Rayleigh scattering and laser spot elongation problems at ALFA

This paper describes the qualitative effects of LGS spot elongation and Rayleigh scattering on ALFA wavefront sensor images. An analytical model of Rayleigh scattering and a numerical model of laser plume generation at the altitude of the Na-layer were developed. These models, integrated into a general AO simulation, provide the sensor sub-aperture images. It is shown that the centroid measurement accuracy is affected by these phenomena. The simulation was made both for the ALFA system and for the VLT Nasmyth Adaptive Optics System (NAOS).