Radio telescopes use a parabolic (or sometimes spherical) dish – similar to the parabolic mirror of an optical reflecting telescope – to focus electromagnetic radiation, in the radio-frequency range of the spectrum. Since radio waves have much longer wavelengths than light waves, a radio telescope’s dish needs to be much larger than the mirror of an optical telescope in order to obtain images of comparable resolution. Radio telescopes are often built in arrays of many similar dishes that can be linked via interferometry to work together as a giant ‘synthetic aperture’, to further increase resolving power.
Since 1932, when Karl Jansky at Bell Telephone Laboratories, first observed radio emissions coming from the Milky Way, radio astronomy has contributed many notable discoveries – in particular, the discovery of the cosmic background radiation, providing evidence for the Big Bang theory of cosmology. Quasars, pulsars and masers, were also first discovered using radio and microwave astronomy.
The Largest Radio Telescopes:
FAST – China
The world’s largest single-dish radio telescope is the Five hundred meter Aperture Spherical Telescope (FAST), completed in 2016, and built into a natural depression in the landscape in Guizhou province China. In fact, only a circular portion of the dish, a maximum of 300 metres in diameter, is used at any one time, with the active surface of the dish composed of separate panels held into the correct parabolic shape. This allows it to reach a maxium of 26.4 degrees from the zenith, or 60 degrees if the aperture size is reduced to 200 metres.
Arecibo Observatory – Puerto Rico
In fact, the world’s second largest radio telescope dish, the Arecibo Observatory in Puerto Rico, actually has a slightly larger usable area than FAST, with a dish of 305 metres in diameter.
Green Bank Telescope in West Virginia, USA
The largest fully-steerable dish radio telescope, is the 100 meter Green Bank Telescope in West Virginia, USA, which can be aimed at any point in the sky from zenith to horizon.
European VLBI Network (EVN)
The largest radio interferometer array in the world is the European VLBI (Very-long-baseline interferometry) Network (EVN) – a part-time array consisting of 21 telescopes in 12 different countries, allowing it to achieve sub-milliarcsecond resolution at frequencies higher than 5 GHz.