Etoiles laser pour les grands telescopes: effet de cone et implications astrophysiques

The performance of an Adaptive Optics (AO) system equipped with a Laser Guide Star (LGS) on 3.6~m and 8~m telescopes are evaluated. The use of an LGS allows to significantly increase the sky coverage (fraction of the sky which can be observed). Indeed, 99~\% of the sky is accessible with a LGS (whereas 10~\% only is accessible with a Natural Guide Star (NGS), at 2.2~$\mu$m, average galactic latitude and longitude with a Strehl ratio of 0.2). The number of quasars which can be observed with a Strehl ratio greater than 0.2 increases from 357 to 6803. The performances of an LGS-AO decrease dramatically towards shorter wavelengths ($<1 \mu$m), due to the cone effect (i.e. focus isoplanatism). A 3 dimensional study of atmospheric turbulence allows to solve this problem. Four LGSs provide a good correction quality in the visible (Strehl of 80~\% for an 8~m telescope). The corrected Field of View can be significantly increased(100'', Strehl of 30~\%). Some low order modes (forms of tilt, defocus, astigmatism) must be measured from a NGS. Due to the finite number of deformable mirrors being used, anisoplanatism appears in the corrected field. The performances of an AO system for correction in the visible on a 100~m diameter telescope are estimated, using 4 laser (and natural) guide stars. The are no physical limitations preventing a high sky coverage, with a milli-arcsecond resolution in the visible. The last chapter is devoted to the study of a few Mira-type stars, with a AO system and an integral field spectrograph, with the aim of detecting shock waves in their atmosphere.