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The Faint Images of the Radio Sky at Twenty-cm (FIRST) is presently the most sensitive large-area, wide-field survey at radio wavelengths. It provides a factor of 50.FIRST is a radio snapshot survey performed at the NRAO Very Large Array (VLA) facility. FIRST covers approximately 10 000°2 of the North and South Galactic Caps with a resolution of approximately 5 arc-seconds. FIRST produces 3-minute snapshots covering a hexagonal grid of the sky, using 2×7 3-MHz frequency channels centred 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. (http://iopscience.iop.org/0004-637X/742/1/49/pdf/apj_742_1_49.pdf)  \section{Trade-off \subsection{Trade-off  between area and depth in FIRST survey}
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