[evlatests] 2019 and 2020 Dish Holography results

[rperley]

Holography data taken in Jan 2019 and May 2020 have been analyzed to 
provide new 'surface images' of the VLA's antennas.  Panel adjustment 
files will be sent to the relevant individuals, and a detailed memo is 
under construction.

Here I include only a brief summary, and one figure to show the 
stability and repeatability of the results.

The attached figure shows the surfaces of ea05 (top row) and ea17 
(bottom row).  The Jan 2019 results are in the left column, the May 2020 
are in the right column.

Ea05 was chosen as it is currently the 2nd worst antenna in terms of 
surface error.  (ea01 is the worst, but it is 'special' ...).  ea17 was 
selected, as it is the best.  The pre-fit rms for ea05 is 0.5 mm, for 
ea17 it is 0.25 mm.

The color wedge runs from -1.5 to +1.5 mm.  The panel layout is 
superposed in black.  Areas under the quad legs (along the X and Y axes) 
are of low weight, due to shadowing.  These areas are downweighted in 
the solution for the panel adjustments.

The main takeaways are:
(1) Nearly all antennas have excellent surfaces, sufficient for our 
shortest wavelength of 6mm.  Only a few antennas really need any futher 
adjustment.  And of these few, only two (ea01 and ea05) could be 
considered high priority.

(2) The surfaces are extremely stable.  Comparison of the left and right 
columns in the figure show that all major features are reproduced, with 
differences at the 0.1 mm level.  Comparison to older data (which will 
be shown in the memo) show the stability likely extends over a decade. 
There are a few counter-examples, but these are fairly rare.

3) Most antennas have curious 'features' (e.g. the dark 'hole' for ea17, 
in the lower right quadrant, and the prominent red spot near the top for 
ea05).  Nearly all are specific to a single panel, and cannot be 
repaired with the existing mounting system (threaded rods at the corners 
of each panel).


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