MARC details
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07143nam a2200229Ia 4500 |
| 003 - CONTROL NUMBER IDENTIFIER |
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IN-BaIIA |
| 005 - DATE AND TIME OF LATEST TRANSACTION |
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20211110142010.0 |
| 008 - FIXED-LENGTH DATA ELEMENTS--GENERAL INFORMATION |
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211028s9999 xx 000 0 eng d |
| 040 ## - CATALOGING SOURCE |
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| Transcribing agency |
IIA Library |
| 080 ## - UNIVERSAL DECIMAL CLASSIFICATION NUMBER |
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| Universal Decimal Classification number |
043:520.8 |
| Item number |
SRI |
| 100 ## - MAIN ENTRY--PERSONAL NAME |
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| Personal name |
Sridharan, R. |
| 9 (RLIN) |
25282 |
| Relator term |
Author |
| 245 #0 - TITLE STATEMENT |
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| Title |
Techiques for achieving higher spatial resolution |
| Remainder of title |
a thesis submitted for the award of degree of doctor of philosophy |
| Statement of responsibility, etc. |
R. Sridharan |
| Medium |
[Ph.D Thesis] |
| 260 ## - PUBLICATION, DISTRIBUTION, ETC. |
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| Place of publication, distribution, etc. |
Bangalore |
| Name of publisher, distributor, etc. |
Indian Institute of Astrophysics |
| Date of publication, distribution, etc. |
2001 |
| 300 ## - PHYSICAL DESCRIPTION |
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| Extent |
xx, 156 p. |
| 502 ## - DISSERTATION NOTE |
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| Degree type |
Doctor of Philosophy |
| Name of granting institution |
Indian Institute of Astrophysics, Bangalore |
| Year degree granted |
2001 |
| 520 ## - SUMMARY, ETC. |
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| Summary, etc. |
This thesis is aimed at developing both hardware and software required for obtaining high<br/>resolution images on a regular basis from ground-based telescopes using speckle and interferometric<br/>imaging techniques. A program (speckle code) for analysing the speckle data has<br/>been developed and used to analyze speckle data obtained from a few solar telescopes.<br/>The first Chapter provides an introduction to the thesis. It starts with an overview of<br/>human endeavors to achieve high angular resolution in telescopic observations. It should<br/>be mentioned that no serious attempt has been made to present the developments in their<br/>chronological order. Another point to be noted is that only technical developments have<br/>been highlighted but the scientific achievements that these developments have led to have not<br/>been described, basically, to conform to the spirit of the title. These technical developments<br/>provide the foundation for the explorations presented in this thesis. The thesis itself finds<br/>its place under the the category of image restoration schemes mentioned towards the end<br/>of the overview. The next few sections have been devoted to explain the adverse effects<br/>of the atmospheric seeing, the importance of phases of an object's Fourier transform, and<br/>resolution as defined in Astronomy. Questions like Why is high resolution needed in Solar<br/>Physics ?, Why should special techniques be developed for ground based observations, while<br/>space telescopes have become the order of the day?, and Why speckle and inte'fjerometric<br/>imaging techniques alone have been addressed in this thesis ? have been answered in the next<br/>few sections.<br/>The second Chapter is concerned with the measurement of the atmospheric coherence<br/>diameter -the so-called 'Fried's parameter'. This parameter severely influences the quality of<br/>the recorded image in ground based observations; the larger its value, the better is the image<br/>quality. Estimation of this parameter is essential in solar speckle imaging. A few methods<br/>of estimating this parameter and their applicability to the nature of the data have been<br/>explored in this Chapter. The details of the speckle and interferometric imaging observations<br/>and the pre-processing methods that are essential for analysing the speckle data and have<br/>been adopted in our speckle code have been presented in this Chapter. The details of the backend<br/>instrument developed at the institute for performing speckle observations at Kodaikanal<br/>Observatory (KO) have also been described. <br/>In the third Chapter, practical methods used in our speckle code to reconstruct an image<br/>from a series of short exposure images have been described. In our speckle code, the Fourier<br/>amplitudes of an object are estimated by Labeyrie's speckle interferometry method and the<br/>Fourier phases of the object are estimated using Weigelt's speckle masking technique. The<br/>reconstructed images of a few sunspot and pore regions, observed with the tunnel telescope<br/>of KO show small scale features up to the diffraction limit of the telescope. The identification<br/>of these indicates the importance of speckle observations in achieving high spatial resolution.