[evlatests] New EVLA Holography Phenomenon

[Rick Perley]

    Two tests were run yesterday, to confirm speculations about the 
origin of the various phase problems previous reported.  The results of 
these tests on this subject will be reported later. 

    The purpose of this note is to record a remarkable 'new' phenomenon, 
which affects *only* affects amplitudes. 

    Both of yesterday's tests consisted of three short holography 
(beam-scanning) blocks, each of which ended with a standard calibrator 
observation.  It was noted for all three blocks, on both tests, for all 
four EVLA antennas, in all IFs, that the visibility amplitude at the 
beginning of the concluding calibrator scan was low by a factor of ~1.6 
for 21 seconds, then jumped to the appropriate full amplitude in a 
timescale of less than 1 second. 
    The phenomenon is completely absent in the VLA antennas. 
    There is absolutely no phase change across the boundary. 

    Ken pointed out that the Tsys correction in the 'fast solar' mode 
that we use for these tests (0.41 seconds averaging time) has no 
smoothing applied, placing suspicion on the Tsys mechanism.  But the 
Tsys values are stable throughout.   So this is not the explanation. 
  
    Upon closer inspection, it is clear that the jump in amplitudes is 
not discontinuous.  Indeed, the first 417 msec integration, in all 96 
measured instances shows an amplitude 1/3 of the way up between the low 
and high state, and the 2nd integration, 833 msec after the transition, 
is at the bottom edge of the noise distribution of the full-strength 
values.  This is (just) compatible with antenna slew motion speeds.  The 
movement was in elevation -- 20 degrees/minute, or 20 
arcminutes/second.  The observed low amplitudes at the beginning of the 
scan are indeed exactly the same as the last  holography amplitude -- 
located at about the half power point, or 15 arcminutes off axis. 

    We thus hypothesize that this effect is due to the EVLA antennas 
being told to move back to the calibrator, after the end of the last 
holographic scan,  21 seconds later than they should have been told. 
    It is interesting to note that this delay does not show up on the 
sequential holography scans. 

    This is no big deal -- it doesn't affect the actual holography data, 
and is easily removed from the calibrator data, which always lasts 
longer than 20 seconds.  But it perhaps indicates some remaining subtle 
timing issues, which would be good to resolve.