Abstract
Artificial guide stars generated by backscattering of laser light in the Earth’s atmosphere are intended to solve one of the main problems of astronomical adaptive optics: the limited sky coverage due to the requirement of having a bright object within the isoplanatic angle, which is needed for wavefront sensing. This article gives a brief introduction into the formation of laser guide stars, either through Rayleigh backscattering in the lower atmosphere, or through resonant backscattering from the sodium layer at an altitude of 90 km. The main limitations of these methods are explained: the tilt determination problem and focal anisoplanatism. The most important properties of the sodium layer are summarized, and some technical issues relevant to the generation of sodium guide stars are discussed.
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