12. THE RADIO TELESCOPE
by Sir Bernard Lovell
(from a television talk in a series arranged by Granada TV)
The radio telescope is in principle very similar to the optical telescope. In its simplest form it consists of a parabolic reflector with the primary feed or antenna at the focus. The incoming radio waves are reflected to this antenna, which is connected by cable to the receiver and recording mechanism. The analogy between optical and radio systems is apparent; they both use parabolic reflecting surfaces. But one deals with wave-lengths in the visible region which are less than a thousandth of a millimetre, while the other works with wave-lengths of up to several metres. It was soon appreciated that the problems facing the scientist investigating the universe with radio telescopes, were similar to the problems facing the ordinary optical astronomer. Each wanted to build bigger reflectors to collect more light or more radio waves and thus to penetrate further into space. Broadly speaking there are two types of radio telescope. The first looks rather like a large array of television aerials - which is not very far from the truth. An arrangement of multiple aerials like this connected to a receiver make possible a broad survey of the strength of the radio signals coming from different parts of the sky. The second type is more clearly akin to the optical telescope and has a parabolic reflector. The Jodrell Bank 250 feet steerable radio telescope, at the moment probably the biggest completely steerable instrument in existence, is of this type. The primary feed or antenna is mounted on a steel mast, at the focus, and the whole system can be driven with very great accuracy to track or point at any part of the sky and study the strength of radio emissions coming in. On the shortest wave-length on which this telescope can be used, the beam width is extremely narrow and quite a highly defined picture of the sky can be obtained.
Whereas in the optical telescopes we either look at the object with our eyes or obtain a photograph of it, when we use radio telescopes we have other means of recording the signals. We use a pen recording instrument which traces out the strength of the radio emissions on a moving chart. There is always a certain amount of random noise in the receiver which is recorded on the chart, but the emissions can be recognised as an increase in intensity when the telescope is pointing directly at a "radio star".
(from Discovery, published for Granada TV by Methuen, London, 1961)