  This article reports on the results of simulations conducted to assess the
performance of a modal Multi-Conjugate Adaptive Optics (MCAO) system on a 10m
telescope with one Deformable Mirror (DM) conjugated to the telescope pupil and
a second DM conjugated at a certain altitude above the pupil. The main goal of
these simulations is to study the dependence of MCAO performance upon the
altitude of the high-altitude conjugated DM and thereby determine its optimal
conjugation. The performance is also studied with respect to the geometry of
the Guide Star constellation when using constellations of Natural Guide Stars
(NGS), which are rare, or constellations of Laser Guide Stars (LGS) which would
allow large sky coverage.
