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We are investigating sodium layer laser guide stars for use with astronomical adaptive optics. A high power dye laser at the Lawrence Livermore National Laboratory is being used to create a guide star in the sodium layer at an altitude of 95 km. This paper describes both the initial experiments aimed at characterizing the guide star spot size, spot motion, and the return signal strength, or irradiance, as well as our first full Hartmann sensor measurements of the wavefront from the guide star.
Because of saturation effects, the observed, or emission, spot size is larger than the laser irradiance spot size. An emission spot size of 3.0 meters was measured, with an implied laser irradiance spot diameter of 2.0 meters. The spot motion was measured with a fast framing camera. The rms centroid motion at the high laser powers, with active beam pointing control, was less than 0.5 arc seconds, and had little effect on the observed spot size. Resonant backscatter from the sodium layer was measured as a function of laser power from 7 to 1100W, to obtain a saturation curve. With a transmitted power of 1100W and atmospheric transmission of 0.6, the irradiance from the guide star at the ground was 10 photons/cm2-ms, corresponding to a visual magnitude of 5.1.
Using Hartmann sensor data from the guide star, we analyze the spatio-temporal statistics of the wavefronts, compare them to measurements from a natural star, and discuss the implications for the performance of an adaptive optics system.
