Image Retrieval in Astronomical Interferometers Affected by Atmospheric Turbulence

Abstract

Single aperture speckle interferometry (Labeyrie, 1970) is a method that de- ciphers 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 sev- eral 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 ob- ject, 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 tech- niques allows to retrieve diffraction limited information from the short expo- sure images. The thesis discusses the development of two image reconstruc- tion 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 infor- mation 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 recon- struction 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 den- sification (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.