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Emerging trends of optical interferometry in astronomy
The current status of the high spatial resolution imaging interferometryin optical astronomy is reviewed in the light of theoreticalexplanation, as well as of experimental constraints that exist in thepresent day technology. The basic mathematical interlude pertinent tothe interferometric technique and its applications in astronomicalobservations are presented in detail. An elaborate account of the randomrefractive index fluctuations of the atmosphere producing randomaberrations in the telescope pupil, elucidating the trade offs betweenlong-exposure and short-exposure imaging is given. Further, the othermethods 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 orlarge telescopes are described as well. The salient features of variousdetectors that are used for recording short-exposure images aresummarized. The mathematical intricacies of the data processingtechniques for both Fourier modulus and Fourier phase are analyzed; thevarious schemes of image restoration techniques are examined as wellwith emphasis set on their comparisons. The recent technologicalinnovation to compensate the deleterious effects of the atmosphere onthe telescope image in real-time is enumerated. The experimentaldescriptions of several working long baseline interferometers in thevisible band are summarized. The astrophysical results obtained tilldate are highlighted.
| Occultations of Stars by Planetary Satellites and Asteroids
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| An Atmosphere on Ganymede from Its Occultation of SAO 186800 on 7 June 1972
On 7 June 1972 the third Jovian satellite Ganymede occulted theeighth-magnitude star SAO 186800. Successful photoelectric observationsobtained at Lembang, Java (Indonesia), and Kavalur, India, shownonabrupt immersions and emersions, indicating the presence of anatmosphere whose surface pressure is greater than about 10-3millibar. By fitting the two occultation durations as chords to a modeldisk, the diameter is found to be 5270 (+30, -~ 200) kilometers, themajor error contribution arising from the uncertain atmosphericthickness below the occultation layer. The derived mean density is 2.0(-0.03, + ~ 0.2) grams per cubic centimeter.
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