Fundamental Astronomy

70
3. Observations and Instruments
signal by a receiver, called a radiometer. This signal is
then amplified, detected and integrated, and the output
is registered on some recording device, nowadays usually
by a computer. Because the received signal is very
weak, one has to use sensitive receivers. These are often
cooled to minimize the noise, which could otherwise
mask the signal from the source. Because radio waves
are electromagnetic radiation, they are reflected and refracted
like ordinary light waves. In radio astronomy,
however, mostly reflecting telescopes are used.
At low frequencies the antennas are usually dipoles
(similar to those used for radio or TV), but in order to
increase the collecting area and improve the resolution,
one uses dipole arrays, where all dipole elements are
connected to each other.
The most common antenna type, however, is
a parabolic reflector, which works exactly as an optical
mirror telescope. At long wavelengths the reflecting
surface does not need to be solid, because the long
wavelength photons cannot see the holes in the reflector,
and the antenna is therefore usually made in the
Fig. 3.24. The largest radio telescope in the world is the
Arecibo dish in Puerto Rico. It has been constructed over
form of a metal mesh. At high frequencies the surface
has to be smooth, and in the millimetre-submillimetre
range, radio astronomers even use large optical telescopes,
which they equip with their own radiometers.
To ensure a coherent amplification of the signal, the surface
irregularities should be less than one-tenth of the
wavelength used.
The main difference between a radio telescope and
an optical telescope is in the recording of the signal.
Radio telescopes are not imaging telescopes (except
for synthesis telescopes, which will be described later);
instead, a feed horn, which is located at the antenna
focus, transfers the signal to a receiver. The wavelength
and phase information is, however, preserved.
The resolving power of a radio telescope, θ, can
be deduced from the same formula (3.4) as for optical
telescopes, i. e. λ/D, where λ is the wavelength
used and D is the diameter of the aperture. Since the
wavelength ratio between radio and visible light is of
the order of 10,000, radio antennas with diameters of
several kilometres are needed in order to achieve the
a natural bowl and is 300 m in diameter. (Photo Arecibo
Observatory)