Analysis of on-sky sodium profile data from Lick Observatory

The next generation of adaptive optics will depend on laser guide stars to increase sky coverage. However, there are a few limitations. The thickness of the sodium layer in the mesosphere at 90 km causes spot elongation, which is more severe for large telescopes. Moreover, the outer-edge subaperture of such large telescopes will resolve variations of sodium atom density seen over the thick layer. We quantify these density fluctuations using real data taken at Lick Observatory. We used the 1 m Nickel telescope to image the return flux due to laser-induced fluorescence from a dye laser launched from the nearby 3 m Shane telescope. This view was from the side allowing the resolution of the sodium return as a function of height. We used drift scanning of the 1 m telescope to provide resolution in time. We show qualitative images of the sodium distribution for different nights and quantitatively study the temporal power spectra of those fluctuations. We conclude that the sodium profiles have an average full width half-maximum of 10 km. However, the extent beyond the nominal 10 km thickness is important for accurate wavefront sensing. The variations in the height of the sodium layer occur on short enough time scales that AO systems for 30 m class telescopes will likely need focus updates on time scales shorter than 100 ms.