Optimum design and development of segmented mirror telescope optics and phasing system

Abstract

Astronomy today is in the path of constructing large telescopes. Due to several manufacturing and maintenance difficulties, large telescopes of more than 8 m are predominantly segmented. To fulfill the need for its growing astronomical community, India is also aspiring to create a 10 m class Optical-NIR observing facility within the country. A telescope of this size requires huge investment along with hundreds of scientists, engineers, and technicians working together about a decade to realize it. Therefore, before embarking on such a challenging megaproject, the segmented mirror technology, which is not yet standardized, needs to be demonstrated. Considering this, it has been proposed to develop a 1 m class Prototype Segmented Mirror Telescope (PSMT). The PSMT is expected to be a perfect test-bed for the primary mirror control and alignment and the phasing system. As a part of the thesis work, we have come up with a cost-effective, optimum design of the optics for the PSMT. The PSMT optics, which uses spherical primary and oblate ellipsoid secondary, is unique in design and simple in construction. We have designed telescope optics considering the segmented primary mirror, and have also carried out a detailed analysis for the sensitivity, tolerance, and the error estimation. In the next step, we also designed the optics for the proposed 10 m class telescope named National Large Optical Telescope (NLOT). Since NLOT will be a large segmented mirror telescope, an extensive study has been carried to understand the effect of segment size, miss-alignment ( tip-tilt,de-center, and clocking), phasing error, segment to segment ROC variations, figure error as well as inter-segment gaps. In this process, we developed many generic tools and techniques which makes segmentation related study simpler. For the 10 m class telescope, we have also explored the possibility of using spherical mirror segments in place of aspheric ones, which are quite difficult to manufacture. A segmented telescope can provide diffraction limited design performance only if their mirror segments are aligned and phased. So, the second part of this thesis is dedicated to developing phasing techniques. We have carried out an extensive study and laboratory experimentation on two different phasing schemes. The first phasing scheme is based on Shack-Hartmann working in the physical optics domain, whereas the other is based on the pyramid sensor. The basic principle of these two phasing schemes are explored, and simulations have been carried out to check their performances. We also attempted to undertake laboratory experimentation on the above phasing techniques. The results of the simulation and the experimentation have been presented and discussed.