A Note on channelized polarization calibration

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4.4.5.2 A Note on channelized polarization calibration

When your data has more than one channel per spectral window, it is important to note that the calibrator polarization estimate currently assumes the source polarization signal is coherent across each spectral window. In this case, it is important to be sure there is no large cross-hand delay still present in your data. Unless the online system has accounted for cross-hand delays (typically intended, but not always achieved), the gain and bandpass calibration will only correct for parallel-hand delay residuals since the two polarizations are referenced independently. Good gain and bandpass calibration will typically leave a single cross-hand delay (and phase) residual from the reference antenna. Plots of cross-hand phases as a function of frequency for a strongly polarized source (i.e., that dominates the instrumental polarization) will show the cross-hand delay as a phase slope with frequency. This slope will be the same magnitude on all baselines, but with different sign in the two cross-hand correlations. This cross-hand delay can be estimated using the gaintype=’KCROSS’ mode of gaincal (in this case, using the strongly polarized source 3C286):

   default(’gaincal’)
   vis                 = ’polcal_20080224.cband.all.ms’
   caltable            = ’polcal.xdelcal’
   field               = ’3C286’
   spw                 =         ’’
   solint              =      ’inf’
   combine             =     ’scan’
   refant              =     ’VA15’
   smodel              = [1.0,0.11,0.0,0.0]
   gaintype            =     ’KCROSS’
   gaintable           = [’polcal.gcal’,’polcal.bcal’]
   gaincal()

Note that smodel is used to specify that 3C286 is polarized; it is not important to specify this polarization stokes parameters correctly, as only the delay will be solved for (not any absolute position angle or amplitude scaling). The resulting solution should be carried forward and applied along with the gain (.gcal) and bandpass (.bcal) solutions in subsequent polarization calibration steps.

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