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Spectrometer Tests of September 10/11, 2002
Mode 4N2-6A-50-9: 1420 MHz; 50 MHz bandwidth; 9-level sampling.
D. Hogg, R. Maddalena, and R. Prestage

Introduction

The GBT spectrometer was used in five configurations during the night of September 10/11, 2002. This report summarizes what was observed in the last configuration and identifies features in the spectrum which are of particular interest.

The data are in the GBT data base as /home/gbtdata/TSPECTEST_05. The data were filled into an area on THALES:
/export/home/thales/scratch/dhogg/ and are in files:
TSPECTEST_05_DCR and TSPECTEST_05_SPECTROMETER_A

SUMMARY

Configuration 5: 4N2-6A-50-9
1420 MHz; 50 MHz bandwidth; 9-level sampling.

Observations were made of the hydrogen line in the galaxy UA93o chosen from the Fisher HI Galaxy Survey. The scans for this configuration are #90 to #99. All scans are of high quality, and all were used to make the average. The baseline on each scan is excellent, and only a linear baseline was removed. The observed value of the rms is in close agreement with the expected value. The average properties of the galaxy derived here are in excellent agreement with those given by Fisher. Although a number of anomalous features are listed for completeness, most are simply the previously known interference spikes. This configuration is ready for general use.

THE ASTRONOMICAL RESULTS

The rms value for an individual receiver in an individual scan is 27.3 mK, with a dispersion in the ten observed values of 0.8 mK. For a receiver temperature of 19 K, using the spectrometer in the 9-level mode with Hanning smoothing, for an OFF/ON observation of total length 10 minutes, and a frequency resolution of 1.526 kHz, the expected rms is 28.1 mK. The observed rms on the averaged spectrum, using five scans and both feeds, is 8.74 mK compared with the expected value of 8.88 mK.

The averaged spectrum shown in Figure 1 (tst5#90avg.ps) is used to estimate the properties of the HI spectrum of the galaxy, and the values are compared to those given by Fisher. The conversion from K to Jy uses the factor 1.91 found by R. Maddalena in an earlier observing session.

SOME COMMENTS ON FEATURES IN THE SPECTRUM

Figure 2 (tst5#90pl.ps) shows the spectra of the individual receivers averaged over the five scans. The strongest interference features seen in these spectra are summarized in the following table, which gives the pixel location in the spectrum, the range of affected frequencies in MHz (eg 1441.28270-1441.28575), and which of the two channels is affected. Channel 2(YY) is more impacted than Channel 1(XX). 
Pixel Range  Frequency Range    Channel 1(XX)  Channel 2(YY)
                    MHz
   476-78    1441.28270-575          yes            yes
  1312-14    1440.00706-1012          no            yes
  7866-68    1430.00645-951           no            yes
  7953-55    1429.87370-675           no            yes
 11230-32    1424.87340-645           no            yes
 14506-08    1419.87462-767          yes            yes
 17783-85    1414.87431-736          yes             no
 20975-77    1410.00371-676           no            yes
 21060-62    1409.87401-706          yes            yes
 24337-39    1404.87370-675          yes            yes
 27613-16    1399.87340-798          yes            yes
 30890-93    1394.87309-767          yes            yes

There are two spurious signals, between 378 and 473 pixels, and
between 760 and 943 pixels, in receiver channel 2(YY) only. These
features are of a much different character than the narrow interference
spikes. In frequency they are located in the range 1441.29033-1441.43529
and 1440.57317-1440.85240, respectively. The features can be seen in both
the OFF spectrum and the difference spectrum.

There is the usual region at the end of the spectrum which is noisy
because of the fall-off in the band pass. This region begins at about
pixel 31500 (1393.94688 MHz) and continues to the last channel, 32767,
at 1392.01360 MHz.

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