Radio astronomy began with a Bell Labs engineer hunting for telephone static
In 1932 Karl Jansky of Bell Telephone Laboratories was asked to track down static that might disturb radiotelephone calls. Besides near and distant thunderstorms, his rotating antenna picked up a faint, steady hiss. It repeated every 23 hours and 56 minutes, which gave away its source: the centre of the Milky Way.
Jansky's antenna in Holmdel, New Jersey, was a rack of dipoles and reflectors about 30 metres across, tuned to 20.5 megahertz and mounted on a turntable, which earned it the nickname Jansky's merry-go-round. A pen recorder in a nearby shed logged months of signals. The hiss kept pace with a sidereal day, the time a fixed star takes to return to the same spot in the sky, rather than the solar day. Comparing its direction with star maps, he found it strongest toward Sagittarius, where the galaxy's centre lies.
The first dish built specifically for the job came in 1937, when the radio amateur Grote Reber put up a 9-metre parabolic antenna in his back garden in Wheaton, Illinois. He confirmed Jansky's result and carried out the first survey of the sky at very high radio frequencies, finding further sources. Radar technology developed during the Second World War then turned radio astronomy into a full branch of astronomy, with universities building large instruments.
Signals from distant planets, stars and galaxies are extremely weak, so radio telescopes need huge collecting areas and very sensitive receivers, and observatories are placed far from cities to avoid interference from broadcasts, radar and cars. Unlike optical telescopes, they work in daylight as well as at night.
Design depends on wavelength. For waves between about 3 and 30 metres long, the reflecting surface can be coarse wire mesh, even chicken wire. Sharpness of view depends on the dish's diameter compared with the wavelength, so dishes for waves between 30 centimetres and 3 metres are usually well over 100 metres wide, while those for shorter waves range from 3 to 90 metres. Many dishes are also linked electronically into arrays.
Source: Radio telescope
