Emerging trends of optical interferometry in astronomy

The current status of the high spatial resolution imaging interferometry in optical astronomy is reviewed in the light of theoretical explanation, as well as of experimental constraints that exist in the present day technology. The basic mathematical interlude pertinent to the interferometric technique and its applications in astronomical observations using both single aperture, as well as diluted apertures are presented in detail. An elaborate account of the random refractive index fluctuations of the atmosphere producing random aberrations in the telescope pupil, elucidating the trade offs between long-exposure and short-exposure imaging is given. The formation of speckles and of ways to detect them in the case of astronomical objects are discussed. Further, the other methods viz., (i) speckle spectroscopy, (ii) speckle polarimetry, (iii) phase closure, (iv) aperture synthesis, (v) pupil plane interferometry, (vi) differential speckle interferometry etc., using single moderate or large telescopes are described as well. The salient features of various detectors that are used for recording short-exposure images are summarized. The mathematical intricacies of the data processing techniques for both Fourier modulus and Fourier phase are analyzed; the various schemes of image restoration techniques are examined as well with emphasis set on their comparisons. A brief account of obtaining diffraction-limited high resolution features of a few extended objects is presented. The recent technological innovation to compensate the deleterious effects of the atmosphere on the telescope image in real-time is enumerated. The experimental descriptions of several working long baseline interferometers in the visible band using two or more telescopes are summarized. The astrophysical results obtained till date using both single aperture interferometry, as well as long baseline interferometry with diluted apertures are highlighted.