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Speckle Imaging: a boon for astronomical observations
S K Saha
Physics , 2000,
Abstract: The speckle imaging is a photographic technique that resolves objects viewed through severely distorted media. The results are insensitive to the errors caused by apparent size of the isoplanatic patch and the telescope aberrations. In this article, a short descriptions of the atmospheric turbulence and its effect on the flat wavefront from a stellar source is presented; the shortcomings of the conventional long-exposure images in the presence of Earth's atmosphere are discussed. The advantages of the speckle interferometric technique over conventional imaging are enumerated. The technical details of the method, basic Fourier optics, data analysis procedures are also described.
Speckle interferometry
S. K. Saha
Physics , 1999,
Abstract: We have presented the basic mathematical treatment of interferometry in the optical domain. Its applications in astronomical observations using both the single aperture, as well as the diluted apertures are described in detail. We have also described about the shortcomings of this technique in the presence of Earth's atmosphere. A short descriptions of the atmospheric turbulence and its effect on the flat wavefront from a stellar source is given. The formation of speckle which acts as carrier of information is defined. Laboratory experiments with phase modulation screens, as well as the resultant intensity distributions due to point source are demonstrated. The experimental method to freeze the speckles, as well as data processing techniques for both Fourier modulus and Fourier phase are described. We have also discussed the technique of the aperture synthesis using non-redundant aperture masks at the pupil plane of the telescope, emphasizing set on the comparison with speckle interferometry. The various methods of image restoration and their comparisons are also discussed. Finally, we have touched upon certain astrophysical problems which can be tackled with the newly developed speckle interferometer using the 2.34 meter Vainu Bappu Telescope (VBT), situated at the Vainu Bappu Observatory (VBO), Kavalur, India.
Emerging trends of optical interferometry in astronomy
S. K. Saha
Physics , 1999,
Abstract: 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 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. 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. 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 are summarized. The astrophysical results obtained till date are highlighted.
Nanodielectrics with giant permittivity
S K Saha
- , 2008,
Abstract:
Parametric Optimization in WEDM of WC-Co Composite by Neuro-Genetic Technique
P. Saha,P. Saha,S. K. Pal
Lecture Notes in Engineering and Computer Science , 2011,
Abstract:
A Finite Element Analysis on MHD Free Convection Flow in Open Square Cavity Containing Heated Circular Cylinder  [PDF]
Sheikh Anwar Hossain, M. A. Alim, S. K. Saha
American Journal of Computational Mathematics (AJCM) , 2015, DOI: 10.4236/ajcm.2015.51003
Abstract: The problem of Magnetohydrodynamic (MHD) free convection heat transfer in a square open cavity containing a heated circular cylinder at the centre has been investigated in this work. As boundary conditions of the cavity, the left vertical wall is kept at a constant heat flux, bottom and top walls are kept at different high and low temperature respectively. The remaining side wall is open. Finite element analysis based on Galerkin weighted Residual approach is used to visualize the temperature distribution and fluid flow solving two-dimensional governing mass, momentum and energy equations for steady state, natural convection flow in presence of magnetic field in side an open square cavity. A uniformly heated circular cylinder is located at the centre of the cavity. The object of this study is to describe the effects of MHD on the thermal fields and flow in presence of such heated circular cylinder by visualization of graph. The investigations are conducted for different values of Rayleigh number (Ra) and Hartmann number (Ha). The results show that the temperature field and flow pattern are significantly dependent on the above mentioned parameters.
Perspective of long baseline interferometry
S K Saha,S. Morel
Physics , 2000,
Abstract: This article is a sort of sequel of the earlier extensive review by Saha (1999a) where emphasis was laid down on the ground based single aperture, as well as on the working long baseline optical interferometers (LBI) situated at the various observatories across the globe that are producing a large amount of astronomical results. Since the future of high resolution astronomy lies with the new generation of arrays, the numerous technical challenges of developing such systems are addressed indicating the current trends and the path to future progress in interferometry. The new generation interferometers such as Palomar testbed interferometer (PTI), Navy prototype optical interferometer (NPOI), Keck interferometer, Very large telescope interferometer (VLTI), Center for high angular resolution astronomy (CHARA) array, Optical very large array (OVLA), Mitaka optical infrared arrays (MIRA), etc., are being developed. A few of them, viz., PTI, NPOI, IOTA are producing results. Among the working interferometers that have been described earlier by Saha (1999a), the expansion of the Grand interf\'erom\`etre \`a deux (two) t\'elescopes (GI2T), Infrared and optical telescope array (IOTA) are in progress. The current status of all these interferometers stated above are enumerated. The data analysis being carried out using the working interferometers are also described. The space interferometry programmes are advancing very fast. Among the notable ones are the Space technology 3 (ST3), Space interferometry mission (SIM), and Darwin; they have already received funds. The technical details of these interferometers and their objectives are highlighted.
Multi-objective optimization of the dry electric discharge machining process
Sourabh Saha,S. K. Choudhury
Physics , 2009,
Abstract: Dry Electric Discharge Machining (EDM) is an environment-friendly modification of the conventional EDM process, which is obtained by replacing the liquid dielectric by a gaseous medium. In this study, multi-objective optimization of dry EDM process has been done using the non dominated sorting genetic algorithm (NSGA II), with material removal rate (MRR) and surface roughness (Ra) as the objective functions. Experiments were conducted with air as dielectric to develop polynomial models of MRR and Ra in terms of the six input parameters: gap voltage, discharge current, pulse-on time, duty factor, air pressure and spindle speed. A Pareto-optimal front was then obtained using NSGA II. Analysis of the front was done to identify separate regions for finish and rough machining. Designed experiments were then conducted in these focused regions to verify the optimization results and to identify the region-specific characteristics of the process. Finishing conditions were obtained at low current, high pulse-on time and low duty factor, where as rough conditions were obtained at high current, low pulse-on time and high duty factor. Focused experiments revealed an additional process constraint based on the flushing efficiency.
Performance of the low light level CCD camera for speckle imaging
S. K. Saha,V. Chinnappan
Physics , 2002,
Abstract: A new generation CCD detector called low light level CCD (L3CCD) that performs like an intensified CCD without incorporating a micro channel plate (MCP) for light amplification was procured and tested. A series of short exposure images with millisecond integration time has been obtained. The L3CCD is cooled to about $-80^\circ$ C by Peltier cooling.
Night time variation of Fried's parameter at VBT, Kavalur
S. K. Saha,V. Chinnappan
Physics , 1999,
Abstract: The night time variation of the Fried's parameter, r$_{o}$ is measured on 28-29 March 1991, at the Cassegrain focus of 2.34 m Vainu Bappu Telescope (VBT), at Vainu Bappu Observatory (VBO), Kavalur, India, using speckle interferometer; the results are discussed.
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