Seeing improvement with ground-layer adaptive optics

Ground-layer adaptive optics (GLAO) promises a significant improvement in image width and energy concentration over atmospheric seeing in a relatively wide field by compensating for low-altitude turbulence only. This approach, which is intermediate between full AO and seeing-limited observations, can benefit many astronomical programs in the visible and near-infrared. We have developed a simple analytical method to compute the compensated image quality over the field and apply it to evaluate future GLAO systems that use either Rayleigh or sodium laser guide stars. It is shown that even with a single Rayleigh laser, a useful correction over a wide field is obtained. We also investigate the dependence of GLAO performance on turbulence profile. Most residual anisoplanatism and ellipticity is caused by the partially corrected turbulence located in the "gray zone" from few hundred meters to 1-2 km above ground.