In 1931, a Bell Telescope laboratories engineer Karl Jansky found a source of radio interference originating from the Milky Way, whilst investigating radio interference in communication links. Following this after World War 2, many experiments in radioastronomy were conducted.
In 1957, the Mark 1 radio telescope (renamed the Lovell Telescope after Sir Bernard Lovell, the British pioneer of radioastronomy) at Jodrell Bank in Cheshire, England was built. It had a 76 metre diameter metal parabolic dish mounted in a special way, so it could point at any part of the sky. This meant that incoming radio waves could be directed into the dish and were then focused onto a detector mounted in front of it. The signal was then amplified and processed.
The largest single radio telescope in the world is the RATAN-600 at Zelenchukskaya in Russia which has a 600 metre dish.
Another type of radio telescope consists of an array of dishes which can be individually steered and then their signals can be added together. An example of this is The Very Large Array which has 28 antennas.
The resolution of a radiotelescope is much less than an optical one because of the much larger wavelengths of radio waves. This resolution can be improved by using interferometry, in which the outputs of more than one radio telescope are combined, giving the resolution of a single dish of the diameter of their separation. It is even possible to link radio telescopes in different countries to produce the equivalent of a radio dish the size of Earth. This is VLBI (very long-baseline interferometry).
The area of the sky that a radio telescope looks at is determined by its resolution at the wavelength it is working. This can be used to record a variation in the strength of the signal with time, or to produce a radio map of the area, which is a "picture" of the area with different colours each representing different levels of radio intensity.
Radio telescopes can be designed to operate at different wavelengths, with some working in the microwave region, where the wavelengths are in the centimetre range rather than not much shorter than a metre. These give information on the chemical compositions of the clouds of matter in space.
Telescopes working in the infrared part of the spectrum have to have special detectors cooled to near absolute zero (-273°C) as infrared radiation is emitted from all warm or hot bodies, including the Earth, its atmosphere and even the telescope itself.
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