Low-order adaptive optics for the meter aperture solar telescope of Udaipur Solar Observatory

Abstract

Meter Aperture Solar Telescope (MAST) is a proposed modern solar telescope equipped with Adaptive Optics (AO) facility for observing the Sun in Optical and infra-red wavelengths. It is planned to develop a low-order AO system at the re-imaged pupil plane of the MAST. Before developing such an AO system, one would like to answer a few questions like what is the size of the sub-apertures required to achieve optimum performance under typical seeing conditions? What is the required bandwidth? Is it possible to operate the system with a narrow bandwidth of 0.1 nm? Is it possible to achieve diffraction limited imaging by using speckle imaging on the low-order AO corrected images? In this paper, we try to answer these questions through extensive computer simulations and arrive at a final optimal specification ot the low-order AO system of the MAST. We simulate distorted wave-fronts for various seeing conditions (for both Kolmogorov and von Karman spectrum) using large phase screens generated using Fourier transfrom method. We find the local slopes of the distorted wave-front over the sub-apertures of different lenslet array geometries using a least square modal recontruction method. Then we estimate the structure functions, optical transfer functions, Strehl resolution of the corrected wave-front and evaluate the performance. We have developed a speckle-masking code and parallelised it using a 16-processor IBM-SP machine. We use a series of AO corrected images to obtain a speckle reconstruction of the object. We evaluate the performance of this hybrid imaging system in achieving diffraction limited imaging of small-scale solar features.