Numerical simulations of MCAO modal systems in open-loop and closed-loop operation

In this work we have performed very detailed numerical simulations of one of the currently proposed experimental setups of a Multi-Conjugate Adaptive Optics (MCAO) systems devoted to assess under real astronomical conditions and on an 8-m class telescope the performance of MCAO systems. Our simulations, both in open and closed-loop, consider all aspects in a real system except for the modeling of the actual Deformable Mirrors (DMs). Although the performances in the K-band attained using 3 Natural Guide Stars (NGS) when correcting a Field of View (FoV) of 30" of radius are very promising, one soon realizes on the very low sky coverage attainable with MCAO systems relying only on NGSs. In view of this we propose a MCAO using a constellation of Laser Guide Stars plus a relatively faint NGS. Our simulations of such a system indicate that this technique is feasible and indeed provides a quite large sky coverage even when no optimization has been made. This leaves good hopes that with a more careful choice of parameters our system with a single NGS plus a constellation of LGS at different altitudes should obtain nearly whole sky coverage.