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Prof. S. K. Saha
Date and Time of Talk

Optical interferometry is undergoing a revolutionary change and is gaining momentum each day. From the
laboratory to the observatory over the past few decades, the surge of activities in developing such systems
reflects the motivation to realize its potential. The implementation of imaging by interferometry began more
than a century ago, though the first successful measurement of the angular diameter of alpha Orionis was
performed in 1920. Subsequent development of intensity interferometry in late 1950's had proved to be the
driving force in ushering in the modern era of quantum optics. Diffraction-limited resolution achievable by
any terrestrial large telescope far surpasses that imposed by atmospheric fluctuations above the telescope
aperture as against lone orbiting telescope. Turbulence and the concomitant development of thermal
convection in the atmosphere distort the phase and amplitude of the incoming wavefront of starlight. Due to
the diffraction phenomenon, the image of a point source blurs at the focal point of the telescope. The blurring
suffered by such images is modelled as convolution with the point spread function. Speckle interferometry
has opened up a new era in modern optics and has made impact in several important fields in astrophysics. It
deciphers diffraction-limited spatial Fourier spectrum and image features of stellar objects from such blurred
images. Following its success, astronomers focused their efforts on developing image processing techniques
and applied them to improve the resolution of astronomical images. Advancements in technology in the last
few decades have brought the hardware to compensate in real time of the wave-front distortion; this
technique is referred to as adaptive optics system. However, a host of basic problems need very high angular
resolution for their solution that can be acquired by using long baseline optical interferometers. An
interferometer of this kind uses two or more apertures to mimic the angular resolution of a single one having
a diameter about the size of the separations. Success in synthesizing images obtained from a pair of
independent telescopes on a North-South baseline configuration in mid 1970's, impelled astronomers to
venture towards ground-based arrays. A novel concept, called hypertelescope, may provide snapshot images
having both a high resolution and a high information content. It involves the blossom of huge flower-like
instruments, which have the potential of extending the angular resolution towards sub-milliarcseconds at
visible wavelengths. However, it requires to incorporate many apertures to provide direct snapshot images
with this high resolution. In this talk, I shall discuss the current trend and the path to future progress in
aperture synthesis techniques in optical astronomy.

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About Speaker

Professor Swapan K. Saha, formerly at the Indian Institute of Astrophysics (IIA), Bangalore, obtained his Ph. D
(Tech) from the Institute of Radio Physics & Electronics, Calcutta University in 1983 and spent a year during
1988-89 at the Observatoire de la Cote d'Azur (formerly C E R G A), Caussols, France, to study the high angular
features of stars, using the long baseline optical interferometer with a pair of 1.5 meter telescopes. He has a
strong interest in Experimental Physics, optical interferometry in particular and has developed various
equipment. Among others, he has developed a speckle interferometer for the 2.34 meter Vainu Bappu Telescope,
Vainu Bappu Observatory, Kavalur, India.
Professor Saha is the author of numerous papers, articles, and books. The books entitled: (i) `Diffraction-limited
Imaging with Large and Moderate Telescopes', Swapan K Saha, 2007, World Scientific, New Jersey, USA, (ii)
`Aperture Synthesis: Methods and Application to Optical Astronomy', Swapan K Saha, 2010, Springer, New
York, USA, (iii) `High Resolution Imaging: Detectors and Applications’, Swapan K Saha, 2015, Pan Stanford
Pub. Co., Singapore, and (iv) `Waves and Oscillations in Nature: An Introduction’, A Satya Narayanan and
Swapan K Saha, 2015, CRC Press, USA, are well cited. At present, he is engaged in writing another book
entitled, `Paradigm of Optical Imaging’, Swapan K Saha, 2022 (tentative year of publication), Pan Stanford Pub.
Co., Singapore.

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Dr. Brijesh Kumar