[evlatests] Spurious sources at X-band too.

[Rick Perley]

    This spurious source did not appear in my tests done this morning ...

    50 MHz, 0.416 sec dumps.  Four IFs.  Antenna 21 dead.  All others 
o.k., with a few minor problems which were flagged out. 

    In IF #1:
    EVLA - EVLA images showed peak rms in Stokes I of 0.7 mJy.  No 
sources anywhere.
    VLA - VLA images showed peak of about 0.4 mJy -- possible source, 
not at phase center.   Looks real.
    All antennas - all antennas showed peak about 0.3 mJy, with an 
obvious background source, not at center. 

    Everything looks fine in this dataset.  No spurious sources noted. 

    Rick

Vivek Dhawan wrote:
> On a tip from Joan I looked at Edo Berger's data from S8531, Apr 20th.
> About 9 hours of data, standard X-band continuum setting.
>
> He used fast switching with only 45s on calibrator. For reasons I have
> not explored fully, the EVLA antennas often had no fringes on the short
> scans (Perhaps to do with 5s dump time and the scan times not being
> aligned with the switch cycle?) The VLA antennas fringe consistently
> for ~35s of each short scan.
>
> Enough data survived on EVLA-EVLA baselines to show a spurious 10-15mJy
> source near the phase center, stretched westward (the beam is nearly
> circular). All 4 IFs show the fake source. All corresponding images
> with VLA-VLA baselines show a field with no obvious artifacts and
> only 0.2mJy at the center. Plots are attached. All 4 IF's are similar.
>
> I have informed Edo.
>
> I have a test running later today at C, K and X band.
>
> What else do we do now? I am cherry-picking people's data (or asking
> them to look carefully, if they are locals) to see if we can identify
> a start time for this bug.
>
> I still don't know if the VLA is completely immune.
>
> Vivek.
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