Rayleigh scattered light of other LGSs, causing a background increase in the subapertures. These perspective and fratricide effects are launching dependent. In this paper, we explore the interactions between the spot elongation and fratricide effects, with classical tomographic wavefront reconstruction techniques to eventually select the 2 launching schemes. We envisage the impact of these effects in terms of SH WFS accuracy and related performance in specific directions in the scientific field of view.

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