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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)  \subsection{Trade-off between area and depth in FIRST survey} survey - this section is incomplete}  Figure 2 : the components of a two-dish interferometer observing in a narrow frequency range. The correlator multiplies and averages the voltage outputs V1 and V2 of the two dishes.  The radio band is approximately five decades in wavelength. This is too wide to be covered effectively by a single telescope. The surface specific intensity and angular sizes of radio sources span an even wider range than the radio band a combination of single telescopes 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 collecting area of circular parabolic radio telescopes is reduced to an effective area because the receiver is on the reflector axis, and together with its supporting legs, the receiver partially blocks the path of radiation falling onto the reflector. One consequence of this blockage is that the effective collecting area is reduced because some of the incoming radiation is blocked.  The effective collecting area Ae (θ, φ) of any antenna averaged over all directions is given by   〈A_e 〉= λ^2/4π  The angular resolution of a diffraction-limited telescope is given by θ ≈λ/D radians (where D is the diameter of the radio telescope dish). Large diameters are required to obtain sub-arc second resolution at radio wavelengths.  The geometric area of a single dish is(πD^2)/4, while the geometric area of an interferometer with N dishes – (with the basic one as shown in figure 2), given by(NπD^2)/4, can be arbitrarily large. Note that an interferometer can comprise of two or more dishes. This arrangement mitigates many complications associated with single dishes, for example, vulnerability to fluctuations in atmospheric emission and receiver gain, and radio-frequency interference.  FIRST’s high angular resolution and faint flux density threshold (the flux density limit of FIRST is ~ 1 mJ) come at a price. Some of the flux from extended sources is resolved out. This leads to a systematic underestimation of extended source flux density and a survey threshold that is a function of source size 
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