Laser Guide Star for 3.6m and 8m telescopes: Performances and astrophysical implications

We have constructed an analytical model to simulate the behavior of an adaptive optics system coupled with a sodium laser guide star. The code is applied to a 3.6-m and 8m class telescopes. The results are given in terms of Strehl ratio and full width at half maximum of the point spread function. Two atmospheric models are used, one representing good atmospheric conditions (20 per cent of the time), the other median conditions. Sky coverage is computed for natural guide star and laser guide star systems, with two different methods. The first one is a statistical approach, using stellar densities, to compute the probability to find a nearby reference. The second is a cross-correlation of a science object catalogue and the USNO catalogue. Results are given in terms of percentage of the sky that can be accessed with given performances, and in terms of number of science object that can be observed, with Strehls greater than 0.2 and 0.1 in K and J bands.