Radio Astronomy 1 



By J. A. Ratcliffe 



Cavendish Laboratory, Cambridge, England 



Our knowledge of astronomical bodies is almost entirely derived 

 from observations of the electromagnetic waves which they emit. The 

 earth's atmosphere forms a very complete absorbing blanket for most 

 of these radiations ; but there are two "windows" in the atmosphere, 

 one in the region of optical wavelengths and one in the region of 

 short radio wavelengths. Radio astronomy deals with the study of 

 those radiations which reach the earth from outer space and are 

 transmitted through the radiofrequency "window" to the surface 

 of the earth. 



If the radiation is received on a tuned radio receiver, it is found 

 that it fluctuates rapidly in an irregular manner, and if the resulting 

 signal is listened to with telephones it sounds like a rushing noise. 

 The reception and amplification of weak irregular noise in a radio 

 receiver present a difficult problem, because the resistances and valves 

 in the amplifier themselves produce similar random noise. The sepa- 

 ration of the received noise from the amplifier noise is usually achieved 

 by switching the amplifier rapidly back and forth between the aerial 

 and a controlled source of radio noise. If the noise produced by 

 the controlled source is not the same as that produced in the aerial, 

 there will be a fluctuating component in the output. This may be 

 recorded, and it provides a measure of the noise received by the 

 aerial. 



Once it has been established that radiofrequency waves are incident 

 on the earth from outside, it is, of course, desirable to use some kind 

 of "radio telescope" to find their direction of arrival. The simplest 

 type of radio telescope consists of a concave mirror with a receiving 

 aerial at its focus and is similar to an optical reflecting telescope. 

 The accuracy with which the direction of arrival of waves can be 

 determined with any apparatus of this kind is, however, limited by 

 the ratio between the width of the mirror and the wavelength of the 



Substance of a Friday Evening Discourse at the Royal Institution (Great Britain) 

 delivered on November 23, 1951. Reprinted by permission from Nature, vol. 169, March 

 1. 1952. 



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