[daip] PCAL and models

[Martin Hardcastle]

Hi folks

We have been trying here to do polarization calibration for VLA data
where there is poor parallactic angle coverage by calibrators, and
have been experimenting with using a model (PMODEL) of a strong
polarized source within PCAL.

The documentation for PCAL certainly suggests that this can be done
for all solution types:

PCAL
Task:  This task reads a UV file, calibrates, subtracts a model and
       determines the effective feed parameters for each antenna and
       IF.  These parameters are then placed in the antenna (AN)
       table.  Polarization corrections can then be applied by setting
       DOPOL=1 in LISTR or SPLIT.
            If a polarized model is given then only the feed
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       parameters are determined.  If no model is given then a point
       ^^^^^^^^^^^^^^^^^^^^^^^^^^
       source is assumed for SOLTYPE='APPR'.  For SOLTYPEs 'ORI-' and
       'RAPR' the source may be resolved but the polarized flux is
       assumed to have the same distribution as the total intensity
       and the polarization angle is assumed to be constant.

[...]

  PMODEL.....A single component model to be used instead of a
             CLEAN components model; if PMODEL(1) > 0 then
                                     ^^^^^^^^^^^^^^^^^^^^^
             use of this model is requested.
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


... you'd think from this that the model can be given for any one of
the solution types. But in fact the code makes it clear that only
'ORI-' uses any of the inputs to PMODEL (other than Stokes I). If
PMODEL is used with SOLTYPE 'APPR' the Stokes I used is supplied but
Q, U and V are fixed to zero in the fits, which of course gives
seriously incorrect solutions for the feed parameters. Is this a bug
in the documentation, or the code?

Thanks

Martin
-- 
Martin Hardcastle
Royal Society Research Fellow & Senior Lecturer
Centre for Astrophysics Research, U. Hertfordshire