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We have designed the experiment to minimize the extraneous sources of radiation enter-
ing the receivers. Some sources, such as the atmosphere and the galaxy, could not be reduced
to negligible levels by equipment design. Instead, we have measured them with the same ra-
diometers used to measure TA.Zrmith in order to minimize the effects of gain calibration errors.
The atmospheric emission is determined by correlating the radiometer output with the amount
of atmosphere observed at different angles from the zenith. Galactic emission is measured from
drift scans we made at the longer wavelengths.
III. DESIGN OF EXPERIMENT AND MODIFICATIONS FROM 1982 WORK
a) Radiometers, mirrors, and ground screens
The five radiometers used to measure TA.CBR operate at 12.0-cm, 6.3-cm, 3.0-cm, 0.91-
cm, and 0.33-cm wavelength. A sixth radiometer monitors atmospheric emission at 3.2-cm wave-
length. All are superheterodyne receivers operated with Dicke switching, and all have low-
sidelobe, corrugated horn antennas as inputs. Each radiometer is described in previous papers
(Sironi et al. 1984; Mandolesi et al. 1984, Partridge et al. 1984; Friedman et al. 1984; and De
Amici et al. 1984).
Since the July 1982 measurements, each of the radiometers has been modified to improve
the quality of the CBR measurement and the ease of operation. The Dicke reference on the 12-cm
wavelength radiometer, previously an antenna pointed at the north celestial pole, was replaced
by a liquid nitrogen load. In addition, the radiometer mount was modified to allow atmospheric
zenith-angle scans in any direction. The configuration of the 6.3-cm radiometer was modified
to resemble that of the 3-cm radiometer in order to minimize the up-versus-down instrument
offset and to permit rapid zenith-angle scans and zenith/cold-load measurements. The 3-cm
radiometer was stiffened and the mirror and ground shields were extended to allow atmospheric
scans to larger zenith angles. The 0.91-cm radiometer was improved with stiffer and flatter mirrors
and more accurate pointing capability for zenith-angle scans. The configuration of the 0.33-cm
radiometer was changed to provide a 60-degree opening angle between the two antennas, so that
atmospheric zenith-angle scans could be made without mirrors. The zenith angles observed by
the atmospheric monitor were decreased somewhat in order to reduce the signal contamination
by ground radiation at the larger zenith angles. The ground shields on all the radiometers were
improved to reduce the extraneous thermal radiation and RF interference entering the sidelobes
of the antennas.
b) Absolute Reference Cold Load
The absolute reference cold load is an ambient-pressure liquid-helium-cooled target viewed
through a large ( 0.7-m) open-mouth dewar covered by two windows of 23-micron-thick polyethy-
lene film. The cold load has been described previously ( Smoot et al. 1983) and has not been
significantly modified. The pressure in the cold load was 487.5 i 2.0 mm Hg, which corresponds
to a liquid helium boiling temperature of 3.776 0.004 K ( Brickwedde et al.). Two Lakeshore
Cryotronics sensors mounted in the target gave readings of 3.773 0.020 K and 3.795 0.020 K.
We calculate the emission from the absolute reference cold load for each wavelength by converting
this target temperature to antenna temperature and adding the emission from the windows and
walls and the radiometer as reflected by the cold load (Table 1). The reflection coefficient of
the load is small, typically 10-3 or less. The antennas of all radiometers except the 12-cm one
have much smaller diameters than the mouth of the cold load, and hence viewed the cold load
by essentially free propagation. The antenna of the 12-cm radiometer has the same diameter as
the cold load mouth; consequently the interfacing and alignment of the antenna and cold load
were more critical, and the walls and interface contributed additional power. In 1983, this extra
source of antenna temperature was measured in each run (see Table 1).
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Smoot, G. F.; De Amici, G.; Friedman, S. D.; Witebsky, C.; Sironi, G.; Bonelli, G. et al. Low-Frequency Measurements of the Cosmic Background Radiation Spectrum, article, November 1, 1984; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc884096/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.