[evlatests] EVLA Polarization stability

[George Moellenbrock]

Rick,

The variation in the cross-hands is observed for data corrected
only by the parallel-hand calibration (CALIB), right?  (I.e., you
did the PCAL for informational purposes, but didn't apply it.  I think
this is what you told me.)

Are you applying the parallactic angle correction with CLCOR?  (or
unwittingly some other way?)  (Applying the PCAL solution presumably
does this, too.)

Application of the parallactic angle (one way or another, including perhaps,
somewhat mysteriously, by the online system or during fill?), will adjust
the
phases such that the source polarization (if any) becomes stationary (in the

frame of the telescope during the observation, the source polarization p.a.
rotated at 2x the parallactic angle rate as rcp and lcp differentially
retard/advance).
The consequence of this is that any uncorrected instrumental polarization
terms
will now rotate (at 2X the parallactic angle rate), and this is something
that should repeat from day to day if the instr pol is stable.

My guess is that this (or some unmodelled [non-linear terms?] residual if
you
are, in fact, applying the PCAL solution) is what you are seeing.   It has
to be
something like this if it is pegged to the parallactic angle and repeatable,

I think.  In an odd way this is a symptom of something quite stable, in
fact.

Or there is lots of crosstalk downstream of the amplifier (e.g., as if
the polarizer were downstream of the amplifier a la Greenhill 190 MHz),
or variable relative R/L gain upstream of (or in) the polarizer.
In this case you simply can't claim to have uniquely isolated
the parallel hand calibration, and a goofy parallactic angle/Dterm
signature will appear superposed on the "corrected" cross-hand data.

That you see variation on *VLA* antennas, too, is confusing, and points to
mishandling of the parallactic angle somewhere rather than some
hardware issue.  That it decreases at higher-freq bands is also confusing...

-George
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