Mullard Radio Astronomy Observatory
An Introduction to Radio Telescopes
A radio telescope consists of one or more directional receiving aerials connected to very sensitive amplifiers. Since radio wavelengths range from millimetres to about 10 metres, the actual form of the aerials can vary greatly; many are parabolic dishes with very precise surfaces, while others, like the Cambridge Low-Frequency Synthesis Telescope (right), are similar to television aerials.
The ability to “see” fine detail in sources depends on the ratio of the size of the telescope to the radio wavelength, and in order to make this as good as possible a method known as aperture synthesis was developed at Cambridge. The principle behind this method is that we use several smaller aerials, linked together, and record the signals as the aerials are moved relative to each other by moving the aerials along a rail track and by the rotation of the Earth. The computer then takes all of the data and synthesises a map with as high resolution as we would obtain if we were able to build a much larger dish. This method has been adopted at observatories around the world, and extended to include telescopes operating on different continents or even on satellites.
The expertise gained in making radio images is now also being applied to the related problems of optical astronomy.
There are astronomical reasons: many phenomena in the Universe show up best in one part of the spectrum, and little or not at all in other parts. Radio astronomers have discovered many exciting types of events in the Universe which had not been suspected before.
And there is a practical one: our atmosphere is transparent to light (wavelengths 300 – 700 nm) and also to radio waves with wavelengths between about 1 mm and 30 m. Shorter-wavelength radio waves are absorbed by molecules in the atmosphere, and longer wavelengths are reflected by the charged particle layers in the ionosphere, high in the atmosphere. (The reflection is the reason why long, medium and short wave signals on your radio can often be received from around the world: all these are “long waves” from a radio astronomer’s point of view.) In order to study other wavebands – X-rays, ultraviolet and infrared – we have to use satellites with specially-constructed telescopes and detectors.
For information about MRAO radio telescopes or other aspects of our work, follow the links on the left.