304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
the band 100-580 Mce./s., but has now been extended to cover nearly 
six octaves of the electromagnetic spectrum, from 25-580 Mc./s. and 
from 2,000-4,000 Mc./s. As with the earlier Australian experiments, 
the Fort Davis equipment uses narrow-band tunable receivers that 
are repeatedly swept across a wide frequency range. The equipment 
comprises six separate receivers, since with existing radio techniques 
it is not possible for one receiver to cover a band of more than one 
octave. Because of its tremendous frequency range and high sensi- 
tivity, it was necessary to place the equipment in a remote section 
of the United States, to avoid manmade radio interference. 
The four very high and ultrahigh frequency receivers at Fort 
Davis are connected to a complex arrangement of primary antennas 
at the focus of a 28-foot paraboloid reflector (pl. 1, fig. 2), which 
tracks the sun daily from sunrise to sunset. The two low-frequency 
receivers are connected to a fixed array that does not track the sun, 
but has a broad beam directed along the meridian at the celestial 
equator, so that the sun can be monitored for the greater part of 
the day. The equipment has now been in continuous operation from 
sunrise to sunset for just over 3 years, and an average of 95 percent 
of the possible observing time has been maintained. This may be 
compared with optical solar observations which are generally limited 
by inclement weather to less than 50 percent of the possible observing 
time. 
All six receivers sweep their respective octave bands three times per 
second, and their outputs are displayed on six cathode ray tubes. 
The spot displacement on each of these cathode ray tubes is propor- 
tional to the instantaneous frequency of the receiver, while the bright- 
ness is proportional to the receiver output. The cathode ray tubes 
are photographed with a 70-mm. camera, in which the film moves 
continuously at a speed of approximately half an inch per minute. 
Time marks are photographed onto this film to an accuracy of 1 
second. Originally the limit on the intensity range of the observa- 
tions was set by the film emulsion, which cannot handle signals that 
differ in intensity by more than 1,000:1. Two years ago, however, 
the response of the amplifiers in the receivers was “compressed,” and 
in this way it was possible to extend the intensity range to 100,000: 1. 
This is typical of the way in which the inherent flexibility of elec- 
tronic equipment is used to offset the limitations imposed on a system 
by its optical, mechanical, or audio sections. 
The frequency coordinate on the spectral films may be interpreted 
in terms of height in the solar atmosphere, according to the model of 
the solar atmosphere shown in figure 2. As would be expected, the 
equipment not only monitors the disturbances which occur at a given 
level in the solar atmosphere, but also gives warning of any disturb- 
ances which are moving inward or outward through the solar atmos- 
