[evlatests] Some Nice ACU Results

[Rick Perley]

     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.

     Overall, the behavior shown from the first test is quite good.  
There is likely a significant issue with the application of referenced 
pointing within the holography function -- I'll review the data this 
morning, and report on this issue separately.

     Rick