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<p> Paul, et al.:</p>
<p> Are you sure it is only *rotation*, as opposed to focus? In
the various test I do, both are involved. I'll soon have better
information, as I'm nearly ready to seriously reduce the 'flux
density' test data.</p>
<p> Rick<br>
</p>
<br>
<div class="moz-cite-prefix">On 04/03/2019 02:15 PM, Paul Demorest
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:5b0b7797eef403c6a7a7f38ec5230a4b@nrao.edu">hi all,
<br>
<br>
This is a report on an analysis of VLA subreflector rotation times
I recently did (some of you will have seen a version of this
already; there is a little new info in here but no change in basic
conclusions). This was motivated by recent anecdotal reports from
operators and analysts about specific antennas often being flagged
due to subreflector rotation for much longer than expected,
sometimes resulting in their missing calibrators, etc. I thought
it would be useful to take a more systematic look at recent data
for problems like this.
<br>
<br>
First, the main conclusions are:
<br>
<br>
- There are several "bad" antennas that frequently spend >~10x
the time flagged due to subreflector rotation as the rest, often
for minutes at a time. These are ea05, ea11, ea22, ea23, and
ea25. These should be prioritized for FRM maintenance if
possible.
<br>
<br>
- There are a few more "marginal" ones that show similar behavior
but not quite as severe (ea09, ea10, ea12, ea13, ea15).
<br>
<br>
- All the "bad" and "marginal" antennas have old ACUs.
<br>
<br>
- Not all old-ACU antennas act badly, for example ea03 and ea04
look generally pretty well-behaved. But even these "good"
examples spend typically ~50% more time flagged than new-ACU
antennas. So the new ACUs and associated mechanical overhaul are
clearly an improvement (this is probably not news to many of
you!).
<br>
<br>
More details about this analysis:
<br>
<br>
I gathered data on this from the SDMs currently available in the
MCAF workspace. Right now this goes back to the beginning of the
year. To avoid confusion from test/maint time, I only counted
real science observations, identified as those datasets that start
with '1' or 'V'.
<br>
<br>
For each day (MJD) I add up all the time each antenna is listed as
being in the SUBREFLECTOR_ERROR state in Flag.xml. This only
counts rotation errors (I haven't looked at focus but could in the
future). Since there will be different numbers of band changes
each day, I then divide all the times by the median of the 10 best
(least flagged) antennas for that day.
<br>
<br>
For a second statistic, I also looked at the duration of each flag
event. For reference, a typical subreflector rotation for a band
change should take somewhere between 5 and 25 seconds depending on
which bands are in use; Rick took a close look at this recently,
see his emails to this list in Nov 2018 titled "Band Change
Times." The assumption that band changes take ~20s is baked into
our software in several places (OPT, observing scripts). I
counted up all the instances where an antenna was flagged for
>30s or >120s, these will be potentially bad for
observations.
<br>
<br>
Both of these metrics are plotted versus antenna number for a
week's worth of data at a time (starting on Wednesday evenings).
The rotation time plot has one point per antenna per day for a
week. The flag duration counts are cumulative for the whole
week. See attached png showing the most recent week, and pdf
showing all available data.
<br>
<br>
This analysis has an implicit assumption that all antennas are
getting commanded to do the same thing. This will occasionally
not be true, for example if an antenna is removed from observing
for part of a day for some reason. So isolated data points away
from 1.0, or small non-zero numbers of long-duration flags can
probably be ignored. But long-term patterns where certain
antennas have consistently high/scattered points or many
long-duration flags are meaningful, for example the "bad" ones I
mentioned above.
<br>
<br>
The other situation that may confuse this analysis somewhat is
subarray observations. To help avoid this, I've excluded all
datasets that used less than 24 antennas. There may be some
residual effect on the first full-array observation following a
subarray project since the antennas will have different starting
subreflector positions. These have not been removed since they
are more difficult to automatically identify. But I think this
happens infrequently enough that it's not a big problem.
<br>
<br>
Please let me know if you have comments, suggestions, or questions
about any of this.
<br>
<br>
Cheers,
<br>
Paul
<br>
<br>
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