Distortion mapping correction in aspheric null testing

Abstract

ABSTRACT We describe methods to correct both symmetric and asymmetric distortion mapping errors induced by null testing elements such as holograms or null lenses. We show experimental resu lts for direct measurement and correction of symmetric mapping distortion, as well as an example result for analytical mapping performed using an orthogonal set of vector polynomials for asymmetric correction. The empirical determination of symmetric distortion is made via calculation from predicted and measured changes to aberrations induced via known changes to the testing point. 1. INTRODUCTION Aspheric primary mirrors are popular for astronomical telescope systems. These mirrors are polished to aspheric shapes to help minimize the overall length of the telescope system and provide additional optical correction for improved telescope performance. Optical testing with high accuracy is necessary, from as early a point in the fabrication process as is feasible, to efficiently converge to the proper finished optical surface. One potential obstacle encount ered when fabricating aspheric su rfaces is distortion present when imaging through null elements in the optical test. Dist ortion arises due to imperfect imaging through a null lens system and other optics in an interferometer or through a transmission CGH. The presence of distortion diminishes accuracy by mapping surface features to incorrect radial positions on the image plane. In this paper, we describe a method used to determine symmetric distortion which may prove practical for large asphere testing, as well as a more general correction method for asymmetric distortion. These techniques can be used to enable efficien t testing of aspheric optical surfaces.