[evlatests] Some Nice ACU Results

[Doug Whiton]

On 6/1/2018 9:29 AM, Rick Perley wrote:
>     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.
>
>     Our initial tests were done Wednesday evening, with rather nice 
> results.  This report is a brief summary.  More details will come later.
>
>     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.
>
>     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.
>
>     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.
>
>     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.
>
>     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:
>
>    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).
>
>     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.
>
>     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.
>
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.
Doug




-- 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20180601/1cba2ad6/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: my sig 1.png
Type: image/png
Size: 13496 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20180601/1cba2ad6/attachment.png>