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Basic Information on gpcomb

Task: gpcomb
Purpose: Combine multiple polarization calibration solutions.
Categories: calibration

        GPCOMB combines the leakage tables and xyphase solutions from
        multiple calibrators. The routine is intended to be used when
        attempting high-precision circular polarization observations at
        the ATCA. Because leakages and xyphases are treated as invariant
        during circular polarization calibration, a more accurate
        calibration may be obtained by averaging several solutions from
        several "leakage calibrators".
        GPCOMB will probably be less useful for linear polarization
        observations, because linear polarization observations are
        usually limited by the xygain amplitudes, not the leakages
        or xyphases.
        If the calibration solutions you are comparing had the circular
        polarization zero-point determined from different observations
        of 1934-638 (eg. on different days), then there may be an
        xy-phase offset hidden in the bandpass, and so averaging the
        leakages and xyphases would probably be a bad idea. Maybe
        try using the same bandpass for both...
        And finally, if you have consistent systematic errors in your
        solutions (eg. you haven't used xyref,polref in gpcal), then no
        amount of averaging is going to help.
        For an overview of high-precision circular polarization
        calibration with the ATCA, see
        Rayner, "Circular Polarization User's Guide", ATNF Technical
        Document Series, 2000, 39.3/102,
        For thorough treatment of the issues, see
        Sault, Hamaker and Bregman, "Understanding radio polarimetry
        II. Instrumental calibration of an interferometer array", A&AS,
        1996, v117, pp149-159.

Key: vis
        The visibility data-set to which the average leakages and
        xyphases will be written. If omitted, the averaged
        leakage values are still reported to screen.
        Existing calibration tables are not applied to vis ie. the
        resultant calibration and leakage tables in vis are REPLACED by
        those from the cals. This is what you would usually want to
        do. If not, apply the appropriate calibration first with uvaver.
        The corrected dataset does retain the original gain
        amplitude and the antenna-based component of the gain phase.

Key: cal
        The data-sets containing the nominally correct polarization
        calibration. No default. May include vis.

Key: select
        Normal uv selection. Only antenna-based selection is supported.
        If an existing leakage/xyphase solution exists for a de-selected
        antenna in vis, the value will be preserved. Otherwise,
        it will be set to zero.

Key: options
        Task enrichment parameters. Several can be given,
        separated by commas. Minimum match is used.
          "noxy"     Do not solve for, or apply, the average XY phase.
          "nopol"    Do not solve for, or apply, average leakages.

Generated by miriad@atnf.csiro.au on 21 Jun 2016