180 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



waves used. The wavelength of radio waves is so much greater than 

 that of light waves that it would require an aerial 150 miles wide to 

 give the same angular accuracy as an optical telescope with a lens 

 1 inch wide. It is obvious that, with radio telescopes, we cannot hope 

 to approach the accuracies which are possible with waves of light; 

 but in an attempt to get the best possible accuracy large mirrors 

 have been constructed. One of the largest, used by the workers at 

 Manchester, has an aperture of about 220 feet and is used on a wave- 

 length of 1.9 m. It is interesting to notice that, if our eyes could 

 observe angles with only the same limited accuracy as the mirror, 

 the sun would not appear as a clear disk in the sky, but it would 

 look like a diffuse blur about twice its present size. All the stars in 

 the sky would appear this same size, so that we should simply see 

 a general diffuse radiation corresponding to the overlapping of all 

 their blurred images. Where there was a particularly bright star 

 there would appear to be a diffuse bright patch of this same size. 



Figure 1. — Receptivity diagram of aerial arrays used in radio astronomy. 

 Roy. Soc, A, vol. 193, 1948. 



From Proc. 



Large mirrors are costly and difficult to construct accurately, and 

 there is a physical limitation to their size. It has long been appreci- 

 ated in optical astronomy that in principle it is not necessary to use 

 the whole of the mirror to achieve the full angular accuracy and that 

 only the portions at the edge are required. This principle has been 

 applied in the following way in radio astronomy. Two aerial arrays 

 are used, separated by as great a distance as possible in the east-west 

 direction. The two arrays are connected to a single receiver at the 

 midpoint, and the combination then has different receiving powers 

 in different directions. The receptivity diagram has the form repre- 

 sented in figure 1, in which maxima alternate with minima. When 

 the earth rotates, the reception pattern is carried around with it, so 



