[evlatests] Hi-Fidelity Polarimetry, the Band Switches, and 'Delay Clunking'.

[Keith Morris]

I would add that we don't know for certain that it is the "band 
switches" -- since this is Ku-band (and we're waiting to learn whether 
this problem manifests at the other high frequency bands), the UX 
converter path-select switches may also need to be investigated. 
(Recall that the "band switches" were replaced on all antennas over a 
year ago; the UX path switches were not).



On 3/11/2014 4:10 PM, Rick Perley wrote:
>       Polarimetric imaging of strongly polarized calibrators, such as
> 3C286, have long shown problems in the dynamic range of the Stokes 'Q'
> and 'U' images.  This has been particularly evident at the higher
> frequency bands, but until recently, I've not looked deeply into the
> problem.  Analysis of the images suggested that, for some scans, the
> amplitudes of the Stokes visibilities Q and U were incorrect by small
> amounts (~10%).
>
>       More careful investigation showed that the issue was not in the
> amplitudes of the (RL) and (LR) visibilities, but in their phases. Most
> interestingly, if the (RL) phase was erroneously high by (say) 10
> degrees, the (LR) phase was low by the same amount.  When the Q and U
> visibilities were formed, the Q and U visibilities had the correct
> phases, but incorrect amplitudes.
>
>       Eric Greisen wrote a little AIPS task (RLCAL) which greatly helped
> the diagnosis -- essentially a polarization  'self-cal', which solved
> for R-L phase error in the data, given a polarization model.  This
> clearly showed that there was an R-L phase error affecting the data
> within some scans.  Typical errors are not large -- 10 degrees -- but
> more than enough to ruin a high-fidelity polarization image.
>
>       The (R-L) phase solutions showed that all antennas were affected
> identically, which tells us right away that it's a property of the phase
> reference antenna.  For the database (at Ku-band) that I was using, the
> reference antenna was ea10 -- chosen because it was the closest to the
> array center.  So what's special about this antenna?
>
>       The answer came in reviewing the (PDif) switched power.   Every
> scan for which we found an (R-L) phase error, also shows the LCP side
> (usually) with switched power low by ~ 25%.  In this experiment, I was
> changing bands every ~25 seconds.  Upon return to Ku-band, some switch
> was not at 100%, and this is apparently accompanied by a small change in
> phase to the affected polarization.  The amplitude loss is corrected by
> application of the switched power (or by self-calibration).  But we have
> (as yet) no regular procedure for correcting the phase change *of the
> reference antenna*.  (A phase change on a non-reference antenna will be
> corrected by self-calibration).
>
>       So what's a user to do to ensure the best possible polarization
> calibration?  Choose a reference antenna that doesn't have this
> problem!  I think the best way to do this is to review the switched
> power.  Antennas with temporally smooth switched power (PDif) appear to
> not have this phase problem. I tested this hypothesis by calibrating my
> data with ea12 as reference.  This antenna has a completely smooth PDif
> plot in both polarizations.    And sure enough, the polarization
> solutions were stable, with no serious issues with the (R-L) phase.
>
>       The switches are not the only source of R-L phase changes -- the
> 'delay clunking' provides a similar effect of similar magnitude -- but
> it is normally not so evident since it operates on a much faster
> timescale (seconds, or shorter) and quickly averages out.
>
>       I will show the evidence at the upcoming Thursday tests meeting
> (10AM, Rm 317).  We will also discuss the origins and fixes -- including
> the annoying 'delay clunking'.
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