Astronomy Principles and Practice Fourth Edition.pdf

Chapter 21
Radio telescopes
21.1 Introduction
Developments in the technology of electronics, such as aeroplane detection by radar, accelerated by
the Second World War, provided the means for the establishment of radio astronomy. The discovery
that energy in the form of radio waves was arriving from space was made a few years prior to this war
by Jansky. Other discoveries of importance to astronomers were made by chance during the war but
it was not until its end that the new techniques could be applied to astronomical research for its own
sake.
Basic measurement of the incoming energy is achieved by using a directional antenna or aerial.
The brightness of a source, as in the optical region, is measured as the flux density or the energy,
E, arriving per unit aperture of the telescope per unit frequency interval and, for example, may be
expressed in units of W m −2 Hz −1 . For convenience, measurements of flux density may be expressed
in flux units or janskys (Jy), with
1Jy= 10 −26 Wm −2 Hz −1 .
For a source with angular extent, its surface brightness is expressed in units of Jy sr −1 .
By using the Rayleigh–Jeans law (see equation (15.4)), any source can be assigned a brightness
temperature, T B ,givenby
2kT B
λ 2 = E (21.1)
where k is Boltzmann’s constant and λ the observed wavelength. However, as radio sources do not,
in general, have black body energy envelopes, the brightness temperature may vary considerably with
wavelength.
The disturbances set up in the antenna may be fed into a receiver, tuned to the frequency of
interest, which amplifies them before rectification to provide the final output signal. In order to remove
the effects of drifts in the equipment, a stabilized calibration source is usually provided. This is
switched periodically to the receiver system and response of the equipment to this known input is
monitored. Such a system, forming the essential part of a radio telescope, is illustrated in figure 21.1.
The output from the receiver can be recorded on magnetic tape or, more likely, is digitized in readiness
for direct computer processing with data storage on disc. These records provide the radio astronomer
with measurements of the strength of the radio radiation and its polarization over the celestial sphere.
As in optical astronomy, one of the aims of the radio telescope is to be able to collect energy from
well-defined regions on the celestial sphere. Because of the longer wavelengths which are involved,
this is not easy and high angular resolution can be achieved only by having large antenna complexes
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