[daip] CXPOLN

[Uwe Bach]

Dear all,

following the suggestion by Bill Cotton I looked more deeply what the 
procedure CXPOLN is doing or better what it is not doing. It seems that 
at some time in the past the OUTCLASS style for images was changed.
The procedure CXPOLN is looking for the MX output files QBEAM, UBEAM, 
QMAP and UMAP, but now the OUTCLASS of dirty images is something like 
[Stokes]IM001 e.g., QIM001 for Stokes Q and UIM001 for Stokes U and the 
beam files have QBM001/UBM001.

I should have noticed that before, because when you run CXPOLN it 
comments that a file is missing but since all the output files were 
there I did not care. I just noticed that I have to rename the OUTCLASS 
of the files, because CXCLN still needs the old style. But due to the 
wrong file names an important step in CXPOLN was not performed. After 
the creation of the complex dirty images and beams in Q and U one has to 
rescale them, so that the reference pixel of the complex beam is 1.0.

In our experiment these factors were always around 1.4 so that we got 
always about 40% less linear polarization and also a rotation of the 
E-vector. I attached a corrected version of the procedure, but I'm not 
sure if this is now valid in any possible case. I changed the OUTCLASS 
from the old to the new style, where it was necessary, but I don't know 
if there are any cases were the input file could be QBM002 instead of 
QBM001 or so. In that case one has to think about a more complex procedure.

I inserted also a section at the end where all files are renamed to the 
old style, since CXCLN needs the files with OUTCLASS: QBEAM, QMAP, UBEAM 
and UMAP. I checked the procedure with several other experiments, where 
I have all antennas with full polarization and the CXCLN images are all 
comparable to the images produced with IMAGR, therefore I'm nearly 
confident that this procedure works properly now.

Thanks for all your help and suggestions, kind regards
Uwe

P.S. Eric Greisen, please tell me if you need a more detailed or more 
structured description of what I changed in CXPOLN for the AIPS changes 
documentation.

-- 

_________________________________________
Uwe Bach
INAF - Osservatorio Astronomico di Torino
Via Osservatorio 20
10025 Pino Torinese (TO), ITALY
http://www.to.astro.it/blazars/bach
phone: +39 011 8101912
fax: +39 011 8101930

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