[evlatests] Pointing Tests

[Rick Perley]

    Ken suggested today that we test referenced pointing with the following
sequence of observations:

   1) On-Axis tracking observation
    2) Pointing determination
    3) On-Axis tracking, as before
    4) On-Axis tracking, with referenced offsets applied
    5) Pointing determination, with referenced offsets applied (e.g. 
secondary reference determination)

    We did this sequence for two sources, one at low elevation, and one 
high, at two bands, X and K.
    Ken introduced into antenna 14 an offset of 1 arcminute in both az 
and el, for X-band only. 

    We tried this at noon today.  It was a total failure, as the 
executor went 'catatonic'.  The
amplitudes for all four EVLA antennas slowly and smoothly declined to 
zero over a few minutes,
indicating either that the antennas weren't moving, or were slowly 
moving in the wrong direction,
rather than tracking.  The phases of 3 of the 4 antennas rotated at a 
quadratically increasing rate,
starting near zero at the beginning.  AS noted by Ken, these are 
consistent with the initial set of
commands to the antennas never being updated. 

    So, after the executor was restarted, and antenna 13 had its L302s 
swapped, we tried the
experiment again. 

    This time, things went a (little) better.  The antennas tracked, 
amplitudes and phases
were more or less as they ought to be.  But not much else worked to plan:

    a) Antennas 13, 14, 16 and 18 gave a pointing solution only once -- 
for the first source
at X-band.  All subsequent pointing offset determinations failed for 
these antennas (but
worked for the VLA antennas).  Ken reports to me that the solutions 
failed because the
antennas didn't `wiggle' -- they stayed on source when they should have 
been moving
to the half power points in az and el. 

    b) To offset this sad news, I offer some better news:  The 
amplitudes from antenna
14 to others improved by 7% when the reference pointing was applied.  
This is close
to the 10% expected (in *amplitude*) for a 1.4 arcminute offset with a 5 
arcminute
FWHM beam. 

    Ken promises to ponder these things over the weekend.