A fixed plate to remove spherical aberration in Rayleigh laser guide stars

Using a Rayleigh beacon as reference source for an adaptive optics system, the latter can be limited by the differential aberrations introduced by the telescope optics on the image of the spot, namely some spherical aberratio ns introduced by the different range of the beacon with respect to the scie nce target. The introduction of new concepts about wavefront sensing along an extended range of altitudes makes urgent the solution of this problem. T n this paper we analyze the possibility to provide such a correction using a simple optical device, actually a Schmidt-like plate. We demonstrate that such a solution exists, for which the residual aberration call be nulled u p to the second order in the displacement from a pre-defined point along th e optical axis. A couple of examples, verified by a ray-tracing analysis, a re also reported. (C) 2001 Elsevier Science B.V. All rights reserved.