The system temperature for the alfa receiver was computed for all data taken with the wapps in spectral line mode (see computing tsys from the archive for a description).
The ratio TsysA/TsysB can show small variations in
TsysA or TsysB since the large scale tsys variations are common to both
(of course you need to worry about polarized sources since the feed is
linearly polarized). Plots were made of the ratio vs frequency, date, az,
za, and alfa rotation angle. Each plot has pixels 0,1,2,3 on the
first page and pixels 4,5, and 6 on the 2nd page. Different project ids
are color coded (projects with fewer than 100 cal measurements were all
grouped into the project "misc"). There were 3337 system temperature measurements
of the entire array (7 pixels).
sky coverage (.ps) (.pdf): This shows the ra/dec , l/b coverage of the data by project. TsysRatio vs Frequency (.ps) (.pdf):This shows the frequencies that each project used. These are the center frequencies of the measurements (Tsys for each spectra was computed using 100 points across each bandpass). The frequencies of the projids are:
A2010 is the only project that is covering much sky. It is doing this with drift scans at the meridian so there is no azimuth or alfa rotation, just za motion. When the ratio is plotted vs ra, the scatter in the a2010 values decreases. There is a small bump at ra 13 to 15 hours and then a dip at 16 hours. Looking at the galatic longitude there is even less scatter. It has a rise up from l= 200 to 360 then a drop down from l= 0 to 30 degrees (remember that we are not certain of the absolute scale (if the cals are off) so changes are what is important). This area is close to the north galactic spur which has polarized power (10 k at 408 Mhz so may .5 K at lband) . The strongest power should be at l=30 b=30. Of course we don't know how the probe angle changes with the the polarization angle of the source. Still this tight grouping of the data seems to say that this change is real , coming from the sky and not some instrumental affect.
1275 Mhz: A1956 1375-1385 Mhz: a1961, a2010, and misc 1405-1420 Mhz: a1963 1410-1420 Mhz: xalfa 1445 Mhz a1943, xalfa TsysRatio vs date (.ps) (.pdf): The data spans nov04 thru feb05. A1963 had a jump in the ratio for pixel 5 between 30nov04 and 18dec04. TsysRatio vs za (.ps) (.pdf): For a1956 you can see the horse shoe shape for pixels 1 and two. This shows an az dependence. TsysRatio vs az (.ps) (.pdf): You can see a clear two az term here (a sine wave with 180 degree period). Some of the pixels do no line up very well with each other. pixel 5 a1963 again has lots of scatter. TsysRatio vs AlfaRotation angle (.ps) (.pdf): There is 180 period for some pixels and about 90 degrees for some others. Notice that pixel 0 has a 180 deg period with an amplitude of 2 %. Pixel 0 is not one of the outer pixels so the variation is not limited to being way off axis. The vertical lines at 0 are those projids that did not rotate the alfa feeds. The vertical line at 19 degrees are the drift scans at the meridian. TsysRatio vs Ra (.ps) (.pdf): The data is plotted versus Right ascension J2000. TsysRatio vs Dec (.ps) (.pdf): The data is plotted versus Declination J2000 TsysRatio vs l (.ps) (.pdf): The data is plotted versus galactic longitude. TsysRatio vs b (.ps) (.pdf):The data is plotted versus galatic lattitude. TsysRatio vs Ra,Dec,l,b (avgPix) (.ps) (.pdf):This plot averages all pixels together.
processing: x101/mbeam/tsys/archive/tsys_gettbl.pro, ratioplot.pro
The drift in a2010 is probably coming from the sky. It has not az or alfa angle changes. There is a 2% amplitude of the TsysA/TsysB vs az with a period close to 180 degrees for some of the feeds. These ratios are probably twice what is happening to TsysA or TsysB by themselves (it is probably out of phase). The variation is happening to the outer pixels and to the central pixel (which is not off axis .. hopefully too much!!) Joanna rankin mentioned that the panels on the dish are 3 by 6 feet long and they are all aligned with the long direction in the east-west direction. It may be that the longer direction of the panels have more warping that the short direction.Where they fit together this may affect the contact that is made between adjacent panels giving an asymetry on the dish. To check this we'd need to know how the probes of alfa are aligned relative to True north. This same 180 period has been seen in other non alfa receivers: Jun02 cband , Apr03 lbw, May03 sbh.