[evlatests] Focus motion failures on ea05 and ea27.

[Rick Perley]

The data taken Saturday night have revealed an occasional failure in 
focus motion on two antennas.

In this run, I did X-band referenced pointing every 50 minutes.  The 
subsequent observations were done at Ka-band.

For ea05, on six out of the eight referenced pointing observations, the 
scan immediately following the X-band referenced pointing was flagged 
out.  Investigation showed that the flag was set by an incorrect 
subreflector position.  The amplitude of that scan shows it down by 
about 30% in power -- about right for a 7 mm subreflector error (which 
is the difference between the X band and Ka band position).  The flag 
table also reveals that the subreflector position error was cleared 
about 4 seconds into the next following observation -- about right for 
the required motion.

     The conclusion seems clear:  the command to move the subreflector, 
following the X-band observation, was either not sent, or not received 
by ea05, for six out of eight situations.

     There was only one other occasion of this -- ea27, which failed the 
same way, but on only one occurrence.

     Although not common, this problem is not rare -- in my earlier 
'beam' holography done last month, this characteristic was seen in most 
of the high frequency runs, on at least one antenna.  I can get better 
statistics (names, numbers ...) if it is useful to aid in the diagnosis 
of this failure mode.

     Rick