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

[Eric Greisen]

Matt Lister wrote:
> 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.

I have forwarded your message to Craig where issues with the instument 
are more likely to get addressed.

Have you tried simply solving for the D terms on these antennas?  It is 
entirely possible that whatever is causing the high Tsys might also 
cause high D terms.  (Something deposited on the feed horn perhaps.)

There has never been a need for a separate gain for cross-hands - that 
is physically unlikely.  Bad polarization separation is - alas - far 
more likely.

Eric Greisen