[evlatests] PDif Correction Failures

[Bryan Butler]

while applying PDif will correct gain, it will not correct the 
associated bandpass changes, so it's not a cure-all...

	-bryan


Rick Perley wrote, On 10/25/11 10:09 AM:
>      The previous note gave the fraction of antennas which are badly
> behaved in their PDif values.  (Note:  Be aware of my definition of
> failure in that previous note -- the 'failure fraction' is not the
> fraction of attempts which gave a different or variable value, but
> rather is the fraction of antennas which -- over time -- give unsteady
> values of PDif).
>
>      Here I tally up the failures for PDif values to correct the
> visibilities.  That is -- following application of PDif to the
> visibility data, which antennas were *not* adequately corrected for the
> changes in visibility amplitude caused by poor switches?
>
>       The list is quite short -- meaning that application of PDif nearly
> always corrects the gain variability associated with the bad switches.
>
>      Band      Notes
> --------------------------------
> L             ea28 in LCP.  PDif here is wildly variable most of the
> time.  There are periods of good behavior.
> S             ea6C -- ~7% changes in PDif which are *not* seen in the
> visibilities.
> C             ea25 in A and C:  Five of the 37 scans showed a 2% change
> in PDif not seen in visibilities.
> X             ea12 in IF 'D' -- PDif is simply large values of random
> noise.
> Ku           ea18 in all IFs:  ~2% variations in PDif not seen in
> visibilities
>                 ea25 in all IFs:  ~10% variations in PDif not seen in
> visibilities
> K             All short-scale changes in visibilities are corrected by
> PDif, to ~2% accuracy.
> Ka           ea01, in RCP:  PDif provides large random values most of
> the time (this is like ea28 at L-band).
>                 Failures at the<5% level cannot be easily distinguished
> from pointing errors.
> Q            Same as K-band, but at the 10% level, set by the pointing
> repeatability.
> --------------------------------------
>
> Bottom Line:  Except for the few failures in PDif noted above,
> application of PDif does an excellent job in correcting for the
> switch-induced gain losses.
>
> Unfortunately, we cannot yet recommend automatic application of PDif to
> our visibility data because of the clear evidence that PDif is reacting
> to changes other than in the gain.
>
>
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