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<div class="moz-cite-prefix">Hi Rick,<br>
What was the azim/elev of this measurement? I wonder if the
target happened to be passing through its point of zero azimuth
velocity, which can be challenging to track accurately (because
the tachometer feedback goes to zero). This situation only
applies to sources that transit between the zenith and the pole.
I worked this out awhile back for GBT servo tests. Here is a
(hopefully) self-explanatory diagram computed for the VLA's
latitude that you can compare to:<br>
<br>
<a class="moz-txt-link-freetext" href="https://safe.nrao.edu/wiki/pub/GB/PTCS/SiderealRateVelocities/azelrate.out.vla.sort.png">https://safe.nrao.edu/wiki/pub/GB/PTCS/SiderealRateVelocities/azelrate.out.vla.sort.png</a><br>
<br>
Todd<br>
<br>
On 6/18/13 8:06 PM, Rick Perley wrote:<br>
</div>
<blockquote cite="mid:51C0F5E8.3010603@aoc.nrao.edu" type="cite">
We are attempting to characterize the behavior of the antenna
motion on ea21, prior to its move later this week to the barn for
the ACU retrofit.
<br>
<br>
To do this, we made four 'holographic' cuts through the main
beam, along the azimuth and elevation direction. This was done at
L and X bands. A 0.1 second integration was used (to search for
rapid oscillations), and 10 seconds per pointing. Each cut is 11
points long, and the oversampling is 5X -- essentially we're
cutting through the main lobe only. <br>
Attached are two plots for X-band, for baseline 17 x 21. Note
that for the vertical cut, the steps are crisp and rectangular,
beyond ~1 second settling. However, for the horizontal cut,
something else is going on, and even after ~10 seconds, there is
no clear sense that the antenna has stabilized. This behavior is
not unique to 21 -- others show the same thing. The same behavior
is seen at L-band, although the amplitude excursion is much
smaller (as expected). <br>
We plan to do 20-second pointings next (to see how long it
takes to really settled), and to do cuts at K-band, for which the
deviations should be much larger. <br>
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