[evlatests] Cross-hand phase convention...

[George Moellenbrock]

Folks,

In Sept, we observed 3C286 and some nearby calibrators at C-band in full 
polarization (C1310plus.55098.blahblah, Sep 24) with WIDAR0.  I reported 
on the polarization calibration (in CASA) of this data in a test meeting 
in Octoberish, illustrating the strong frequency-dependence of the 
instrumental polarization, differences between old and new C-band 
polarizers/receivers, and the effectiveness of deriving the D-terms 
(channelized!) and correcting data with them, including transfer among 
sources.

Recall that the _net_ phase of the WIDAR0 parallel hands had the incorrect 
sign at this time, and so Stokes I images (with sufficient structural 
detail to tell) came out reflected through the origin. This _polarization_ 
dataset is a good and more subtle test of the phase sign convention, at 
least for the state of the correlator, etc. at that time.  This is because 
the cross-hand phase for a polarized source should show the 'correct' 
evolution with time (where 'correct' here means being consistent with the 
sign convention for parallactic angle phase correction in CASA or AIPS). 
Also, the RL phase differences between sources of known polarization 
position angle should be consistent with prior knowledge.  If the 
cross-hand phase has the wrong sign, then the source poln p.a. will rotate 
in the wrong direction, and polarization imaging that integrates over 
significant parallactic angle coverage will de-cohere. (An unpolarized 
source, or a snapshot of a polarized source [no parang coverage], will 
image coherently, but suffer the same image reflection that the Stokes I 
image does, assuming all correlations are 'organized' the same way through 
the correlator and labelled correctly in the output.  We've confirmed most 
of the latter via good bandpass corrections of the cross-hands.)

For my reduction, I did NOT negate the visibility phase a la PHNEG. The 
D-term calibration relied upon an unpolarized source because, despite 
about 75min of data, it was observed at an hour angle far from transit 
that yielded very little parallactic angle coverage on 3C286.  However, 
I've had a closer look and there is, in fact, sufficient parallactic angle 
coverage (and SNR) on 3C286 (11% polarized) to detect the parallactic 
angle rotation in the cross-hand phase.  The cross-hand phase of 3C286 
calibrated for everything but the parallactic angle phase (bandpass, 
delays including R-L delay [these were quite large then], complex gain and 
D-terms) shows about 4 or 5 degrees of monotonic variation.  When the 
parallactic angle correction is also applied, the phase rate doubles, 
rather than going to zero.  Similarly, the position angle difference 
between 3C286 and 1310+323 has the incorrect sign---note that the small 
parallactic angle variation enabled a reasonably coherent average of these 
data over time, even with the spuriously doubled parallactic angle phase 
rate.  (I recall reporting in Oct that the difference had the ~correct 
magnitude, but perhaps being a bit vague about the sign.  Today, I am less 
impressed with the magnitude of the difference.  The difference is off by
~10 degrees cf POLCAL results of the ~same epoch---this is probably due
to a modest difference in the parallactic angles of these sources that 
gets doubled by the spurious parang correction.  But I think the 
difference is close enough to be reasonably confident about the _sign_ of 
the difference.  In any case, the p.a. phase-rate double is the better 
evidence.)  These facts do not change the conclusions about D-term
calibration effectiveness reported previously.  It worked.

So, this shows that the WIDAR0 cross-hands required the same phase flip as 
the parallel hands.  This should be confirmed for the current WIDAR.  So 
far, only the (recently revised) parallel-hand phase sign has been 
confirmed for WIDAR.  Michael expects soon to take some full polarization 
data on 3C286 and its neighbors to re-confirm the parallel-hand phase 
sign, confirm the cross-hand phase sign, confirm the D-term solution 
veracity, and confirm the transferability of R-L phase among sources 
(i.e., the source poln. p.a. differences).

-George

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