Radiotelescopes detect radio waves, produced for example from the interactions of charged particles with magnetic fields. Radio waves interact with matter in a different way than visible light, so radiotelescopes are different from optical telescopes.
The strength of a radio source is measured by the spectral flux density, S(ν), in units of energy per second and unit frequency. To determine the total amount of energy per second (i.e. the power P) detected by the receiver, we have to integrate over the telescope’s collecting area and over the frequency interval to which the detector is sensitive (bandwidth).
If we assume that S(ν) is constant:
Typically S(ν) is in the order of 1 Jansky (Jy), 1 Jy = 10-26 erg s-1 cm-2 Hz-1
Radio waves have longer wavelengths than visible light. According to Rayleigh criterion,
we need much larger diameters for good resolution in radiotelescopes (of the order of km, to achieve a resolution of 1”). One advantage is that long wavelengths also allow larger deviations from the parabolic shape (λ/20).
Instead of building huge radiotelescope dishes, a doable alternative is to use more than one radiotelescope to detect the same source of radio waves.
Groups of radiotelescopes are called arrays.
Antenna pattern: An antenna is able to detect signals even if they don’t come directly from the direction at which it is pointing. The lobes in an antenna pattern indicate regions from which waves can be detected, and also the relative sensitivity (the larger the lobe, the higher the sensitivity).
The largest individually operating radiotelescopes have diameters in the order of fractions of a km.
Groups of radiotelescopes operating together are arrays.
Very Large Array: 27 radiotelescopes arranged in a Y shape. Near Socorro, New Mexico.
Square Kilometre Array: Beginning of construction scheduled for 2016, expected to be fully operational in 2024.
Very Long Baseline Array: Completed in 1993, consists of 10 radiotelescopes scattered around some regions of North America, Hawaii, and the Virgin Islands, and is operated from the computer center in the Very Large Array.