[evlatests] Correlator offsets versus number of VLA antennas

[Ed Fomalont]

Regarding the stability of Correlator Offsets:

The observing test on Tuesday morning corroborated our guess that the
correlator offsets would change with VLA antennas in the array.  See
attached plots for correlator offsets for 1RR and 2RR.

Yellow dots are 1 hour vector averages of the amplitude visibility 
versus baseline length of the 'blank' field versus for all vla antennas. 
  The slight increase at the shortest baselines are caused by faint 
sources in the field.

White dots are the same plot after removing antennas 17 and 18 (EVLA)

Red dots are the same plot after removing antennas 2 and 6 (VLA)

The baselines with gross errors in the yellow dots are vitually the
same as that for the previous multi-frequency observations using the
same set of antennas.  Removing two antennas does decrease the strength
of the correlator offset.  The offsets of correlator 1RR are 
significantly worse than those of 1LL, 2RR and 2LL, as usual.

I should have tried the experiment with more antennas removed.  A
previous test suggests that removing about 8 antennas will remove all
offsets.  Hence, the offset level seems to depend on the number of
telescopes used.

I don't think these results come as a surprise, and suggest that the 
cause is the clearing of some correlator residuals after the self test, 
and gets worse for more antennas.  What should be done???

Ed
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