<br/>The reconstructed images of two sub-flare regions of the NOAA AR8898, observed with the<br/>15 cm Coude telescope of the Udaipur Solar Observatory indicate that 'frame selection' can<br/>be one of the ways of improving the resolution. The high redundancy in the estimated phase<br/>of the Fourier transform of the object increases the signal-to-noise ratio of the reconstructions<br/>and implies that the reconstruction from a few selected 'good' frames can significantly improve<br/>the quality.<br/>The fourth Chapter is concerned with the application of speckle imaging. The speckle<br/>imaging technique described in the previous Chapters was used to analyse near-simultaneous<br/>filtergrams obtained in the G-band ().=4305 A ) and the K line of Ca II ().=3933 A) at a<br/>plage region, quiet Sun region and the NOAA AR8923. As the seeing conditions were poor,<br/>a reconstruction was obtained from three best images of the sequence of recorded images of<br/>each region in the G-band and the K line of Ca II. The G-band bright points (GBPs) were<br/>extracted from the corresponding reconstructed images using image segmentation techniques.<br/>Then the morphology of the GBPs and the Ca II K network bright points were studied in<br/>each region. The prime objective of this study was to see whether this data can offer a clue<br/>on the mechanism that leads to the preferential heating at the chromospheric level (network<br/>boundaries) while the source, if assumed to be the GBPs, is distributed everywhere. We<br/>suggest the possibility of having two classes of GBPs, those present at all locations and those<br/>swept by the supergranular horizontal motions to the network boundaries. While the former<br/>are perhaps generated continuously and observed at any given time, the latter may cause the<br/>preferential heating at the Ca II K network boundaries. The intra-network GBPs could be<br/>associated with the intra-network Ca II K bright points, not resolved in the present data. In the fifth Chapter, the basic principle of the interferometric imaging observations has been<br/>described briefly. The possibility of having two kinds of transfer functions in such observations<br/>has been indicated. A laboratory experiment performed to understand the details of 'closure<br/>phase imaging' has been described. A program was developed to simulate phase screens.<br/>Specklegrams and interferograms were generated from the simulated phase screens and used<br/>for simulating fringes that could be formed by bright features residing inside pores. These<br/>fringes were then compared with those obtained from real observations at KO by placing a<br/>non-redundant mask at the re-imaged pupil plane of the telescope. Such observations can be<br/>useful, at least to resolve isolated bright points." The problem of "source confusion" can be<br/>minimised by restricting the field-of-view to about an arc second.<br/>The last Chapter provides the summary of the thesis work. The speckle code developed by<br/>the authors is compared with that developed by others and the advantages are highlighted.<br/>This is followed by a brief description of the future plans of the authors. |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
Ph.D. Thesis |
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1344 |
| 650 ## - SUBJECT ADDED ENTRY--TOPICAL TERM |
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| Topical term or geographic name entry element |
spatial resolution |
| 9 (RLIN) |
25283 |
| 700 ## - ADDED ENTRY--PERSONAL NAME |
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| Personal name |
P. Venkatakrishnan |
| Relator term |
Supervisor |
| 9 (RLIN) |
48859 |
| 856 ## - ELECTRONIC LOCATION AND ACCESS |
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| Uniform Resource Identifier |
<a href="http://prints.iiap.res.in/handle/2248/2904">http://prints.iiap.res.in/handle/2248/2904</a> |
| Link text |
Click Here to Access eThesis |
| 942 ## - ADDED ENTRY ELEMENTS (KOHA) |
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| Koha item type |
Thesis & Dissertations |
| Source of classification or shelving scheme |
Universal Decimal Classification |
