[evlatests] optimal rotation settings

[Ken Sowinski]

Earlier this month test observations were made to see how well the
rotation trick was working and to verify that FRM rotation setting
were all correct.  This note summarizes the methods and results.

The correctness of the rotation settings was checked by a series of
scans which measured the response of all antennas at a sequence of
offsets from the nominal setting and looking for the maximum amplitude
response.  this was done at a high elevation, around 64 degrees, and a
low elevation, around 23 degrees to check the correctness of the
rotation adjustment over a range of elevations.  Pointing offsets form
an X band pointing scan were applied in all cases; no second order
referenced pointing was done.  Two sequences were observed at each
band, the first with the rotation trick disabled and the second with
it enabled.  Only the four high frequency bands were checked.

There are three conclusions.
1.  Only one antenna at one band had a large error in rotation
setting.  ea05 at Ku band was in error by about five degrees.

2.  Most rotation setting errors for all antennas and all bands were
under a degree, and only a few exceeded 1.5 degrees.

3.  At Ku band the rotation trick hurts more than it helps.  The
evidence is that for almost all antennas the amplitude response was
lower with the rotation trick applied than with it disabled.  Further
the response at the rotation offset which maximizes amplitude response
was in all cases greater when the rotation trick was disabled than
when enabled.  The interpretation is that the Ku band feed is so far
removed from the elevation axis (47.6 degrees) that the damage we do
in the X direction outweighs any improvement in the Y direction.

All the indicated corrections have now been made, even those for Ku
band.  Another check will be made in the next few days to see that we
have done what we expected and not inflicted any new damage.  After
that we should set the Ku band rotation values to the optimal value
for 45 degrees elevation and disable the rotation trick for that band.