First analysis of artificial guide star focus anisoplanatism

Abstract

The backscatter from a laser beam focused in the atmosphere can be used as an artificial guide star reference for the wavefront sensor of an adaptive optics astronomical imaging system. The wavefront information thus provided represents probing of the turbulence along a somewhat different propagation path than the path the starlight to be imaged will follow, with a resultant discrepancy in the wavefront sensor’s output. This discrepancy has become known as focus anisoplanatism. In the period Dec. 81-Jan. 82 we carried out a detailed analysis of the effect of focus anisoplanatism on image resolution (or rather on antenna gain). Our results indicated that the effective diameter, corresponding in a diffraction-limited sense to the achievable antenna gain could be interestingly large, e.g., of the order of 0.5 m for operation at a wavelength as short as 0.35 µm, if the backscatter altitude were 20 km. A copy of the original manuscript of our analysis is exhibited and the analysis reviewed. A graph of the originally obtained results is presented.