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Title: Development of faster algorithm for Shack-Hartmann wavefront sensor
Authors: Valsan, V
Issue Date: Aug-2010
Publisher: Indian Institute of Astrophysics
Citation: M. Tech. Thesis, University of Calcutta, Calcutta
Abstract: Atmospheric turbulence reduces the quality of the image from the optical telescope. The plane wavefront becomes distorted because of turbulence. Adaptive Optics is a technique that corrects the distorted wavefront to obtain the diffraction limited performance of the ground-based telescope. An Adaptive Optics system mainly consists of three main components, a wavefront sensor, a wavefront corrector and a control system. The most commonly employed method of wavefront sensing is Shack-Hartmann wavefront sensing. It consists of an array of micro lenslets, which samples the incoming distorted wavefront. Due to non-planarity of the sampled wavefront at each lenslet, the image displaces its position from the reference position. By measuring the displacement from the reference point, the slope of the wavefront can be calculated. By using the slope of the wavefront, its phase is estimated. Since the turbulence variations are taking place in the order of milliseconds, the necessary algorithm has to be faster than the atmospheric variations. The project work is carried out in two parts. In the first part, a study of different centroid algorithm for the point source, is done and it is found that the Iterative Weighted Center of Gravity (lWCoG) method is more efficient and fast for Shack-Hartmann sensor. The second part focuses upon developing a fast phase retrieval algorithm which reduces the noise and improves the sensitivity. From the centroid the wavefront is reconstructed through modal approach, which has been done taking care of the amount of time consumed. At first the slope is measured from the centroid data. From the slope measurements, the wavefront has been reconstructed through Zernike polynomials, which are orthogonal over a unit circle. The program has been written in MatLab 7.
Description: Restricted Access
Thesis Supervisor Dr. J. P Lancelot
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Appears in Collections:Integrated M.Tech-Ph.D (Tech.)

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