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.
Why radio?
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.
Last modified: October 2008
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