Documentation on gpnorm

Task: gpnorm
Purpose: Correct the polarisation leakage terms for an offset.
Categories: calibration

        GpNorm is a MIRIAD task which corrects gains and polarisation
        leakage tables for XYphase and leakage offsets. It is based on the
        supposition that the leakage and gain tables are offset from the
        true values. This is often the case when they have been determined
        from observations of a weakly polarised calibrator. GpNorm
        compares the leakage terms with nominally correct terms, and determines
        an leakage and XY phase offset to add which will minimise the
        difference between the two sets of leakages. The determined leakage
        and XY phase offset can then be used to correct the leakages and
        gains.

Key: vis
        The visibility data-set to correct. This must have a leakage table.
        Additionally, it should have a gains table if the XY phase correction
        is to be applied. No default.

Key: cal
        The data-set containing the nominally correct leakage table. No
        default.

Key: options
        Task enrichment parameters. Several can be given, separated by commas.
        Minimum match is used.
          "noxy"     Do not solve for, or apply, the XY phase offset.
          "apply"    Apply the solved for corrections to the vis data-set.