Spectral-line case

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Next: A Note on Calibrater Tool Operation Up: Solving for complex gain, G Previous: Continuum polarimetry case

Spectral-line case

To obtain gain calibration solutions for the spectral-line data set: select the data you want to find solutions for (the inner 55 channels, starting at channel 3, for the gain and flux calibrators in FIELD_IDs 1 & 2), set up to derive scan-based solutions for phase and amplitude, and then solve for the calibration solutions. Write the gain solutions to a file called ngc5921.gcal and plot the solutions:

calS:=calibrater(filename='ngc5921.ms'); # Create calibrater tool for 
                                         #  spectral line data set
calS.setdata(msselect='FIELD_ID <= 2',   # Select data for calibrators 
             mode='channel',             #  (Fields 1 & 2)
             start=3,                    #  and drop the outer channels 
             nchan=55);                  #  that may bias the gain solution.
calS.setsolve(type='G',                  # Arrange to solve for G for 
              t=0,                       #  each integration.  
              refant=14,                 #  Choose reference antenna 14: 
              table='ngc5921.gcal');     #  a well-behaved antenna
                                         #  near the center of the array. 
calS.state();                            # Review setsolve settings
calS.solve();                            # Solve for the net complex gains 
                                         #  and write solutions to the table 
                                         #  ngc5921.gcal located on disk.
calS.plotcal(tablename='ngc5921.gcal');  # Inspect solutions

Note that we have omitted application of P since there is no polarization data. The state function provides the current state of the calibrator tool in a logger message:

  The following calibration components will be applied:
    None.
  The following calibration components will be solved for:
    G table=ngc5921.gcal t=0 preavg=0 phaseonly=F refant=14 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, 1331+30500002, spw=1: 13-Apr-1995/09:19:00 to 13-Apr-1995/09:24:30
  G Jones    Initial fit per unit weight = 1.94099 Jy, sum of weights = 6.32923e+06
  G Jones    Final   fit per unit weight = 0.0274396 Jy after 8 iterations
  G Jones Slot=2, 1445+09900002, spw=1: 13-Apr-1995/09:27:30 to 13-Apr-1995/09:29:30
  G Jones    Initial fit per unit weight = 0.324512 Jy, sum of weights = 2.65356e+06
  G Jones    Final   fit per unit weight = 0.00756852 Jy after 8 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. 1.12). If you want solutions plotted for each antenna separately, choose MULTIPLOT=True.

$\begin{figure} % latex2html id marker 997 \epsfig{file=cookbook.dir/vla.plotcalS... ..., the last four are for the gain calibrator, 1445+099.} \hrulefill \end{figure}$

Next: A Note on Calibrater Tool Operation Up: Solving for complex gain, G Previous: Continuum polarimetry case Contents