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<div class="moz-cite-prefix">On 6/1/2018 9:29 AM, Rick Perley wrote:<br>
</div>
<blockquote type="cite"
cite="mid:a25f687f-8c6a-88ce-1426-438302d9506e@nrao.edu"> With
the arrival of my REU Summer Student (Lucas Wilkins), we are
continuing the antenna optics alignment program which was started,
but not completed, last summer.
<br>
<br>
Our initial tests were done Wednesday evening, with rather
nice results. This report is a brief summary. More details will
come later.
<br>
<br>
This test comprised two cuts through the main beam and
innermost sidelobes at each band from C through Q. One cut in
azimuth, the other in elevation. The elevation of the target
source (3C273) was 46 degrees. We used the holography function,
with a 10-second step duration, and a 5X oversampling, so the
stepsize in the holographic cuts varied from ~33 arcseconds at
X-band to ~6 arcseconds at Q band.
<br>
<br>
The major purpose of the test was to see if the new ACU
antennas had any oddities in their beam profiles arising from from
optical misalignments, or from the various 'tunings' needed to
smooth the response to the step motions.
<br>
<br>
There are now eight 'new ACU' antennas: 1, 2, 7, 8, 14, 16,
21 and 28. For all of these, the response to both the azimuth and
elevation steps was extremely good, with virtually no overshoot
visible, except at the shortest stepsizes (Q-band). But even
here, the overshoot is very small, and much better than the 'old
ACU' response.
<br>
<br>
In terms of optics alignments, it's clear that some small (few
millimeter) motion of the subreflector will be needed to optimize
performance. Antenna 1 is definitely the worst. There are small
but significant offsets visible in both azimuth and elevation cuts
in most antennas (both 'new' and 'old'). The azimuth offsets will
require small horizontal 'nudges' to the subreflector to correct.
We'll provide a table of recommended offsets once analysis is
complete. The elevation offsets should be correctable through
modification of the 'subreflector rotation trick' coefficients.
More tests are needed to evaluate these -- the first were taken
last night, the remainder should be completed this evening.
<br>
<br>
For the 'old ACU' antennas: Three of these were used as
reference (5, 9, 19) so we have no information on their
performance. Amongst the others, there are two antennas that
warrant special mention:
<br>
<br>
1) ea10's azimuth performance is quite exceptional: For the
larger stepsizes (say, 15 arcseconds and larger), its settling
looks like the other antennas. (Which is to say there's a
significant overshoot, but quick settling after 4 or 5 seconds).
But for small stepsizes, the overshoot appears larger and the
settling not nearly completed after 10 seconds. (I'd estimate the
overshoot amplitude to be at least 6 arcseconds at Q-band, which
has a 6 arcsecond stepsize).
<br>
<br>
2) ea22 is similar to ea10 in that the overshoot becomes much
more pronounced for the smaller stepsizes, but is different in
that the oscillation is quickly damped out.
<br>
<br>
It would be good to understand and repair the ACU issues
leading to these curious performance characteristics, but an
alternate solution would be to place these two antennas at the top
of the 'ACU Retrofit' list.
<br>
<br>
</blockquote>
Seeing that ea27's ACU was an unremarkable 'old ACU' performer, we
will put that ACU in ea10 when ea27 gets a new ACU in a few
weeks. We will replace ea22's ACU with one of our spares. We
will try to reproduce your results before and after to verify an
improvement. We will definitely consider AZ/EL pointing performance
along with F/R performance and maintenance history when choosing ACU
upgrades.<br>
Doug <br>
<br>
<br>
<br>
<br>
<div class="moz-signature">-- <br>
<img src="cid:part1.E729D5B3.2925CB00@nrao.edu" border="0"></div>
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