Polarization-dependent Gain (G)

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4.4.3.1 Polarization-dependent Gain (G)

Systematic time-dependent complex gain errors are almost always the dominant calibration effect, and a solution for them is almost always necessary before proceeding with any other calibration. Traditionally, this calibration type has been a catch-all for a variety of similar effects, including: the relative amplitude and phase gain for each antenna, phase and amplitude drifts in the electronics of each antenna, amplitude response as a function of elevation (gain curve), and tropospheric amplitude and phase effects. In CASA, it is possible to handle many of these effects separately, as available information and circumstances warrant, but it is still possible to solve for the net effect using calibration type G.

Generally speaking, type G can represent any per-spectral window multiplicative polarization- and time-dependent complex gain effect downstream of the polarizers. (Polarization independent effects upstream of the polarizers may also be treated with G.) Multi-channel data (per spectral window) will be averaged in frequency before solving (use calibration type B to solve for frequency-dependent effects within each spectral window).

To solve for G on, say, fields 1 & 2, on a 90s timescale, and do so relative to gaincurve corrections:

gaincal(’data.ms’,
        caltable=’cal.G’,       # Write solutions to disk file ’cal.G’
        field=’0,1’,            # Restrict field selection
        solint=90.0,            # Solve for phase and amp on a 90s timescale
        gaincurve=True          # Note: gaincurve=False by default
        refant=3)               #

plotcal(’cal.G’,’amp’)          # Inspect solutions

These G solution will be referenced to antenna 4. Choose a well-behaved antenna that is located near the center of the array and is ever-present for the reference antenna. For non-polarization datasets, reference antennas need not be specified although you can if you want. If no reference antenna is specified, an effective phase reference that is an average over the data will be calculated and used. For data that requires polarization calibration, you must choose a reference antenna that has a constant phase difference between the right and left polarizations (e.g. no phase jumps or drifts). If no reference antenna (or a poor one) is specified, the phase reference may have jumps in the R–L phase, and the resulting polarization angle response will vary during the observation, thus corrupting the polarization imaging.

To apply this solution to the calibrators and the target source (field 2, say):

applycal(’data.ms’,
         field=’0,1,2’,         # Restrict field selection (cals + src)
         opacity=0.0,           # Don’t apply opacity correction
         gaintable=’cal.G’)     # Apply G solutions and correct data
plotxy(’data.ms’,xaxis=’channel’,datacolum=’data’,subplot=211)
plotxy(’data.ms’,xaxis=’channel’,datacolumn=’corrected’,subplot=212)

The calibrated data is written to the CORRECTED_DATA column, with calwt=True by default.

Alert: Current (as of June 2010) JVLA data has no weights to the data. To avoid trouble, calwt=False should be set for those data sets. Older VLA data should still be calibrated with calwt=True.

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