300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
source in the constellation of Cassiopeia. Since these pioneer dis- 
coveries, many radio sources have been cataloged. A number of 
these coincide in position with outstanding objects in the sky: some 
with supernovae, others with interacting galaxies, nebulous conden- 
sations, and soon. It is, however, one of the outstanding enigmas of 
radio astronomy that the great proportion of these radio sources defy 
optical identification. This has led to a great deal of speculation, 
some of it premature, concerning the nature of the radio sources, not 
to mention the nature of the universe in which we live. It is clear, 
however, that the radio astronomer’s universe bears a marked simi- 
larity to the optical astronomer’s universe. 
The radiations from outer space known as “light” cover a very small 
section of the electromagnetic spectrum—one octave, to be precise. 
By contrast, the radio waves reaching the earth from outer space 
cover approximately 12 octaves of the spectrum. For receiving equip- 
ment on the earth, the boundary to observations at short wavelengths 
is set at 0.5 cm. by the absorption effects of oxygen and water vapor 
in the terrestrial atmosphere. At long wavelengths, greater than 
about 30 m., the radio waves are reflected back into outer space by the 
terrestrial ionosphere. The incoming radio signals do not have the 
properties of speech or music: they have the mathematical form and 
audible characteristics of random noise. 
The basic instruments of radio astronomy comprise a directive an- 
tenna, which is often a paraboloid of large aperture, and a sensitive 
radio receiver provided with means for accurate calibration. In 
radio telescopes of paraboloidal design, the focus is occupied not by a 
photographic plate, as in optical astronomy, but by a simple primary 
antenna system, which passes the radio signals along to a radio re- 
ceiver. Permanent records are generally obtained with pen re- 
corders, or, in some cases, by photographing cathode ray tubes, 
One of the great difficulties confronting radio astronomers is that 
the radio wavelengths, which are of the order of a million times 
longer than those of light, make the resolving power of radio tele- 
scopes greatly inferior to that of optical telescopes. The 250-foot 
paraboloid at the Jodrell Bank Experimental Station, in England, 
at its shortest operating wavelength has a resolution which is not so 
good as that of the human eye. To some extent, it has been possible 
to develop special antennas for obtaining high directional discrimina- 
tion. Most of these antennas make use of interference techniques, and 
are subject to confusion, if the brightness pattern of the part of the 
sky under examination is complex. In their efforts to achieve im- 
proved resolution, radio astronomers have contributed greatly to the 
development of antennas. One particularly interesting example is 
a cruciform array developed by Mills, and another is an elaborate 
periodic array developed by Christiansen. 
