Astronomy Principles and Practice Fourth Edition.pdf

Antennas 353
Figure 21.1. The basic elements of a radio telescope.
and collecting areas extending over many hundreds of square metres rather than by using single dishtype
collectors. An appropriate substitution of the value of λ in equation (16.14) shows why this is
so.
With the introduction of special techniques of very long base-line interferometry, in which
telescopes are used thousands of kilometres apart, the angular precision of measurement is now of
the order of fractions of milli-arcseconds.
21.2 Antennas
The purpose of antennas used in radio astronomy is to collect waves arriving from particular directions
and provide at their terminals a disturbance which can be detected by a radio receiver. Because
of their fundamental nature, antennas are selective to a fairly narrow range of frequencies and
are also sensitive to the polarization of the radio waves. The range of frequencies to which the
antenna responds is, however, usually much wider than that accepted by the receiver. The broad
range of radio waves available for measurement is covered by using antennas of different sizes in
combination with appropriately tuned receivers. Some particular frequencies are, indeed, a special
interest as they correspond to energy transitions of identifiable atoms and molecules. The monitoring
of these frequencies has led to investigations of the distribution in interstellar space of such atoms
and molecules. Study at these radio spectral lines, e.g. the hydrogen line at 21 cm (1427 MHz), has
provided a major contribution to our knowledge of the structure of our own galaxy.
It is important in most types of observation for the radio telescope to have high sensitivity and
directional discrimination. When a radio telescope is directed to a particular source, a signal appears
at the output of the receiver. If now the telescope is allowed to drift relative to the source’s direction,
the output signal does not fall to zero as soon as the telescope is just off the source’s position. The
signal may only fall to zero when the telescope is directed away from the source by several degrees.
The rate at which the signal falls with angle describes the directional quality of the telescope. A
simple representation of a directional sensitivity of a radio telescope can be given by drawing a polar
diagram. In this representation, the length of a vector is used to represent how the magnitude of the
output response would vary according to the apparent direction of a distant point source of constant
radio brightness. The polar diagram may be said to be the instrumental profile of the radio telescope.
A two-dimensional polar diagram for a typical telescope is depicted in figure 21.2. The diagram may
not necessarily be symmetrical about the z-axis and, in general, a three-dimensional polar diagram is
necessary to describe the directional sensitivity of a telescope.
It will be noted from this same diagram that a radio telescope has some sensitivity outside the
main beam. These side-lobes may sometimes be troublesome by picking up sources which are way