Adaptive optics with 4 laser guide stars: cone effect correction on large telescopes

In this paper, we study the performance of an adaptive optics system with 4 laser guide stars (LGS) and a natural guide star (NGS), and compare it to the system with 1 LGS, both installed on an 8-m telescope. Focus anisoplanatism is obtained with a numerical simulation. The typical errors of NGS wavefront are computed with analytical formulae. The entire system is studied to obtain its performance in terms of achievable Strehl ratio. This 4-LGS method allows to push adaptive optics system towards the visible part of the spectrum without tomographic reconstruction of 3D atmospheric perturbation. The cone effect is two times smaller with 4-LGS than with 1-LGS, allowing to reach almost a Strehl ratio of 0.5 at 500 nm. Considering the NGS errors and the focus anisoplanatism, the Strehl ratio (SR) can reach 0.45 at 1.25 mu m under good seeing (GS) conditions with the Nasmyth Adaptive Optics System (14 x 14 sub-pupils wavefront sensor) at the Very Large Telescope.