
Overview of Difmap polarization mapping.

Mapping polarization data in Difmap.
-----------------------------------

Difmap has limited support for polarization mapping. The main
limitation is that there is currently no way to combine two maps of
different polarizations to form a percentage polarization map or other
combinations. Such post-processing of images must be done in other
programs such as AIPS. Also the only polarization calibration
performed by Difmap is self-calibration. All absolute calibration must
be performed outside of Difmap, say in AIPS.

While it is only possible to make a map of one polarization at a time
in Difmap, self-calibration corrections and editing applied while
making one map are always applied to all other polarizations. This is
a necessary feature, designed to ensure that polarization calibration
and consistent UV coverage over all polarizations is preserved. So, if
self-calibration is required, all polarizations can simultaneously be
self-calibrated while making a single map of the strongest
polarization. The same is true of the scope of self-calibration
corrections wrt spectral-line data.

The polarizations recorded in the originating UV FITS file can
be displayed with the 'header' command.

If an observation is recorded with multiple polarizations, the
'select' command must be used to select the polarization to be
processed. This selection affects what is displayed in all plotting
commands, and forms the data upon which all processing is based until
the 'select' command is again called upon to change the selection.

The selected polarization may either be one of these polarizations, or
be the name of a polarization that can be derived from them.

The names of recognised polarizations are:

  I  Q  U  V  RR  LL  RL  LR  XX  YY  XY  YX

The following is a list of polarizations that may be contructed from
other polarizations:

Name       Derived from:           Derived as:
   
 I         RR and LL               (RR + LL)/2
 Q         RL and LR               (RL + LR)/2
 U         RL and LR              i(LR - RL)/2
 V         RR and LL               (RR - LL)/2

If you need to save one polarization map before making a second of a
different polarization, you will have to save as a FITS file, for
post-processing in another package, with the 'wmap' command.
