[evlatests] Focus change with elevation

[Walter Brisken]

I tested the focus change with elevation in a test , day 60546, 
LST 2130-0030, project code TBRISK.  I spent alternate 12 minute periods 
observing calibrators 0038+416 and 0238+166 at K-band in mode 2AC, 5 
second int times.  On each source I alternated between setRefocus(1) (for 
sources without an F in the name) and setRefocus(0) (sources with F 
appended).  Due to an unfortunate editing error, the EVLA antennas were 
never pointed toward 0038+416 so only the low elevation measuments (20-50
degrees) are usable.

All 4 EVLA antennas were put into the subarray.  18 produced no 
visibilities (unless they were autoflagged upon fitld -- haven't checked 
for that).  13 showed no fringes, as if it weren't pointed.  Ants 14 and 
16 worked well.

Calib solutions on 14 and 16 showed minute-to-minute changes in both 
amplitude and phase as expected to be caused by the subreflector's motion.

There are two amplitude plots in 
http://www.aoc.nrao.edu/evla-internal/techdocs/tests/testresults/20060402.wb 
showing the amplitudes modulated with the focus.  The first (amps123.ps) 
shows three time groups at elevations 22, 28, and 33 degrees; the later 
(amps456.ps) at elevations 38, 42, 47 degrees.  At 33 degrees, ant 14 
showed a 7% gain increase with refocus set to ON.  At 28 degrees, 
ant 16 shows a 10% improvement.  At 22 degrees the gains are too variable 
to make a quantitative measurement, but the improvement is ~ 12%.  Above 
35 degrees the improvement shrinks to the couple percent level.

Thus the corrections in the online system have the correct sign.

-Walter