Concept and performance of multiple laser guide stars for 8 m class telescopes

In this paper, we describe a concept of multiple laser guide stars system based on tomographic reconstruction and multi-conjugate correction for 8-m class telescopes. We show that this type of adaptive optics (AO) systems can be considered as the next generation of AO systems for 8-m telescopes and represents a necessary intermediate step toward AO systems for Extremely Large Telescopes. Multiple guide stars allow to correct the cone effect which affects single LGS systems and prevents from going to wavelengths shorter than similar to 1 mu m. With 4 LGS plus 1 NGS, it is expected that correction with a high Strehl can be obtained at least in the R and I bands, with an extended corrected field of view (FOV). An analytic AO model is used to assess the expected on-axis performance. Using recent results on 3D mapping of turbulence (i.e. tomography), we estimate the sky coverage of such a system. We also discuss the implications of a large corrected field of view on the system design: large wavefront sensor field, constrains on the optics and deformable mirrors, and size of the science detector. With an MCAO system, large telescopes would be able to observe faint extragalactic objects and wide crowded fields (2 arcmin) at the diffraction limit.