Astronomy Principles and Practice Fourth Edition.pdf

362 Radio telescopes
(a)
Paraboloid section
(b)
Figure 21.10. (a) A horn antenna with feed output. (b) A horn antenna with a reflection collector.
of antennas covering the same area. However, the calculation of power gain usually requires an
adjustment to allow for the fact that the feed horn (see later) is designed to prevent ‘spill-over’ to
the ground with the result that the dish is effectively under illuminated. A typical loss is of the order of
a factor of two so that A e ≈ A/2, where A is the full geometric area of the dish. If the absolute power
gain is expressed in terms of the diameter of the dish, it is given by
G ≈ π 2 D 2
2λ 2 . (21.7)
Thus, if a 10 m dish is used at a wavelength of 1 m, the absolute power gain is close to 10 3 .
As with the case of optical telescopes, some radio dishes are constructed to support a secondary
reflector prior to the primary focus, so operating as a Cassegrain system. With this configuration, the
position of the feed is more accessible being just behind the dish and the system’s length is reduced.
21.5 Horn collectors
For frequencies ²GHz, horns are commonly used to act as collectors. Apertures are normally º15λ.
A basic design is depicted in figure 21.10(a). An alternative design might have a reflective paraboloid
section attached to the horn lip to increase the effective area (see figure 21.10(b)). One of the features
of horns is that their beams can be calculated accurately which, in turn, ensures good calibration of
signal strengths and absolute measurements of bright sources. In some telescope designs, a collection
of horns might be used and their signals combined.
A horn is sometimes placed at the focus of a radio dish to accept the collected radiation. Obviously
it is important that the horn only ‘sees’ the dish collection aperture and not beyond its perimeter
otherwise it would respond to illumination from the ground. The design is, therefore, engineered
so that its beam tapers rapidly at its edges.
21.6 Interferometry
21.6.1 A basic interferometer
By using interferometric techniques, analogous to those used in optics, it is possible to obtain higher
angular resolutions than those which result from telescopes in the form of two-dimensional arrays and
reflecting dishes. The simplest form of radio interferometer consists of two antennas which point in the
same direction but are separated by a considerable distance in relation to the radio wavelength which
is being used. This system is equivalent to Young’s double-slit interference experiment in optics. The