For those interested, Rick's test data set discussed in this and the last
few e-mails is WICloopCtr
and is available in the usual formats in
though I believe it's now also accessible from the archive.
> > 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
> > 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
> > ...
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