[evlatests] Carilli/Reichers problem

[Vivek Dhawan]

What we are calling the "Carilli/Reichers" problem is the
appearance of a fake source in a blank image field. To cut a long
story short, the working hypothesis right now is some kind of
interference; K-band is most affected, but it is also where we have
been looking hardest. Observations at C and X band have been looked
at, and show little if any problem, but careful tests on a blank field
are needed.

The main features are:

A fake source appears just north of phase center, stretched out
towards N.

EVLA-EVLA baselines show it much stronger than VLA-VLA.

Continuum and line data both show it, but not all frequencies
and times are affected equally badly. In some test observations,
the artifacts appear as higher than expected rms, but not at the
phase center. In all cases, an image made from VLA-VLA data is
much cleaner than EVLA-EVLA data.

--------------- example -  recent data ----------------

Some short continuum observations done over the weekend;
At each frequency, 8 images were made, one per IF/polarization,
with and without EVLA baselines. Expected rms is about 200uJy
1-sigma on V-V images, and about 400uJy on E-E images.

f_GHz    image with E-E baselines                   V-V baselines

23.4649  +_10 mJy peak; near center                 no VLA data

23.8649  +_10mJy; near center                       +_0.5mJy peak

24.2649  +_8mJy; not near center                    +_0.5mJy peak

24.6649  RR=+_7, LL=+_2mJy, not near center         +_0.5mJy peak

22.4851  +_2mJy not near center
22.4351  +_2mJy RR not centered; LL at center.      +_0.5mJy peak

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