[evlatests] FRM focus

[Ken Sowinski]

OPtimal focus can be dtermined by measuring the antenna response at
woth the focus set to the nominal position and offset a fraction of
a wavelength up and down from the nominal position.  In an effort
to learn why we consistently see inconsistent results from a few
antennas a check of focus motion was made.

The difference between commanded and current measured position was
plotted as a function of time while the subreflector was moved from
0.0 to 5.0 cm and back to 0.0 cm positions.  This was done with the
antennas stowed; gravity was not seen to affect the speed of focus
motion.  The nominal speed of focus motion is 0.5 cm/sec.

Antennas with new ACUs take nine or ten seconds to complete five cm
of motion, which is consistent with the nominal speed.  All antennas
behaved the same except for ea21 which took about 20 seconds and did
not move with uniform speed during that time.

Antennas with old ACUs took 12 or 13 seconds, so move at about
0.4 cm/sec.  Again all behaved the same way except for ea03 which
took about 16 seconds to move 5 cm, about 0.3 cm/sec.  ea26 was
special in that when commanded to reutrn fro five to zero cm moved
by a few millimeters and then stopped.

ea03 and ea21 ought to be adjusted.  I am sure that ea26 is already
on the list of badly behaving antennas.

I have plots if anyone is interested:

    /users/ksowinsk/alioth/focus/new.ps
    /users/ksowinsk/alioth/focus/old.ps