Abstract Details

Affiliation: Indian institute of astrophysics
Conference ID: ASI2017_801
Title : Pyramid Sensor for Alignment and Phasing a Segmented Mirror Telescope
Authors and Co-Authors : Annu Jacob (Indian Institute of Astrophysics, Koramangala, Bangalore-560034), Sindhu Divakaran(Manipal Institute of Technology, Manipal 576104), Padmakar Parihar(Indian Institute of Astrophysics, Koramangala, Bangalore-560034)
Abstract Type : Poster
Abstract Category : Instrumentation and Techniques
Abstract : For any segmented mirror telescope (SMT) , to achieve a seeing limited performance it is necessary that all mirror segments are aligned. However, for diffraction limited performance which gives highest spatial resolution as well as sensitivity, every mirror segment need to be phased. In an SMT, alignment and phasing is done with help of an active optical system which is basically some kind of wavefront sensor, having special ability to measure piston (phase discontinuty) error. One of such sensor which is being extensivly explored by various group around globe is Pyramid based Alignment and Phasing System (APS). Pyramid sensor appaers to be ideal for this purpose due to its many merits such as: it has a capability to measure piston error between two segments, measure miss-alignment of mirror segments, capable of measuring wavefront aberration caused by deformation of the mirror segment, avoid bad effects of charge diffusion in CCD detectors, ability to naturally filters out high spatial frequency information etc. Over more than an year in ITCC laboratory of of IIA, we have been exploring the possibility of using pyramid sensor for aligning and phasing proposed PSMT and NLOT telescopes. In this effort we could derive basic mathematical formulations required for the pyramid sensor as well as developed a tool using MATLAB® to simulate its operation. Pyramid sensor basically works like a modified Knife edge test. In which the PSF formed by any imaging device such as telescope is split into four parts by pyramid sensor and then subsequently made to form four pupil images using a collimating device. From these four pupil images, two images which is called signal maps are synthesized. Using these signal maps and the mathematical formula which also includes SVD based inversion matrix, we find the wavefront. The simulations have been carried out for both non-aberrated as well aberrated wavefronts and we found that in both cases we could fairly recover the injected wavefront. As a next step we designed the experimental optical setup using ZEMAX®. Finally a crude laboratory experimentation have been conducted to check over all functioning of a pyramid based wavefront sensor and its application on aligning and phasing a segmented mirror telescope. In this paper, we will present the basic theory behind pyramid sensor, some of the results of our MATLAB® simulations, optical design and preliminary experimental results.