Design and development of closed-loop AO system for 2-m class telescope at IIA

Abstract

Adaptive Optics (AO) is a real time wave-front correction technique, through which the light coming from the stars can be corrected for the effects of atmospheric turbulence. This enables the ground based telescope to reach diffraction limited image quality and thereby improving its resolution. The main objective of the work is to develop a laboratory model of closed loop Adaptive Optics system for Indian Astronomical Observatory (IAO) Hanle telescope (having a primary aperture diameter of 2m). The mean value of Fried Parameter at IAO Hanle is 15 cm at a wavelength 500 nm. So, Dr/0 value of Hanle telescope is 12. In order to achieve the same value in the laboratory with deformable mirror actuator size of 450 μm the beam size be 5 mm approximately. Strehl ratio is another important parameter defined as ratio of measured point spread function (PSF) to theoretical PSF. It is calculated theoretically at 1.1μm. Since AO system design in IR band gives better Strehl ratio compared to visible band. In order to understand the performance of Adaptive Optics system, a laboratory model has been proposed which is scaled down version of 2-m class. Components including Shack-Hartmann wave-front sensor (SHWFS), continues membrane deformable mirror (CDM) have been studied experimentally. A control algorithm converts the local slope measurements in the wave-front sensor caused by random phases into command values that can be addressed by the correcting element. This step called wave-front reconstruction is considered as the heart of an AO system since it controls the accuracy of wave-front sensing. An algorithm for wave-front reconstruction is developed by using Fried's geometry with vector multiplication method and it is tested for various light intensity levels for different Zernike polynomials. sCMOS which is a low noise and high speed camera, is calibrated. For a CDM, slope influence function is developed for assessment of CDM performance by giving a specific voltage to an actuator and bias voltages to all other actuators. The performance of slope influence function is tested for various higher order Zernike polynomials. The purpose of the real-time closed-loop control system is to interpret the WFS data to produce a set of meaningful control signals to position elements of the WFC, usually by reconstructing the wave-front. The wave-front is measured and data electronically processed by the RTCS, which calculates and generates signals used to apply the phase conjugate to the corrector.