[daip] Error in parallactic angle

[Andy Biggs]

Hi there,

I'm analysing VLA data from two monitoring campaigns that were observed 
over the same 3-month period. One used 3C119 as the PPA calibrator and 
the other 3C48. 3C119 is far superior as it's unresolved in polarized 
flux and doesn't pass as close to the zenith. If I compare the R-L phase 
corrections, there is a very interesting systematic offset between the 
two within an hour or two of transit - see attached plot.

I showed this to Rick and he suggested an error in the assumed 
parallactic angle due to misalignment of the antenna pads and, sure 
enough, the functional form of that agrees well with what I see (blue 
line on plot). However, to match the data I have to assume an offset of 
~10' which is about an order of magnitude too big. Ken Sowinski says 
that a few pads were erroneously built parallel to local gravity as 
opposed to the centre of the array and that an error of ~10' would be 
possible for a misaligned pad at the end of an arm, but repeating the 
analysis deleting each antenna one by one makes no difference.

Although very unlikely, I am wondering therefore if it might be possible 
that AIPS is making an error with the source or array position when 
correcting for the parallactic angle. This would be a very neat 
explanation, particularly as it's not clear to me how random 
pad-alignment errors would lead to a systematic offset - I would expect 
them to all cancel out.

Might such a thing be possible? Note that the 3C48 R-L corrections were 
derived from Q and U fluxes measured from maps made using IMAGR.

Thanks,

Andy

-- 

Andy Biggs
ARC Astronomer, ALMA Regional Centre
European Southern Observatory
Karl-Schwarzschild-Straße 2
D-85748 Garching
Germany

tel. +49 89 3200 6471
fax. +49 89 3200 6898
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