[evlatests] Delay/phase jumps on ea13 and 26 -- Sticky Subreflector!

[Rick Perley]

     The cause of the 'jumps' seen in phase and delay on ea13 and ea26 
is now known -- they are due to the subreflector moving late -- or not 
at all -- following a change of band.

     The X-band observations follow Ka-band, which thus necessitates a 
small change in subreflector focus position. Review of the on-line flags 
immediately showed that the subreflectors for these two antennas are 
consistently late in completing the required motion.

     The observed phase change (200 degrees) and delay change (40 ps) 
are both consistent with a ~7mm change in subreflector position.

     Examination of the phase plots shows that the problem is *not* that 
the subreflector motion is slow, but rather that the command to move (or 
the actual start of the movement) is late.  In some cases, 'late' is 
defined by more than 2 minutes (!), which is the length of the scan.  
(In other words, there are some scans where the subreflector never moved 
at all.

     *The loss of data is quite large:  For ea13, 40% of the data were 
flagged due to this.  For ea26, the fraction is 76%.*

     There is evidence that, for ea13 at least, this problem has been 
with us a long time.  I have data dating from mid-2014 whose phase 
solutions for ea13 show exactly the same phenomenology.

     (For those who wonder why I'm looking at flagged data -- I wondered 
why ea13 and ea26 had such a large fraction of the data flagged, when 
the amplitudes looked perfectly fine.  Unflagging these data uncovered 
the effect).

     I have not yet been able to examine the data from the other bands 
(Ku, K, and Ka).  This will be done shortly.