[evlatests] Subreflector positioning (focus) errors at X-band

[Rick Perley]

     I have a 6.5 hour block of data, taken on February 04, with X, Ku, 
K, Ka, and Q bands.

     An extraordinary number of system flags were set *at X-band only*, 
for some antennas.  Examination of the data showed that the visibility 
amplitudes were fine.

     Closer examination revealed the following curiosities:

     1) Antennas 3, 13 and 26 were flagged for most of the run with 
'focus errors'.  Examination of the amplitudes showed nothing amiss.  
But examination of the phases showed that indeed, the subreflectors were 
in the wrong position.  I can tell this because, for some of the scans, 
the subreflector moved to the correct position partway through the 
scan.  For all three antennas, the transition from 'off' to 'on' took 5 
seconds, and the phase changed by 120 degrees.  This corresponds to a 
~6mm error  -- the same for all three antennas.  Not enough to  affect 
the amplitudes, but definitely enough to dramatically change the 
phases.  The key point here is that for all three antennas, the move 
from the wrong to right position took place well into the scan (a couple 
minutes).  It rather looks like a command got 'waylaid'.

     2) Antenna 5 is different.  Here the noted reason for flagging was 
'subreflector error'.  But there was no noticeable change in amplitude 
or phase when the flags were set, or not set.

     In the script, the observation prior to the X-band observations was 
always at Q-band.

     I emphasize that *no other band* showed this focus offset behavior.

     An earlier run, taken in mid January, showed exactly the same 
behavior.


     rick