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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〗degrees〗^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.   SDSS's most used data products are imaging, and spectroscopy. Image parameters include positions, fluxes, and morphology of detected objects.The spectroscopic parameters include redshift, and spectral classification. The number of objects loaded in the databases, observing conditions for imaging fields and for spectroscopic plates, and other parameters are included in the data products. \cite{Stoughton_2002}  Parameters used for imaging or spectroscopy (respectively), are grouped into classes. Table X (table of parameters of the SDSS) indicates some of the parameters associated with imaging data, their type/ class and gives a description of each parameter.  \subsubsection{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 survey at radio wavelengths. FIRST covers approximately 10,575 degrees squared.It 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 000° squared  with a resolution of approximately 5 arc-seconds. Coverage is shown in Figure  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) \subsubsection{Trade-off between area and depth in FIRST 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. 
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