[daip] PCAL and models

[Bill Cotton]

Eric,

   This used to work but it's been "fixed" since then.

-Bill

Eric Greisen writes:
 > Martin Hardcastle writes:
 >  > 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?
 >  > 
 > 
 > Actually subroutine PCLFLX is called for all modes and uses PMODEL
 > (XSMOD inside the task).  PCLFLX exits abruptly on ORI- and does a lot
 > more on the other modes.  PCLFLX is called by PCLSEL from PLCUV ahead
 > of the long if mode ORI- region.  For mode ORI- XSMOD is used within
 > that region instead.
 > 
 > Having said all this, do not think that I have any idea what the task
 > is actually doing.  I am including Bill Cotton in the reply because he
 > may have some memory of this.
 > 
 > I am completely ignorant about polarization - if you find things that
 > need fixing let me know and I will see hwat I can do.
 > 
 > Eric Greisen