Sky Projected Shack-Hartmann Laser Guide Star T. Butterley a , D.F. Buscher b , G. D. Love a , T.J. Morris a , R. M. Myers a and R. W. Wilson a a University of Durham, Dept. of Physics, Rochester Building, South Road, Durham, DH1 3LE, UK; b University of Cambridge, Cavendish Astrophysics Group, Cambridge CB3 0HE, UK ABSTRACT We describe SPLASH (Sky Projected Laser Array Shack-Hartmann) which is a method of laser guide star (LGS) wavefront sensing with reduced focal anisoplanatism (FA). We present the results of a semi-theoretical analysis and a semi-geometrical simulation of SPLASH, allowing a direct comparison between SPLASH and a conventional laser guide star system. We show that SPLASH is significantly less susceptible to focal anisoplanatism than a conventional LGS. Keywords: Adaptive optics, Laser guide stars, ELTs 1. INTRODUCTION Focal anisoplanatism (FA) is a limitation of the current generation of laser guide star adaptive optics (LGS AO) systems and will render a single LGS AO system on an extremely large telescope (ELT) virtually unusable. Multiconjugate adaptive optics (MCAO) will mitigate some of the problems by essentially combining data from different lasers, however a LGS system without focal anisoplanatism would be a huge bonus with or without MCAO. Here we outline a possible alternative implementation of a LGS in which the atmosphere is sensed on the upward path of the laser through the atmosphere. SPLASH, or Sky Projected Laser Array Shack-Hartmann 1 is a pseudo-reverse of the usual method of Shack-Hartmann wavefront sensing. An array of converging beams is launched monostatically from the telescope to produce an array of Shack-Hartmann spots projected onto the sky, which are then imaged by the whole telescope. A schematic outline of a SPLASH is shown in Fig. 1. An array of converging beams is focused on the sodium layer, conceptually the same as placing a huge lenslet array over the top of the telescope. The system could also be implemented with a Rayleigh laser. The beam size, at the telescope, is ∼ r 0 as in a conventional Shack-Hartmann system and therefore the focal spot quality will be largely unaffected by the atmosphere but the position will be shifted due to the local wavefront gradient. On the downward passage of light through the atmosphere the light is collected by the whole telescope and conventionally imaged without any kind of wavefront sensor. The array image will be both distorted and shifted due to the global tip and tilt. The relative positions of the spot images will therefore be a measure of the local tilt minus the global tilt — i.e. exactly the same quantity as measured in a conventional LGS with a Shack-Hartmann wavefront sensor (WFS). The major advantage of this system is that the cone effect will be much reduced. The system proposed in Fig. 1 appears rather similar to the concept of stitching and butting (see, e.g., Fried, 2 Parenti and Sasiela 3 ) whereby an array of LGSs is used in order to reduce focal anisoplanatism. However, in this approach the atmosphere is still sensed on the return path, and the light is sent to a wavefront sensor. Problems then occur in trying to join the resulting phase maps together. There are a number of issues associated with SPLASH wavefront sensing. 1. A major one is the fact that Fig. 1 is drawn assuming geometrical optics whereas in reality the beams will diffract. Assuming no aberrations, a 10cm sodium beam focused at 90km has a full beam width of 1.3m, which is clearly much too large for the system to work. Furthermore, the spots will further merge due to the actual displacements due to the atmosphere, which will be of a similar magnitude. There are two potential solutions to this problem: Further author information: E-mail: timothy.butterley@durham.ac.uk