[daip] VLA polarization calibration, cookbook section 4.6

[Jim Ulvestad]

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