As part of TSPECTEST_02, seven pairs of 300 sec OffOn scans were performed with the L-band receiver, observing a galaxy which had previously been observed as part of the Fisher GBT HI galaxy survey. To the extent tested by only ~1 hour of observing, we consider this mode to have passed its astronomical checkout. Baselines look reasonably good, the rms noise values integrate down as expected, and the HI galaxy line profile is a good match to that obtained by Fisher. Further details are given in the sections below.
Data were acquired on 18th August 2002 (EDT) as part of commissioning project TSPECTEST_02. Data were acquired using M&C version 3.5.0, but with a test version of the LO1A synthesizer manager, which had upgrades to improve the frequency switching behaviour. This was expected to have no impact on position-switched observations. The version of the Spectrometer Manager used was the build of Tue Aug 13 13:55:10 GMT 2002. The "daily" version of aips++ was used for data reduction. This corresponds to Version 1.8, and builds between approximately 165 and 180.
These observations were made on U9179o, a galaxy chosen from the Fisher GBT HI Galaxy Survey. Due to an observing error, the source name in these scans is given as 3C295, the previous pointing calibrator. The data are available in raw form at /home/gbtdata/TSPECTEST_02. An aips++ measurement set at containing the scans discussed below is available at /home/thales/scratch/gbtmsdata/T12_5_MHZ_9LEVEL_1N2-0A-12-9 in MS TSPECTEST_02_SPECTROMETER_A/ .
For the purposes of Spectrometer checkout, seven pairs of 300 second OffOn scans are relevant, as follows:
45-46 50-51 52-53 54-55 56-57 58-59 60-61
Figure 1 (postscript) shows a typical total-power passband for this spectrometer mode. It can be seen that the IF filter band edges are not quite aligned with the spectrometer passband. For this (and other) reasons, the first ~0.5MHz (~360 pixels at this resolution) of the reduced spectrum is significantly noiser than the remainder. Results for a typical scans (46) are shown in Figure 2. (postscript).
Our result:
Fisher result:
Table 1 is the comparison of the derived parameters, using the K/Jy value found by R. Maddalena of K/Jy = 1.91
Table 1
Comparison of Derived Profile Properties
Quantity Fisher GBT TSPECTEST_02
Peak (Jy) 0.860 0.843
Systemic Velocity (km/s) 303.5 304.3
Line Width (km/s) 160.7 158.4
at 20% of peak
Profile Integral (Jy.km/s) 115.8 110.4
(199-401 kms/s)
Table 2
RMS Values (mK)
(a) Individual Scans of 300 Seconds
Scan Number Chan X Chan Y Both
46 27.0 27.4 19.2
51 30.0 29.6 22.6
53 26.7 26.4 19.3
55 28.4 28.3 20.5
57 26.8 26.4 19.0
59 27.3 26.8 19.2
61 30.0 33.0 22.1
Mean 28.0 28.3 20.3
Dispersion 1.5 2.4 1.5
(b) Averaging Scans
Scan Number Chan X Chan Y Both
46&51 20.5 20.4 15.1
53&55 19.6 19.8 14.3
57&59 19.2 19.0 13.7
46&51&53 16.0 16.1 11.6
55&57&59 16.2 15.9 11.7
53&55&57&59 14.1 13.8 10.2
46&(51-61) 10.7 11.2 7.94
Assuming that the Tsys is 19.5 K, from scan #46, and that the
channel width is 1526 Hz, the rms expected in an observation
of 300 seconds is 28.8 mK. All of the observed values of rms
are plotted in Figure 6 (postscript).
The solid line shows the expected
variation, and the observed values are in excellent agreement
with the expectations.
After the data were inspected interactively, the following glish procedures were used to automate the production of the above figures. Note some interaction with dish is required to set baseline regions.