Static telescope aberration measurement and correction using lucky imaging techniques

Abstract

ABSTRACT A procedure has been developed to compute static aberrations after the PSF measured with the lucky imaging technique, using a nearby star as the point source to probe the optical system. This PSF is ite ratively turned into a phase map at the pupil using the Gerchberg-Saxton algorithm, and then conver ted to the adequate actuation information for a deformable mirror having low actuator number but large stroke capability. The main advantage of this procedure is related with the ca pability of correcting static aberration at the specific pointing direction and without the need of a wavefront sensor. Keywords: Static aberrations, lucky imaging, phase retrieva l, Gerchberg-Saxton, diffraction limit imaging. 1. INTRODUCTION Most present-day telescopes were designed bearing in mind th e seeing statistics as the refere nce for the error budget in static aberrations. It was expe cted for the telescope mirror supports flexures not to introduce significant aberration when pointing to the different directions on sky, in comparison with mean seeing figures. New observing techniques like Lucky Imaging (FastCam)[1][2] have demonstrated their capability to routinely provide diffraction limited imaging in the I-Band, at telescopes in the range 1to 4 meters, and even to obtain Airy rings at the 1.5 meter TCS at Observatorio del Teide (Tenerife, Canary Is. SPAIN). Once the atmospheric turbulence is removed, a remaining aberration due to mirror misalignment is the limiting factor in spatial resolution, provided the shape of the primary mirror has been adequately adjusted whenever feasible. This aberration depends on the pointing direction, mostly due to the effect of grav ity on the mechanical structure. A procedure has been developed to compute these static aber rations after the Point Spread Function (PSF) measured with the lucky imaging technique, using a near by star as the point source to probe th e optical system. This PSF is iteratively turned into a phase map at the pupil using the Gerchberg-Saxton algorithm[3], and will be converted in the future to the adequate actuation information for a deformable mirror ha ving low actuator number but large stroke capability. This algorithm has been developed and testd using MATLAB, since it is a nice development and debugging platform and to tests its capabilities easily, but the future aim of the project is to use FPGAs (Field Programmable Gate Array) to execute the algorithm in real time, w ith all the advantages this means. The main advantage of this procedure is related with the ca pability of correcting static aberration at the specific pointing direction and without the need of a wavefront sensor. This paper describes the PSF measuring and phase recovery that have been accomplished and tested at th e 1.5 TCS at Obs. del Teide (Canary Is.) *LRR@iac.es; phone +34 922 605 200; fax +34 922 605 210; www.iac.es