[evlatests] Delays and Closure

[Michael Rupen]

For those interested, Rick's test data set discussed in this and the last
few e-mails is

     WICloopCtr_001.55236.00132274305

and is available in the usual formats in

     /home/casa-dev-11-2/mrupen/WIDAR0

though I believe it's now also accessible from the archive.

              Michael

>    Spurred by a question from Frazer and Sanjay, I've investigated the
> closure issues a little more closely.
>
>    The BLCAL that I ran on the 4 Jy source was applied to the SPLITed
> data -- the only deviant points remaining were those from the 'wobble'
> baselines.
>
>    I thus ran SPLIT *without* the BLCAL -- and got a *very* different
> picture ...
>
>    Both the RCP and LCP visibilities now show significant offsets in
> the visibilities of some baselines.  These are easy to separate from the
> 'wobble' baselines -- wobbleing baselines always oscillate about the
> mean amplitude of phase.  The true closure baselines are always offset
> and steady -- and are only seen in amplitude.  The offsets in LCP are
> much larger -- up to 10%!
>
>    It was immediately found that *all* of the visible offset (closure)
> baselines are identified with those antennas with large delays.  This is
> a one-to-one relation -- there are no baselines with large delays
> without offsets.  Running BLCAL on this database perfectly removes the
> visible closure offsets.
>
>    The trouble is:  It's my understanding that AIPS corrects for the
> decorrelation from these large delays.  I followed the recommended
> procedure:  FRING and CLCAL, which created a CL table which was then
> applied for further processing.
>
>    Furthermore, comparing the amplitude closure offsets with the
> measured delays shows a curious relation:  All the negative offsets
> arise from baselines whose delays are positive.  All the positive
> offsets arise from baselines where the delays are of opposite sign.  Yet
> these latter offsets are much smaller than the negative ones, when they
> should be larger (the two contributing antennas are 'further apart' in
> delay space).
>
>    This indicates that the delay correction is not being applied
> correctly (or there is a pilot error by your faithful scribe).
>
>    All of this will be eliminated once we get these pesky delays out of
> the system!  We might then be able to measure the *true* closure offsets
> ...
>
>
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