Development of alignment and phasing system for India’s large segmented mirror telescope

Abstract

The segmented mirror technology has become the preferred choice for building moderate to large-size optical telescopes. For the segmented primary mirror to function like a single monolithic mirror, each segment must be co-aligned, co-focused, and co-phased. In any segmented mirror telescope, these tasks are accomplished by an Alignment and Phasing System (APS). The India’s upcoming large optical-NIR telescope, will use a segmented primary mirror. This thesis explores the technology required to align and phase its mirror segments. To begin with, an end-to-end Pythonbased code is developed to carry out very detailed simulations on co-alignment, cofocusing, and co-phasing. The code generates realistic images for the telescope + APS system. The simulation results are used to estimate the achievable measurement accuracy by the APS. As a part of the thesis work, a Segment Alignment Device (SAD) has also been designed, developed, and tested for the one-meter class Prototype Segmented Mirror Telescope (PSMT). For any large segmented mirror telescope to achieve diffraction-limited performance, its mirror segments must be phased with very high accuracy (in the order of a few nanometers). Phasing the mirror segments is one of the most challenging tasks, and as a part of this project, we have explored two different phasing techniques: the Dispersed Fringe Sensor (DFS) and the Optical Transfer Function (OTF) based phasing. For both techniques, we have first performed extensive simulations to test their feasibility under realistic conditions. And then, the simulation results are verified through laboratory experimentation. From our research work we have shown that the DFS, which is conventionally considered as a coarse phasing technique, can also be used for fine phasing. In laboratory experimentation, we have achieved a piston measurement accuracy of 2–3 nm. Similarly, we have explored the OTF-based phasing for which we achieved a piston measurement accuracy of 1.5-18nm through laboratory experiments. Through our extensive simulations and laboratory experiments, we have confirmed that both DFS and OTF can be used for coarse and fine phasing of mirror segments, thus eliminating the need to use two different phasing techniques as is done conventionally. This thesis explores all three aspects of an APS and aims to understand and improve the alignment and phasing techniques so that this knowledge can be later implemented on the India’s large segmented mirror telescope.