[daip] Cross-hand amplitude problems for BR and FD at 15 GHz

[Matt Lister]

Hi, I've been reducing BL178BI, a continuum VLBA 15 GHz data set with full 
polarization from Aug 20 2013.

I've noticed that the system temperatures for BR and FD are 2-3 times higher 
than the other
antennas. The weather was fine at both of those sites. It appears however that 
these had higher Tsys as
well in other 15 GHz runs in July and August, so maybe this is an issue you are 
already aware of.

The LL and RR amplitudes for BR and FD calibrated just fine with APCAL, and are 
consistent with the other antennas.
However, the RL and LR cross-hand amplitudes for BR and FD are a factor of 1.5 
-2 too high. This makes the
polarization data unusable for those antennas, as I am unaware of any AIPS task 
that can adjust the cross-hand gains
without also altering LL and RR.  The cross-hand phases look fine.

You can find a UV file for one of the bright sources
at http://www.physics.purdue.edu/~mlister/2200+420.u.2013_08_20.uvf_raw_edt

You can clearly see the effect in FD by plotting amplitude vs uv distance for LR 
or RL.

Other possibly useful plots are
http://www.physics.purdue.edu/~mlister/POSSM_2.PS
http://www.physics.purdue.edu/~mlister/POSSM_CROSS.PS
http://www.physics.purdue.edu/~mlister/APCAL.PS
http://www.physics.purdue.edu/~mlister/BPASS.PS

Any insight you might have on the cause of this problem and whether the BR and 
FD cross-hand data are recoverable would be helpful.

Thanks,
Matt Lister