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\subsubsection{Trade-off between area and depth in FIRST survey - this section is incomplete}  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 sensitivity of the collecting area is given by  σ=(2k_B T_sys)/(A_e 〖(Δν_RF τ)〗^(1/2) )   where T_sys the temperature of the interferometer system, Δν_RF is the receiver radio-frequency bandwidth, τ is the duration of the signal received from the interferometers.  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.