Continuum polarimetry case

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Continuum polarimetry case

To obtain gain calibration solutions for the continuum polarimetry data set, create the calibrator tool, select which data you want to find solutions for (e.g. the gain and flux calibrators in FIELD_IDs 9 & 11), set up to apply pre-computed polarization calibration, set up to solve for phase and amplitude solutions, and then solve for the calibration solutions. The actual solutions will be written to a file (here called ap366.gcal) that will be written to disk. The dataset itself will not be written to at this point. Thus:

calC:=calibrater(filename='ap366.ms');      # Create Calibrater tool

calC.setdata(msselect='FIELD_ID IN [9,11]');
                                            # Select calibrator sources 
                                            #  in MS fields 9 & 11
calC.setapply(type='P',                     # Arrange to apply parallactic 
              t=5.0);                       #  angle correction (for polarization 
                                            #  only).  Choose a 5 sec timescale.
                                            #  The P-table corrections are 
                                            #  computed in AIPS++.  
calC.setsolve(type='G',                     # Arrange to solve for G 
              t=60.0,                       #  on a 60.0 sec timescale
              refant=4,                     #  Choose reference antenna 4, 
              table='ap366.gcal');          #  a well-behaved antenna
                                            #  near the center of the array. 
calC.state();                               # Review setapply/setsolve settings
calC.solve();                               # Solve for the net complex gains 
                                            #  and write solutions to the table 
                                            #  ap366.gcal located on disk. 
calC.plotcal(tablename='ap366.gcal');       # Inspect solutions

The state function provides the current state of the calibrator tool in a logger message:

  The following calibration components will be applied:
    P table=<pre-computed> t=5 select=[]
  The following calibration components will be solved for:
    G table=ap366.gcal t=60 preavg=0 phaseonly=F refant=4 append=F

While the calibrater.solve function is running, the logger will report the status of the convergence with messages like:

  G Jones Slot=1, 0917+624, spw=1: 22-May-1998/22:40:05 to 22-May-1998/22:41:00
  G Jones    Initial fit per unit weight = 0.989737 Jy, sum of weights = 6210
  G Jones    Final   fit per unit weight = 0.013267 Jy after 7 iterations
  G Jones Slot=10, 1328+307, spw=1: 23-May-1998/00:53:15 to 23-May-1998/00:53:55
  G Jones    Initial fit per unit weight = 7.41213 Jy, sum of weights = 4920
  G Jones    Final   fit per unit weight = 0.0821705 Jy after 7 iterations

Good solutions will have a final fit per unit weight that is a few percent of the initial fit per unit weight, as shown above. You can plot the solutions graphically using the calibrater.plotcal function, e.g. (Fig. 1.11):
If you want solutions plotted for each antenna separately, choose MULTIPLOT=True.

$\begin{figure} % latex2html id marker 978 \epsfig{file=cookbook.dir/vla.plotcalC... ... gain calibrator, 0917+624, and 3C\,286 are plotted. } \hrulefill \end{figure}$

Note that all calibrater.setapply and calibrater.setsolve executions prior to the calibrater.solve execution will be in effect when obtaining the solutions. Thus, the parallactic angle correction, P, is applied before the G solutions are obtained. The parallactic angle correction is required for polarimetry reduction, and may be omitted otherwise. If data imported from AIPS already had the parallactic angle correction applied using the AIPS task CLCOR, then the calibrater.setapply step for P should be omitted (it is the user's responsibility to keep track of this).

If your calibration solutions do not converge well, try longer solution intervals. You may also have bad data remaining in the calibrators: take a closer look at your data and flag anything that looks suspect.

NOTE: To specify scan-based solutions (e.g. determine gain solutions for each scan) set t=0.0 in calC.setsolve.

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