Polarization-independent Gain (T)

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4.4.3.2 Polarization-independent Gain (T)

At high frequencies, it is often the case that the most rapid time-dependent gain errors are introduced by the troposphere, and are polarization-independent. It is therefore unnecessary to solve for separate time-dependent solutions for both polarizations, as is the case for ’G’. Calibration type ’T’ is available to calibrate such tropospheric effects, differing from ’G’ only in that a single common solution for both polarizations is determined. In cases where only one polarization is observed, type ’T’ is adequate to describe the time-dependent complex multiplicative gain calibration.

In the following example, we assume we have a ’G’ solution obtained on a longish timescale (longer than a few minutes, say), and we want a residual ’T’ solution to track the polarization-independent variations on a very short timescale:

gaincal(’data.ms’,               # Visibility dataset
        caltable=’cal.T’,        # Specify output table name
        gaintype=’T’,            # Solve for T
        field=’0,1’,             # Restrict data selection to calibrators
        solint=3.0,              # Obtain solutions on a 3s timescale
        gaintable=’cal120.G’)    # Pre-apply prior G solution

For dual-polarization observations, it will always be necessary to obtain a ’G’ solution to account for differences and drifts between the polarizations (which traverse different electronics), but solutions for rapidly varying polarization-independent effects such as those introduced by the troposphere will be optimized by using ’T’. Note that ’T’ can be used in this way for self-calibration purposes, too.

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