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Report on the Spectrometer Tests of September 10/11, 2002
Configuration: 4N2-6A-12-3; Rcvr1_2GHz.

D. Hogg and F. Ghigo

Summary

The GBT spectrometer was used in five configurations during the night of September 10/11, 2002. This report summarizes what was observed in the second configuration.

The observed profile is in good agreement with that of Fisher, but the observed rms is smaller by 27% than the expected value. The origin of this discrepancy should be understood before this correlator configuration is released for general use at L Band.

Analysis

The data are in the GBT data base as /home/gbtdata/TSPECTEST_05. The data were filled into an area on EUCLID:
/export/home/euclid/scratch/dhogg/ and are in file:
TSPECTEST_05_SPECTROMETER_A
Configuration 2. 4N2-6A-12-3
L Band, at 1429 MHz; 12.5 MHz bandwidth; 3-level sampling.

Observations were made of the hydrogen line in the galaxy UA93o chosen from the Fisher GBT HI Galaxy Survey, Scans #45 to #54. 

Scans #46,#48, #50, #52, and #54 are all comparable, and are all
good scans. We have averaged them to get a best estimate of
the HI spectrum. Figure 1 (ttday5#46pla.ps) shows the spectrum for
each receiver averaged over all scans, with intensity as a function
of pixel number. Figure 2 (ttday5#46plb.ps) shows intensity as a function
of frequency. 


For one 300 second observation in one receiver, with a
channel width of 95.4 Hz, and a system temperature of 21 K, we expect
the rms to be 169 mK. The observed values are shown in the following table.

Effective                  RMS, in mK                     Ratio
Int. Time           Expected     RX1    RX2    Both        O/E
   150               169.       123.   125.               0.74
   300               119.                      87.6       0.74
   750                75.5       54.7   55.9              0.73
  1500                53.4                     39.1       0.73

The receivers are of comparable sensitivity. The observed rms is
about 27% less than the expected value. This is a much greater
discrepancy than is experienced in other configurations, and it
should be investigated before the correlator configuration is
released for general use at L Band. The rms does decrease with
increasing integration time exactly as expected.

The final spectrum obtained by integrating all data is given in
Figure 3 (ttday5#46ava.ps), which shows intensity as a function of
barycentric radial velocity. The figure has been cropped to give
a display similar to that in the Fisher catalog. 



The properties
of the profile are compared in the following table. We use the
average of the two channels for each IF, and we convert from
antenna temperature to flux density using the conversion factor
found in early August by R. Maddalena, 1 Jy = 1.91 K.

Quantity                     Fisher          This Test

Maximum in profile (Jy)                         0.48

Peak Intensity     (Jy)       0.38              0.44

Line Width @ 20%   (km/s)     194.7             196.6

Systemic Velocity  (km/s)     717.7             718.4

Flux Integral   (Jy km/s)     57.30             58.74

The velocity parameters are in fair agreement, in view of the
high noise on the spectrum. The peak intensity and the flux integral
are somewhat higher in the test than in the Fisher profile, by 16%
and 2.5% respectively.  It should be noted that the peak intensity
in the test is an estimate, and may not be at the same point in the
profile as the Fisher value.

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