[evlatests] Optics Alignments on EA01

[rperley]

Antenna ea01 has always been 'different'.  The subreflector alignment 
process that Ken and I have been working on for some time has always 
wanted the subreflector for this antenna to be shifted to the left (from 
the point of view of looking outwards, from behind) by 10 mm or so.  But 
Mike Romero reports that when his crew went to that antenna to do this, 
they found that the subreflector had already been moved all the way to 
the left, and there was no practical way move it further.  This must 
have been done long before the alignment project started (~3 years ago), 
and quite possibly was done back in the 1970s!

So what to do?  It turns out that a small rotation of the subreflector 
(about a horizontal axis with the antenna pointing at the zenith) is 
nearly equivalent to a horizontal shift.  Ken and I calculated that a 
small rotation, of about 0.5 degrees, would be equivalent to the 
horizontal offset we wanted.  This was done March 12.

Beam cuts, done on March 29, showed that the beams were nearly 
completely destroyed -- at high frequencies, there was no beam at all!  
So we naturally thought that the rotation was done with the wrong sign, 
and asked that the subreflector be rotated 0.5 degrees the other way.  
This was done April 08.

The following beam cuts (taken 23 April) showed much improvement, but 
not what we had expected (or at least hoped) to see.

At this point, on April 16, Vivek had a brilliant idea --  .  If the 
shim used to 'tilt' the subreflector was put in at the top of the 
barrel, rather than at the point where the subreflector is attached to 
the barrel, the effect would be to offset the subreflector, in addition 
to the rotation.  The sense of offset would be to move the subreflector 
horizontally the wrong way -- towards the middle, and would be 
sufficient to explain why the beam was destroyed in the first tests.  
Mike Romero shortly confirmed that the shim was indeed put in at the top 
of the barrel.

However, this confusion led to a realization that the fundamental 
problem with ea01 was likely the 'tilt' of the subreflector, which had 
been compensated for 'way back when' by shifting the subreflector.  
Examination of the beam profiles from the April 23 observation suggested 
that a horizontal shift of the subreflector to the right (back towards 
the center) combined with the existing rotation (done the 'wrong way') 
might do the trick.  This was done May 05.

The subsequent beam cuts show great improvement at all bands.  Ku and K 
bands look just about perfect.  Ka and Q bands, however, show there is 
some significant misalignment still.  The beam cuts suggest a further 
horizontal motion of 5 to 10 mm would suffice, although it's not 
entirely clear this is all that's needed.  Some further thought is 
needed.

I've attached some beam profiles to illustrate the process.  There are 
two plots in each file, the first is the elevation cut of ea01, the 
second the horizontal cut of ea01.  The latter is what is relevant here.

The files are named by band and date:

March:  These are the cuts after the initial +0.5degree rotation.
April:  These are the cuts after the following -0.5 degree rotation.
May:    These are the cuts after the 12 mm horizontal shift of the 
subreflector, combined with the -0.5 degree rotation.

All suggestions welcome.

Rick
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