Basic Information 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: select
Normal uv selection. Only antenna-based selection is supported.

Key: options
Task enrichment parameters. Several can be given, separated by commas.
Minimum match is used.
  noflag   Treat leakages that are identically 0 as good.
  noxy     Do not solve for, or apply, the XY phase offset.
  apply    Apply the solved for corrections to the vis data-set. The
           default is to solve for them only.
  xyleak   GPNORM can determine the xy phase differences from both the
           gains table and polarisation leakages. By default, it assumes
           the gains-derived value should take precedence. The xyleak
           option causes GPNORM to take the leakage-derived value as the
           preferred on. This only has an effect if options=apply is used
           and options=noxy is not used.

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