Performances of natural and laser guide star adaptive optics for 8m class telescopes

We have studied the performances of natural and laser guide star adaptive optics systems for the ESO Very Large Telescope. Analytical formulae are used to derive the Strehl and FWHM of the PSF obtained with NGS and LGS, for two atmospheric models (20 % of the time and median). Sky coverage was computed with stellar densities, leading to a 75 % sky coverage at a Strehl of 0.3 in K band at average galactic coordinates with LGS and 8 % with NGS. A cross correlation approach was also used: reference stars were searched in the USNO-A catalogue around science objects found in the Veron-Cetty 96 and SIMBAD catalogues. This approach shows that the number of quasars with strehls greater than 0.2 is increased by a factor of 20 with LGS. In J band LGS is much less efficient, but in good atmospheric conditions, it allows to observe 720 quasars with Strehl greater than 0.2. In median conditions, this number drops to 0, mainly because of the cone effect. This leads to the the need for on-line atmospheric turbulence profile measurement and flexible scheduling of the observation, to optimize the LGS performances in J band.