Radio telescopes are used to study naturally occurring radio emission from stars, galaxies, quasars, and other astronomical objects. As shown in the figure 1.2, most of the radio telescopes are dish-shaped, having metal parabolic reflecting surface to focus the incoming radiation to the focal point where the receiver is mounted. Radio telescopes have two basic components (1) a large radio antenna and (2) a sensitive radiometer or radio receiver. The sensitivity of a radio telescope i.e is the ability to measure weak sources of radio emission which in turn depends on the area, efficiency of the antenna and the sensitivity of the radio receiver which is used to amplify and detect the signal. The detected signal produces an electric current that is amplified, filtered for noise and then recorded.
A radio dish need not have a perfect surface; it can even be made of
separate panels covered by a perforated mesh. This is because the surface
accuracy of the reflector surface needs to be of
the order of 
of the observed wavelength. Therefore, while radio
telescopes designed for operation
at millimeter wavelengths need a high accuracy dish surface and are
typically only about ten meters or a few tens of meters across, those
designed for operation at centimeter and meter wavelengths need not be
so accurate and are made of larger size, such as the 45 m dish of the
Giant Metrewave Radio Telescope (GMRT), north of Pune. There are also
expensive dishes built, such as the new 100 m Green Bank telescope
in the USA, which goes from low frequencies to
very high frequencies.
