Single NGS for Extremely Large Telescope MCAO

Most of the MCAO systems planned for astronomical telescopes use laser beacons to create at will a star constellation having the geometry required for a good tomographic reconstruction. The problem of undeter- mined LGS tip tilt prevents one from measuring the tilt of the reference sources. As a consequence the global tilt of the NGSs over the FoV and the altitude and amplitudes of the low order modes are not correctly estimated. In general to solve these problems several NGSs are used. However the use of a few NGSs can reduce the sky cov- erage of the LGS MCAO system by a significant factor. In this paper we present a technique to estimate the low order modes including overall Tilt in the tomographic distribution directly from the LGSs and a single NGS. In this way the sky coverage of the MCAO system is improved. This paper reports the performance of this technique for the case of a 42 m diameter telescope calculated using numerical and analytical simulations.