title: 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 40 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 nm in the worst case at the subaperture level and a small error of the order of 10 nm for six LGSs after wavefront reconstruction.
authors: Onera-The French Aerospace Laboratory, Châtillon, France. nicolas.muller@onera.fr
authors: Nicolas
authors: N
authors: Muller
authors: Vincent
authors: V
authors: Michau
authors: Clélia
authors: C
authors: Robert
authors: Gérard
authors: G
authors: Rousset