Comparison of the GBT 4 beam and Single Pixel performance

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.

kbandTip1.gif kbandTip2.gif
Intensity versus Elevation plot for first Polarization. The GBT was at fixed aximuth and scanned in Elevation from 6 degrees to 80 degrees. Intensity versus Elevation plot for second Polarization

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.

tmcSinglePixelNH3Spec.gif

Spectral Line Images

TMCCP-HC7N.gif tmcSinglePixelNH3.gif
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. Image of the NH3 molecule distribution. Same mapping parameters as before.

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

Topic attachments
I Attachment Action Size Date Who Comment
TMCCP-HC7N.gifgif TMCCP-HC7N.gif manage 65 K 2008-11-05 - 09:17 GlenLangston  
kbandTip1.gifgif kbandTip1.gif manage 5 K 2008-11-04 - 21:25 GlenLangston  
kbandTip2.gifgif kbandTip2.gif manage 5 K 2008-11-04 - 21:26 GlenLangston  
observingLog08Oct12-II.txttxt observingLog08Oct12-II.txt manage 4 K 2008-12-03 - 11:18 GlenLangston Tsys comparision Tests
tmcSinglePixelNH3.gifgif tmcSinglePixelNH3.gif manage 66 K 2008-11-05 - 09:17 GlenLangston  
tmcSinglePixelNH3Spec.gifgif tmcSinglePixelNH3Spec.gif manage 18 K 2008-11-05 - 09:17 GlenLangston  
This topic: KPAF > DirectorsOffice > EngPlanning > SinglePixelTesting > KbandTests08Oct12
Topic revision: 2008-12-03, GlenLangston
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