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The Faint Images of the Radio Sky at Twenty-cm (FIRST) is presently the most sensitive large-area ($\gg {1^{\circ}}^2$)survey at radio wavelengths \cite{Becker_1995}. It is a radio snapshot survey performed at the NRAO Very Large Array (VLA) facility. FIRST covers approximately ${10 000^{\circ}}^2$ with a resolution of approximately 5 arcseconds. Coverage is shown in Figure 6 and Figure 7. FIRST produces 3-minute snapshots covering a hexagonal grid of the sky, using $2 \times 7$ 3-MHz frequency channels centered at 1365 and 1435 MHz The survey catalogue contains around one million sources, and it is estimated that nearly $15\%$ of these sources have optical counterparts. The FIRST survey area has been selected to correspond with that of the SDSS. (Sky Survey). FIRST provides a database that is uniform in angular resolution and flux density sensitivity and it offers the opportunity to produce the largest unbiased survey for statistical analysis. FIRST’s design enables the search for radio variability of sources on timescales of minutes to years \cite{Thyagarajan_2011}   \subsubsection{Trade-off between area and depth in FIRST survey - this section is incomplete} survey}  The radio band is approximately five decades in wavelength. wavelength - tens of meters to millimeters.  This is too wide to be covered effectively by a single telescope &/ \& /  receiver. The specific intensity and angular sizes of radio sources span an even wider range than the radio band a combination of single telescopes (&/receivers) \& /receivers  and aperture-synthesis interferometers are therefore required to for effective detection. The basic interferometer is a pair of radio telescopes whose voltage output are correlated (multiplied and averaged).The larger the collecting area of an ideal radio telescope, the more it can detect faint radio sources. The sensitivity of the collecting area is given by σ=(2k_B T_sys)/(A_e 〖(Δν_RF τ)〗^(1/2) )