Observations were made with the 4 beam K band receiver and with the single pixel
receiver in order to directly compare the performance of the two systems.
The "High" K-band system was used, so the performance was only compared above
22.5 GHz.
The observations consisted of Peak and Focus observations of 3C48 and
by spectral line observations of the Taurus Molecular Cloud.
The first 36 scans were performed using the 4 Beam receiver and the
remaining scans used the single pixel receiver. All observations
were performed without Doppler tracking.
The test
report compares the
single pixel and K band 4 beam receiver system temperature as a function of observing
frequency.
Summarizing the results: the system temperature of the single pixel receiver
was higher than the current GBT 4 beam receiver. This is believed
to be due to the strong IF spurs in the down conversion unit
and insufficient gain at the higher frequencies. This issues are
being addressed by the down converter redesign work.
Frequency L System Temp R System Temp
(GHz) (K)
23 2.161*(8.995/0.350) 2.178*(9.022/.339)
55.5 57.96
24 1.944*(7.380/0.289) 2.223*(7.659/0.284)
49.64 59.95
25 1.818*(6.302/0.245) 2.311*(6.257/0.222)
46.76 65.135
26 1.714*(7.589/0.193) 2.021*(7.557/0.176)
67.40 86.78
Measurement of Atmospheric Attenuation
The atmospheric attenuation was measured by "tipping" scans, holding the telescope
azimuth constant, and moving continously in elevation, between 80 and 6 degrees.
The data for the two polarizations is shown below. In the model fit, the
sky temperature was assumed to be 290 Kelvin. The fit yields the receiver
temperature and the atmospheric opacity. It is assumed that
the atmospheric opacity is the same for both polarizations, so the
difference in measurements is assumed to be the measurement
uncertainty. The opacity value for these observations is 0.062+/-0.005.
The center frequency of the observation was 24000 GHz.
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Intensity versus Elevation plot for first Polarization.
The GBT was at fixed aximuth and scanned in Elevation from 6 degrees to 80 degrees.
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Intensity versus Elevation plot for second Polarization
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Spectral Line Test
After the amplitude comparisons, we configured the GBT for a single 200 MHz wide band, centered on the
ammonia (NH3) 1-1 and 2-2 lines. In addition, the molecule
HC7N was coincidentally in this same frequency
range. The plot below shows these lines.
Spectral Line Images
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Image of the HC7N molecule distribution in the region surrounding the Taurus Molecular Cloud. The image
region is 0.5 degrees square. The integration time for each pixel is 1 second.
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Image of the NH3 molecule distribution. Same mapping parameters as before.
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Observing Log
Start Stop Source Proc. # # Sky Freq
Scan Scan Name Scans Data
====== /home/gbtdata/T_KFPA_08OCT12 ==== Glen Langston
1 4 3c48 Peak 4 576 24000. 2008_10_12_04:11:44
5 5 3c48 Focus 1 294 24000. 2008_10_12_04:14:04
6 9 3c48 Peak 4 576 24000. 2008_10_12_04:16:38
10 10 3c48 Focus 1 294 24000. 2008_10_12_04:18:58
11 11 3c48 Tip 1 1494 24000. 2008_10_12_04:24:31
12 12 3c48 Tip 1 1494 24000. 2008_10_12_04:31:42
13 16 3c48 Peak 4 576 24000. 2008_10_12_04:38:29
17 17 3c48 Focus 1 294 24000. 2008_10_12_04:40:50
18 21 3c48 Peak 4 576 24999. 2008_10_12_04:43:20
22 22 3c48 Focus 1 294 24999. 2008_10_12_04:45:46
23 26 3c48 Peak 4 576 25980. 2008_10_12_04:48:12
27 27 3c48 Focus 1 294 25980. 2008_10_12_04:50:40
28 31 3c48 Peak 4 576 23000. 2008_10_12_04:53:15
32 32 3c48 Focus 1 294 23000. 2008_10_12_04:55:44
33 36 3c48 Peak 4 576 23000. 2008_10_12_04:58:00
37 40 3c48 Peak 4 576 12950. 2008_10_12_05:09:58
41 41 3c48 Focus 1 294 12950. 2008_10_12_05:12:31
42 45 3c48 Peak 4 576 12950. 2008_10_12_05:14:52
46 46 3c48 Focus 1 294 12950. 2008_10_12_05:17:24
47 50 3c48 Peak 4 576 13450. 2008_10_12_05:20:19
51 51 3c48 Focus 1 294 13450. 2008_10_12_05:22:51
52 55 3c48 Peak 4 576 13450. 2008_10_12_05:25:14
56 59 3c48 Peak 4 576 13950. 2008_10_12_05:30:27
60 60 3c48 Focus 1 294 13950. 2008_10_12_05:33:00
61 64 3c48 Peak 4 576 13950. 2008_10_12_05:35:24
65 68 3c48 Peak 4 576 14450. 2008_10_12_05:39:59
69 69 3c48 Focus 1 294 14450. 2008_10_12_05:42:32
72 75 3c48 Peak 4 720 14450. 2008_10_12_05:49:15
76 76 3c48 Focus 1 294 14450. 2008_10_12_05:51:47
77 80 3c48 Peak 4 576 13950. 2008_10_12_05:54:24
81 81 3c48 Focus 1 294 13950. 2008_10_12_05:56:55
82 85 3c48 Peak 4 576 13450. 2008_10_12_06:00:07
86 86 3c48 Focus 1 144 13450. 2008_10_12_06:02:34
87 90 3c48 Peak 4 576 12950. 2008_10_12_06:05:44
91 91 3c48 Focus 1 144 12950. 2008_10_12_06:08:10
92 95 3c123 Peak 4 576 12950. 2008_10_12_06:13:58
96 96 3c123 Focus 1 144 12950. 2008_10_12_06:16:18
97 98 TMCCP OnOff 2 60 13241. 2008_10_12_06:23:25
99 100 TMCCP OnOff 2 60 13191. 2008_10_12_06:27:33
101 102 TMCCP OnOff 2 60 13191. 2008_10_12_06:30:17
103 108 TMCCP RaLongMap 6 270 13191. 2008_10_12_06:34:15
109 110 ORIONA OnOff 2 90 13191. 2008_10_12_06:48:50
113 213 TMCCP RaLongMap 101 4590 13191. 2008_10_12_06:57:47
214 215 TMCCP OnOff 2 60 13191. 2008_10_12_09:57:15
--
GlenLangston - 03 December 2008