Isoplanatic angle and optimal guide star separation for multiconjugate adaptive optics

We propose a first order performance estimation of multiconjugate adaptive optics (MCAO) systems. An important and restrictive parameter is the angular guide star (GS) separation for a field of view (FOV) of interest to be compensated. An analytical approach is proposed to estimate the residual variance of a MCAO systems for a given position in the FOV as a function of GS separation. This approach allows us to define expected isoplanatic angles for MCAO systems as a function of the atmospheric and observing conditions (turbulence profile, telescope diameter and GS separation). The analytical results are also compared with the numerical simulation of a MCAO system, in which a great care has been taken in the 3D wavefront reconstruction from 2D wavefront sensor (WFS) measurements. For a 8m telescope, we show that 3 GS's and 3 deformable mirrors provides very good performance in a 200 arcseconds FOV at 2.2 mu m.