Optical design of FOSC and wide field aberration corrector for the prototype segmented mirror telescope

Abstract

The Prototype Segmented Mirror Telescope (PSMT) is a 1.3m segmented mirror telescope that aims to develop and demonstrate the segmented mirror technology indigenously. The telescope design includes a spherical primary with seven hexagonal segments of size 500mm each and an ellipsoidal secondary. Since the telescope has a spherical primary, hence it suffers from spherical aberrations as well as large off-axis aberrations, thus limiting its field of view. In order to improve the image quality over relatively larger field, an aberration corrector is required. The Faint Object Spectrograph and Camera (FOSC) is a widely used back-end instrument for any telescope12 and PSMT is also supposed to be equipped with such an instrument. Therefore, we have designed the optics of the FOSC in such way that it meets dual requirements i.e. it works as a science instrument as well as an aberration corrector. The FOSC instrument consists of multi-element collimator and camera lenses and a grism is used as a dispersive element. The FOSC optics design is optimized for the visual wavelength range of 4500-8500Ao and up to 10 arc-min field of view. Here, we present the optical design of the FOSC and outcome of the analysis carried out using the ZEMAX optical design software.