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The word “survey” in the astronomical context initially referred to what could be called a sky atlas - these were initially hand-drawn sky charts, and later photographic images. Currently the word survey largely denotes catalogues of astronomical sources and their properties (positions, fluxes, morphology, etc.). They are used to systematically map the universe and characterize its astrophysical components with the aim of discovering new types of objects or phenomena. Surveys are often preceded by the development and introduction of new technology. Where the technological advancement allows us to observe the sky in some new way, for example, viewing images in a previously unexplored wavelength range. The Sloan Digital Sky Survey (SDSS) is one such astronomical sky survey.  The SDSS is a five-filter imaging survey consisting mostly of the Northern Galactic Cap combined with a spectroscopic follow-up program. The survey and its extensions (SDSS-II and SDSS-III) cover approximately〖〖14 500〗dgrees〗^2, which is nearly a third of the entire sky. SDSS’ main telescope is at Apache Point, New Mexico, and it is was specially designed to take wide field (3°×3°) images using a mosaic of thirty 2048×2048 pixel CCDs\cite{Djorgovski_2013}. The survey includes a spectroscopic survey of approximately 1.3 million objects — mostly galaxies, and quasars.   Figure 1. Histogram of peak flux densities for sources in the FIRST survey. (http://iopscience.iop.org/0004-637X/742/1/49/pdf/apj_742_1_49.pdf)  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{Faint Images of the Radio Sky (FIRST)}    Figure 1. Histogram of peak flux densities for sources in the FIRST survey. (http://iopscience.iop.org/0004-637X/742/1/49/pdf/apj_742_1_49.pdf)  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)   
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