... manual.% latex2html id marker 8268 \setcounter{footnote}{1}\fnsymbol{footnote}
For list of all calibrators see : http://www.vla.nrao.edu/astro/calib/manual/index.shtml 
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... GMRT.% latex2html id marker 8582 \setcounter{footnote}{2}\fnsymbol{footnote}
For details kindly refer: http://www.ncra.tifr.res.in/$\sim$ngk/primarybeam/beam.html 
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... depth
optical depth gives a measure of how opaque a medium is to radiation passing through it. 
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... frequencies.% latex2html id marker 9539 \setcounter{footnote}{2}\fnsymbol{footnote}
The spectral index is defined as, the exponent $\alpha$ in $S_\nu \propto \nu^{-\alpha}$ where $S_\nu$ is the flux density at frequency $\nu$ and $\nu$ is the frequency. 
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