Adaptive Optics for the SALT Matthew A. Kenworthy a , Andrew Sheinis b and David A.H. Buckley c a Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA; b University of Wisconsin, Department of Astronomy, 475 N. Charter Street, Madison, WI 53706, USA; c SAAO, PO Box 9, Observatory 7935, Cape Town, South Africa ABSTRACT We present a cost-effective solution for adaptive optics (AO) correction on the Southern African Large Telescope (SALT), where each primary mirror segment has compensation for tip-tilt atmospheric errors and a slower, active optic loop for sensing piston and correcting for focus drift. By treating the telescope as 91 independent tip-tilt corrected units, we compute the encircled energy gains for different seeing conditions at the SALT. Finally, the optical design for a simple AO demonstrator camera is presented, using seven tip-tilt correctors to directly measure and compare closed loop and open loop performances, which will help lead a full SALT AO system design. Keywords: adaptive optics, tip-tilt, SALT, segmented primary mirror, active optics 1. INTRODUCTION The Southern African Large Telescope (SALT) 1, 2 is a fixed elevation telescope (based on the Hobberly Ebberly Telescope 3 (HET)) whose aperture is formed from 91 hexagonal mirrors in a close packed hexagonal array, generating an 11 meter diameter telescope. The aperture has a spherical figure with radius of 26165mm, and a Spherical Aberration Corrector 4 (SAC) that provides a field flattened, aberration corrected 10 arcminute diameter field of view. Optical path differences introduced by the atmosphere prevent many ground based telescopes from reaching their diffraction limited performance, as the plane wavefront from a distant celestial object is distorted by its propagation through temperature-induced changes in the index of refraction to the telescope aperture. These peturbations vary on a timescale of milliseconds at the wind moves across the telescope’s line of sight. Many large telescopes use adaptive optic (AO) systems to improve the delivered image quality to the final science instruments by using a deformable mirror to provide path length compensation at a rate of up to 1kHz. A standard measure of the optical strength of the turbulence is Fried’s parameter, r 0 , which is defined as the diameter over which the optical phase distortion has a mean-square value of 1 rad 2 at a wavelength of 0.5μm. The Fried parameter is a function of wavelength λ, varying as λ 6/5 . Longer wavelengths have larger Fried parameters, with typical values of r 0 = 20cm for 1 arcsecond seeing in the visible, up to r 0(2μm) =2m in the infrared - furthermore, the Fried parameter can rapidly vary (up to a factor of 2) over several seconds. An adaptive optic system provides partial correction for the aberrations introduced by the atmosphere. The atmospheric aberrations are measured with a wavefront sensor (WFS) camera and a set of deformable optical elements are subsequently supplied with corrective signals from the WFS camera, forming a closed loop whose rate of correction varies between different systems, but is typically 500Hz to 2kHz. The reference wavefront is either supplied by an astrophysical object, either a nearby bright astrophysical source or the science target of interest itself, or from a projected laser beam which produces an artificial guide star. The former systems are called Natural Guide Star 5, 6 (NGS) AO systems, whilst the latter are Laser Guide Star 7, 8 (LGS) AO systems. Further author information: (Send correspondence to M.A.K.) M.A.K.: E-mail: mkenworthy@as.arizona.edu Telephone: +1 520 626 6720 A.A.S.: E-mail: sheinis@astro.wisc.edu, Telephone: +1 608 262 0492 D.B.: E-mail: dibnob@saao.ac.za, Telephone: +27 21 4606286 Fax: +27 21 447639 Adaptive Optics Systems, edited by Norbert Hubin, Claire E. Max, Peter L. Wizinowich, Proc. of SPIE Vol. 7015, 701563, (2008) 0277-786X/08/$18 · doi: 10.1117/12.789623 Proc. of SPIE Vol. 7015 701563-1 Downloaded from SPIE Digital Library on 13 Jul 2011 to 150.135.115.80. Terms of Use: http://spiedl.org/terms