Please use this identifier to cite or link to this item: http://hdl.handle.net/2248/6980
Title: Image retrieval in astronomical interferometers affected by atmospheric turbulence
Authors: Arun Surya
Issue Date: Jun-2015
Publisher: Indian Institute of Astrophysics
Citation: Ph.D. Thesis, University of Calcutta, Kolkata
Abstract: Single aperture speckle interferometry (Labeyrie, 1970) is a method that deciphers diffraction limited spatial Fourier spectrum and image features of stellar objects by counteracting blurring effect caused by the atmospheric turbulence. Together with pupil-plane techniques, as well as hybrid methods (speckle techniques with non- redundant pupils), it has made impacts in several important fields in astrophysics. The field of research that has benefited the most from such high angular resolution techniques and will still benefit in the future, is the origin and evolution of stellar systems. However, classical speckle interferometry falls short of obtaining phase information of the object, but provides a second-order moment (power spectrum) analysis which is the modulus of the object Fourier transform. Triple Correlation technique and other advanced image retrieval methods has been developed which also allows the reconstruction of the phase information. Such algorithmic techniques allows to retrieve diffraction limited information from the short exposure images. The thesis discusses the development of two image reconstruction algorithms based on triple correlation to be used with direct images from optical interferometers. Direct bispectrum algorithm uses a computationally intensive yet efficient triple correlation technique to reconstruct object information from two dimensional speckle images. Tomographic speckle masking algorithm has been developed to offer a computationally efficient method to reconstruct images from speckle data. It uses one dimensional projections and Radon transform to gain considerable savings in computational time and memory. Both these algorithms were tested with numerical simulations, real data and experimental simulations. Numerical simulations of different optical interferometric techniques was built to understand the possibility of reconstruction using these methods in multi-aperture optical interferometry. In the simulations the advantages of the all-in-one beam combination is over the use of pair-wise combination was also analyzed. We layout a study of the signal advantage we obtain at low light levels through all-in-one beam combination in cophased and non-cophased speckle mode. The usability of these algorithms with diluted aperture interferometers which use pupil densification (Hypertelescopes) is also explored in this thesis. It is seen from the numerical simulations of image recontructions that the developed algorithms can be used to restore atmospherically degraded images from hypertelescopes with good signal to noise ratio.
Description: Thesis Supervisors Prof. Swapan Kumar Saha
URI: http://hdl.handle.net/2248/6980
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Appears in Collections:IIAP Ph.D.Theses

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