[evlatests] Problems with Shadowing

[George Moellenbrock]

Rick, Eric-

If the sign convention for the w coordinate is consistently applied, then
this
sign can be used to determine which antenna (first or second in the
baseline)
should be flagged as the shadowed one  (if the w axis is positive toward
the sky, then the 2nd antenna in the baseline is the shadowed one when w
is positive---I think).  Since AIPS FILLM re-sorts ("flips") baselines in
order to guarantee
that the lower numbered antenna always comes first (so the ant numbers can
be the
indices), there is the potential for getting this backwards, as you've
observed.   E.g.,
does the sign of w get reversed when a baseline is flipped?

This is not unlike the nominal sensitivity error discovered in FILLM a few
months
ago, wherein ancilliary info (in that case, the nom sens data) wasn't
properly flipped
when the data was flipped.

-George

On Wed, Nov 12, 2008 at 1:20 PM, Bryan Butler <bbutler at nrao.edu> wrote:

>
> if you tell FILLM not to use the flag in the archive record, but to
> rather calculate its own shadowing, do you see exactly this same effect?
>
>        -bryan
>
>
> Rick Perley wrote:
> >     I reported at the Monday meeting that there appear to be troubles
> > with the identification of shadowed antennas, in that some antennas,
> > when clearly in a shadowed geometry, were not being flagged as such.
> >
> >     I have now a much clearer picture of the problem.  I filled the data
> > with all shadow flags off, and selected a time at which the source 3C138
> > was setting at an elevation of 20 degrees -- the azimuth is very close
> > to 275 degrees.
> >     Antenna 1 is at W1, antenna 22 is at W2, antenna 11 is at E3,
> > antenna 21 is at E2, and antenna 20 is at N1.
> >
> >     By looking at the (u,v) coordinates, it is easy to see that:
> >
> >     Antenna 1 is shadowed by antenna 22
> >     Antenna 11 is shadowed by antenna 21
> >     Antenna 21 is shadowed by antenna 20.
> >
> >     So, antennas 1, 11, and 21 should be flagged, and antennas 20 and 22
> > should not.
> >
> >     But in the database, the situation is exactly reversed!  Antennas
> > 20, 21 and 22 are flagged, and antennas 1 and 11 are not.   It seems
> > clear that the shadow algorithm has the direction of the baseline -- at
> > least in some cases -- backwards.    I suspect that in cases where the
> > lower-numbered antenna is the one that is being shadowed, the shadow
> > flags are attached to the higher numbered antenna instead.
> >
> >     Rick
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