Sky coverage and Strehl ratio uniformity in layer-oriented MCAO systems

Abstract

The sky coverage and the Strehl Ratio (SR) uniformity over Field of Views (FoV) of some arcmin are two key points for the development of Natural Guide Star (NGS) based Multi-Conjugate Adaptive Optics (MCAO) systems. We developed a numerical code able to simulate the behavior and to evaluate the performance of MCAO Layer-Oriented systems. In this paper we study the two issues and present the results of the simulations. In particular we consider the Multiple Field of View (MFoV) version of the Layer-Oriented concept. This technique allows looking for the reference stars in sky-areas bigger than the FoV corrected by the adaptive system, with a considerable gain in term of sky coverage. In the MFoV approach of the Layer-Oriented the NGS are selected in two or more concentric annular FoVs. In the configuration we take into account the guide stars are chosen in two different FoVs. The references for the ground layer loop are chosen in an annular FoV which inner diameter has the dimension of the scientific one. This annulus has only technical purposes and only a ground layer correction is applied in those sky directions. The reference stars for the highest loop are selected, as usual, in the corrected FoV. First we take into account the sky coverage considering different galactic latitude cases and studying the results distribution of the simulated cases. In order to enable a statistical approach to the problem we considered a big number of NGS configurations for each galactic latitude case considered. Then we take into account the level of SR uniformity in the cases where a useful correction is achieved. We define a function to quantify the SR uniformity over the FoV and we study its distribution using the results computed by the numerical simulations.