[evlatests] One phenomenon explained ...

[Doug Whiton]

Rick,
We should expect the antennas with new ACUs to move with precision (within
several milliseconds).  The biggest mystery for me is why 6 and 10 were
not behaving as expected.  I'm sure Bruce will want to look into that.  We
are aware of the fact that the old ACU antennas are running on upper edges
of the slew speed spec and setting their slew speed is far from precise.

Doug


> An addendum to the previous post:
> In examining the DFTIM 'waterfall' plots carefully, I think I can
explain the apparent 'bad data' for the moon for the 'speedy' antennas.
I notice that the reflected circularly polarized signal is visible, at
the normal strength, for the 40 seconds of data preceding the apparent
arrival of the 16 new ACU antennas.  For these 40 seconds, only the
'speedy' antennas are claimed to be on source.  What I thought was
evidence of bad data for these 40 seconds -- easily visible at the
bottom of the attached screenshot, is actually just higher noise, as
there were only nine antennas on source at the time.
> The attached screenshot shows just the first scan -- this is the V
vector sum, with frequency on the horizontal axis, time on the
> vertical.  The bright circularly polarized reflection is easily seen
throughout.  The bottom 40 seconds shows notable noisier residuals --
which I originally thought represented bad data, but which is now
interpreted to be just the higher noise due to the reduced number of
antennas contributing to the vector sum. The evidence is the continuance
of the V signal.  For the (flagged, therefore not visible) data prior
to
> the bottom row, this V signal is not present.
> The sudden transition to better data is easily seen, 40 seconds up from
the bottom row, and is caused by the amazingly simultaneous arrival of
all the other antennas (or, perhaps more likely, the on-line flagging
deciding this was a good time to declare things normal).
> So I think the only 'mystery' is how 16 'new ACU' antennas can arrive at
the target source, after so long a journey (nearly 180 degrees of
azimuth rotation) at exactly the same time.
> Rick
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-- 
Doug Whiton
Hancock VLBA
National Radio Astronomy Observatory
603-525-4332