GLAO 4 ELT : a trade study and SAM experience

It is expected that a substantial fraction of wave-front distortions at EL Ts will be isoplanatic, being produced jointly by near-ground and dome turbulence, wind bu ffeting, flexures, and vibrations. We compare various strategies for sensing this ”ground” component using (i) seve ral sodium LGSs, (ii) one or several Rayleigh LGSs and (ii) NGSs in a wide field. The experience gained so far with the RLG S GLAO system SAM at 4m telescope is presented. The results include wide-field partial correction with NGS, vibrations in focus and astigmatism, PSF modeling, and first tests of SAM with UV RLGS.