Very low flux adaptive optics using spatial and temporal priors

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2 85748 Garching, GERMANY
² Centre de Recherche Astrophysique de Lyon - Observatoire de Lyon, 9 Avenue Charles André 69561, Saint Genis Laval cedex, FRANCE

^a e-mail: cbechet@eso.org

Abstract

Although laser guide stars are great candidates to increase the sky coverage of adaptive optics (AO) systems, their use on an Extremely Large Telescope leads to difficulties that can be avoided by using natural guide stars (NGS). Therefore it is interesting to optimize the behavior of an AO system running on very faint NGS. Very low flux conditions with a Shack-Hartmann wavefront sensor (WFS) leads, according to Poisson statistics, to a certain amount of sub-apertures receiving no photons during each WFS exposure. Such extinctions are considered in current AO systems as measurements of null slopes. Even if the AO loop can still be closed with extinctions in a third of the sub-apertures or more, we show that spatial priors in the reconstruction are not sufficient to guarantee a good AO correction. Temporal priors become essential in this regime. New control approaches are investigated. It relies on a fast maximum a posteriori reconstruction which takes into account the extinctions occurred in every loop step. Temporal priors help to optimize the closed-loop correction. The resulting performance is compared to the classical matrix-vector multiplication with an integrator, thanks to end-to-end simulations.