The first dish (or radio) telescope was made in 1942 by an American Grote Reber, of Wheaton, Illinois. He constructed his apparatus after studying the experiments of K.G. Jansky, another American. Jansky discovered in 1935 that the intensity of radio waves increases as a highly sensitive aerial is directed progressively nearer to the Milky Way. The maximum intensity is reached when the antenna is pointing towards Sagittarius that is to say, towards the galactic centre.
Radio telescopes are called dish telescopes because of the steerable dish-shaped or parabolic reflector which gathers the radiation and focuses it on to a centrally mounted aerial. The surface of the dish is made of a good electrical conductor and the radio waves are reflected from it. The parabaloid shape ensures that all the reflected rays arrive at the central point, where they are “swallowed” by an electromagnetic horn and fed into a receiver.
Since the Second World War the development of radio telescopes has gone ahead rapidly. A 250-foot diameter instrument was installed at the Nuffield Radio Astronomy Laboratories at Jodrell bank, Cheshire, England. It is under the direction of professor under the direction of Professor Sir Bernard Lovell and has already contributed a great deal of new information to astronomy.
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