[evlatests] Holography Phase Oddities

[Rick Perley]

    A 1.3 hour test in holography mode was done yesterday afternoon, to 
try understand better the characteristics of the phase behavior.  The 
test did the following:

    1) A calibration scan
    2) Six consecutive rasters, from 'on-axis' position to the 2nd null, 
heading 'up', and oversampling by a factor of 9 (so we had 18 points).
    3) A calibration scan.
    4) Five more rasters, the same as in (2) above.
    5) Another calibration scan.
    6) Six rasters, in the E-W direction, going back and forth, from 
half-power to half-power, oversampling by a factor of 9 (so we have 9 
points).
    7) Another calibration scan.
    8) Five more rasters as in (6) above.
    9) A calibration scan to end the test. 

    All observations were of 3C286, at L-band, in continuum.  The 
duration for each holographic pointing is 10 seconds.  The integration 
dump time is 0.418 seconds.  I made every even-numbered antenna a 
reference, so that the 'moving' antennas are the odd-numbered ones. 

    I looked only briefly at the amplitudes -- all seems normal here, so 
I'm quite certain that all timing and positioning are fine for both EVLA 
and VLA antennas. 

    Unhappily the phases are not so good:

    1)  All looks normal for the calibration scans (except that the last 
calibration scan wasn't observed -- see the explanation later).   There 
are no 'global' phase jumps between the EVLA and VLA. 
    2) For every holographic scan, every EVLA antenna (both the 
'reference' and 'moving') had its phase jump, on all IFs, on the second 
ten-second observation.  This is not a 'global' jump between the two 
arrays (EVLA and VLA) -- the phase jump on each EVLA antenna is 
different than on any other.  The jump is the same between IFs A and C, 
and between IFs B and D.  The phase jump on IF1 is different than on IF 
2.  The phase returns to the correct value on the third ten-second 
record.  No other jumps are seen. 

       It is important to note that this second ten-second record is 
actually the first record, as seen by the holography program UVHOL.  
Apparently, there are ten seconds of good data (visible to LISTR, but 
not to UVHOL) preceding the first valid holography point. 

    3) Following the end of the first calibration, the phases on ALL the 
EVLA antennas jumped by 20 degrees, in IFs A and C only.   The antennas 
did not move (the holographic scan started 'on axis').  But, this odd 
jump did not occur again. 

    Finally -- an annoying feature of the new system was found.  The 
file I submitted was in stop times.  I had added 10 or 20 seconds to 
each holography scan, in order to give the system a little extra time to 
get reorganized (and also to see what would happen).  What happened was 
this:
          The executor ignored all my commanded stop and start times.  
On its own accord, it decided not to honor my requested extra time for 
each holographic scan, and essentially made up a new file with no padded 
time added -- it executed each holography scan in the minimum time 
possible (10 seconds per requested point).  However, this override did 
not happen for the calibration scan -- the full duration was given, even 
though the scan was not at the time requested.  Because of the removal 
of the extra times, the file ended 4 minutes and 40 seconds early.  For 
reasons unclear to me, the final calibrator observation never took 
place.  Instead, the system lingered at the final holographic position 
for 80 seconds, then decided to close up shop (so to speak), without the 
final calibration scan being observed.