[daip] Re: VLA polarization calibration, cookbook section 4.6

[gtaylor]

Hi Jim,

   Its a little more complicated than changing one entry in the Cookbook.
The EVPA of 3C48 changes as a function of frequency.  The correct values
to use are given in a table in section 3.3 of the VLA calibrator manual 
(see http://www.aoc.nrao.edu/~gtaylor/calman/polcal.html).  We can either
provide this information about the EVPA as a function of frequency
or point people to the VLA calibrator manual.  Ciao,
                                                            - Greg

On Fri, 25 Aug 2000, Jim Ulvestad wrote:

> I've been calibrating X-band VLA polarization data using
> 3C 286 as a position angle calibrator.  But I also
> observed 3C 48 in the 24-hr run.  When I checked the
> position angle calibration on 3C 48, I got about
> -132 deg. for twice the source position angle
> (-134 deg. in IF 2, -131 deg. in IF 1.)  The Cookbook
> says this should be -140 deg., and I thought the
> difference was big enough to look into it.  So I
> found that in the Myers/Taylor VLA/VLBA polarization
> calibration data base, they assume -132 deg. for this
> value, exactly what I get, but noticeably different 
> from the cookbook.
> 
> Since people use the cookbook to do their polarization
> calibration, and many people probably use 3C 48 as
> their calibrator, it seems to me that we ought to change
> the cookbook.  It will make a difference of only 4 deg.
> in polarization p.a., but this is significant relative
> to errors that can be on the order of 2 deg.
> 
> jim
>