Differential focal anisoplanatism in laser guide star wavefront sensing on extremely large telescopes

Abstract

Laser guide stars (LGSs) aim at increasing the sky coverage of adaptive optics (AO) as this is highly restricted when using only natural guide stars. With such three-dimensional extended objects, spot elongation may limit the measurement accuracy of wavefronts. We evaluate the effect of differential focal anisoplanatism, induced solely by the longitudinal extension of a side-launched LGS, on the slope measurements performed by a Shack–Hartmann for a $\mathrm{40}\mathrm{m}$ class telescope. We also take this effect into account in the wavefront reconstruction and derive estimations of the resulting wavefront error in a multi-LGS AO system. We find an error of $100\mathrm{nm}$ in the worst case at the subaperture level and a small error of the order of $10\mathrm{nm}$ for six LGSs after wavefront reconstruction.

© 2011 Optical Society of America

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