280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



galaxy and radio signals have been detected from the neighboring 

 galaxies in the universe. Nearer home, the sun, Jupiter, and even 

 Venus have been found to be powerful radio emitters. The cause of 

 these signals and the nature of the invisible stars are unknown, and 

 much research effort is being expended at the present time to solve 

 these mysteries. 



THE EQUIPMENT USED BY RADIO ASTRONOMERS 



Almost every observation so far has been made with the equipment 

 shown schematically in figure 1. Signals are picked up from space 

 by the radio telescope to be magnified in the receiver and fed to a 

 suitable display unit. 



RADIO 

 RECEIVER 



\ 



DISPLAY 

 UNIT 



Figure 1. — The equipment used by radio astronomers. 



Radio telescopes fall into two categories, those with a single direc- 

 tional beam and those with multiple beams. A single beam is formed 

 by the parabolic reflector, as shown in plate 1, which acts like an auto 

 headlight in reverse. Waves from a radio star are focused by the 

 paraboloid to form a spotlike image which has a diameter inversely 

 proportional to the aperture of the telescope. Large apertures are 

 expensive and one of the best images that has so far been obtained is 

 y 2 degree, given by the new 60-foot disk at Harvard. This em- 

 phasizes the main disadvantage of radio telescopes; the definition is 

 extremely poor, not even as good as that of the human eye, but as we 

 shall see later there are ways of overcoming this defect. At the 

 focus of the paraboloid the image is allowed to fall on a dipole element 

 which is formed from two metal rods similar to one side of an 

 H-shaped TV antenna. Electric voltages and currents are induced 

 in the dipole and are fed down a cable into the receiver. 



