[evlatests] Q-Band Holography Results

[Rick Perley]

     Analysis of the Q-band holography imaging has been completed, and 
surface adjustment files are ready for the implementation.

     A brief summary follows -- all the gory, excruciating details will 
be explained in an upcoming memo.

     Four extensive observations were utilized:  Two taken in October 
2011, separated by 10 hours, and two in January 2015.  One of the 
October observations was in daytime, the other in nighttime. Interesting 
differences are noted -- see below.  The January observations were 
nighttime.  Unfortunately, due to weather and re-configuration issues, 
the last observation was delayed until after the CnB move -- the north 
arm antennas were at too great a distance for phase calibration.

     Some interesting and useful conclusions follow:

     1) There is great stability over the 3.2 year separation between 
the observations, to the level of tenths of millimeters.  There is every 
reason to believe that, once reset, there will be no need to re-adjust 
for many years.   The holography program predicts a ~ 1.5 dB increase in 
gain by readjusting for most antennas.  This is likely too high, but for 
most antennas, readjustment should significant improve gain.  There are 
about 5 antennas which *must* be adjusted, as they show significant 
corrections.  (The worst, by far, is ea20).

     2) There is an interesting difference in surface shape between 
night and day.  This was found by Michiel Brentjens in the October 2011 
data -- the surface difference image clearly shows the backup 
structure.  The difference is small (tenths of mm).  A difference image 
between the daytime image in October 2011, and the nighttime image from 
last January shows the same pattern -- and the difference between the 
October 2011 night-time and January 2015 shows no rib-like pattern.

     3) The 'HOLGR' program fits for the effective subreflector 
position, with an accuracy much better than 1mm.  The focus fits for the 
two October 2011 data observations clearly show a 1.0 -- 1.5 mm 
difference in focal length between night and day.  All antennas show 
this -- there are no exceptions.  One suspects the day-night temperature 
change (and indeed, the coefficient of expansion of steel, combined with 
a ~20 -- 30 F temperature change, predicts a change of 1.5 mm in the 9m 
focal length.  However, a homogeneous antenna must stay in focus for all 
changes of temperature.  So -- do we have different steel in the support 
legs, or are there significant temperature gradients in the structure?  
(The night-time observations were centered near 9PM, long before the 
antennas would normally in in thermal equilibrium).

     4) Some antennas seem to be 'missing panels', or have otherwise 
very peculiar illumination amplitudes.  ea26 is the most obvious -- at 
all frequency bands, one of the outer ring's panels is 'missing'.  This 
means the amplitude is effectively zero.  Of course the panel is there 
-- but I suspect it is significantly tilted from the correct position.  
A visual inspection has already been made, but no suspicious tilt  has 
yet been found.  (It doesn't have to be a big tilt -- the physical 
resolution we use is about 80 cm, not much smaller than a panel, so tilt 
of only 1 cm over the panel width would make it become invisible ...)

     These, and other curious things, will all be in the major memo now 
under construction.