[evlatests] Report on AJ329 observations, August 9

[Jim Ulvestad]

Brief data report from AJ329
=============================
AJ329 was observed Wednesday morning, for 1.5 hours,
at about 1230 to 1400 UTC.  It was the simplest sort
of observation, all X band default continuum, 50 MHz
bandwidth, with no frequency or band changes.  Default
frequencies were 8435 and 8485 MHz.  Three
EVLA antennas were used -- 13, 14, and 16.
10-second integrations were used.  The target source was
a weak source of a mJy or yes, with a 2.3 Jy point source
calibrator also observed every 10 minutes or so.

(1) All EVLA-EVLA baselines appeared to be bad for the
first 10-second records of each scan.  QUACK appears to have
an option only for flagging data by antenna and not by baseline,
so antennas 13, 14, and 16 were flagged completely for the first
record of each scan.  The values in these records typically
are low by about 20%, but not at zero.

(2) Both antenna 14 and antenna 16 had periods of about 10-20
minutes with no significant amplitude, and were flagged bad
for these periods.  For antenna 14, there were no comments about
this in the observing log.  For antenna 16, this coincided with
a 10-minute period noted in the operators log for lost data
when the technicians were checking the cryo subsystems.  Despite
this notice in the log, the data were not flagged by the on-line
system and had to be flagged by hand.

(3) Other than the flagging above, the EVLA antennas appeared
to perform well.  There were no obvious phase jumps due to
anything except weather, though I have not yet looked for
gradual phase drifts.

(4) EVLA-EVLA baselines were high by about 4% on the calibrator
after the data were flux calibrated.  This is reasonably
consistent with the expected closure errors due to mismatched
bandpasses.

(5) The total integration time on the target source was about
45-50 minutes (7 scans of 7 minutes each).  For a VLA-only
array of 21 antennas, the rms noise in the image (robust=0)
was 45 uJy.  Adding the 3 EVLA antennas, the rms was 40 uJy.
Although I have not checked sensitivities or AIPS weights,
this is roughly consistent with the expected improvement
from 21 to 23.5 antennas (accounting for the flagging above),
if all the antennas have about equal sensitivity.

(6) The naturally weighted image with 23.5 antennas has an
rms of about 30 uJy.  The Observational Status Summary predicts
45 uJy for 10 minutes with 27 antennas.  Scaling to 23.5
antennas for 40-45 minutes, I should expect 25-30 uJy, so
the overall image with VLA+EVLA appears to be near the
theoretical noise level.

The attachment is a uv plot of the data on the phase calibrator,
after the bad data are edited out, showing the effect of
the bandpass closure errors on the EVLA-EVLA baselines.

Jim U.

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