Optimum design and development of segmented mirror telescope optics and phasing system a thesis submitted for the degree of doctor of philosophy (technology) in the department of applied optics and photonics, University of Calcutta Annu Jacob [Thesis]

Doctor of Philosophy University of Calcutta 2020

Astronomy today is in the path of constructing large telescopes. Due to several
manufacturing and maintenance difficulties, large telescopes of more than 8m
are predominantly segmented. To fulfill the need for its growing astronomical
community, India is also aspiring to create a 10m 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 1m 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 10m
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 10m 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.