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18.8
Things We Always Assume But Don’t Always Measure
In the following two sections, we look at two aspects of temperature calibration which are
always assumed to be of minimal importance, but are rarely measured.
18.8.1
Zenith Angle Dependence of the Primary Beam Response
Two separate analyses of the change in zenith angle of the primary beam response have
been made. In the first, I present the results from a dedicated experiment where I measured
the change in T∗A for Jupiter as a function of zenith angle during one night. In the second
analysis, I have extracted the zenith angle dependence from all of our ηMB measurements
using Jupiter and the moon.
Analysis #1: On February 7, 1992 measurements of the total power emission from Jupiter
were made at zenith angles ranging from 33◦ to 72◦ with the 230 GHz receiver tuned to a
frequency of 220.3987 GHz. The measurements were made by consecutively monitoring the
total power emission of the planet, a position free of source emission located 5′ away from
the planet in azimuth, and the emission from a hot load inserted directly in front of the
receiver. This allows us to directly calculate T∗A
T∗A = 2.0 × 280.0 ×
Von − Voff
Vhot − Voff
where the factor of 2.0 puts T∗A on the single-sideband scale and we assume (to be consistent
with the CSO on-line system) that the hot load temperature is 280.0 K.
Skydip measurements were made at the beginning and end of the test which yielded the
following results...
τ (220.3987) = 0.078 at start of test
τ (220.3987) = 0.060 at end of test
ηhot = 0.60
Therefore, the atmosphere was quite stable for the duration of the test. The pointing was
checked at the beginning, in the middle, and at the end of the test with the following results...
FAZO = -19.9, FZAO = 267.9 at the start of the test
FAZO = -19.7, FZAO = 268.8 in the middle of the test
FAZO = -25.3, FZAO = 275.7 at the end of the test
mean values → FAZO = -21.6±3.2, FZAO = 270.8±4.2
Therefore, the pointing was stable throughout the duration of the test with an rms uncertainty of ∼ θMB /8.
The results from this test are shown in Figure 6.1. For zenith angles less than 68◦
T∗A = 124.8±2.5 K
which indicates that the CSO main beam efficiency at 220 GHz varies by <
∼ 2% for zenith
angles < 68◦ .
Note that this value for T∗A is about a factor of 1.4 lower than other values for T∗A derived
from other 230 GHz receiver measurements made during this observing run. This is probably
due to a zero offset in the mixer. Therefore, the T∗A values derived from this analysis
should not be used to derive ηMB .
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