[evlatests] Pointing Gone Wild -- Jan 2019 Examples

[rperley]

Here are four more examples of the strange drift in pointing.  All have 
the same characteristics -- the antennas start heading away from the 
intended position.

The plots show four antennas, ea13, 16,  18, and 19,  from Ka-band 
observations on the night of 20 Jan, 2019.  Weather was ideal.  The 
black 'splotches' show the antenna gains at a particular time.  The 
script used referenced pointing (which finds and corrects for pointing 
errors) every seventh observation -- about every 50 minutes.  The 
'sawtooth' appearance of these plots is due to the referenced pointing 
correcting the error.

The elevations of these observations started and ended near 30 degrees, 
and reached 57 degrees at transit (at Hour Angle = 0).  For three of the 
antennas the 'sawtooth' = pointing drift, is near transit.  But the 
single spectacular instance for ea16 was at HA = -3, far, far from 
transit.  In any event, none of these observations was near zenith.

The sizes of the offsets is large,and always in elevation.

For ea13, the largest correction was +18 arcsecond.  Note also that the 
two small steps early in the runs had jumps *upward* -- indicating the 
referenced pointing made things worse.  (But the corrections are small 
-- less than 10 arcseconds).
For ea16, the correction was +21 arcseconds.
For ea18, the three significant jumps correspond to -42, +36, and +16 
arcseconds.  *note the reversal in sign!!*
For ea19, the two obvious steps arise from pointing errors of 16 and 10 
arcseconds.

The script was repeated the following night, at the same LST.  None of 
the antennas above behaved as badly, but instead, different ones did.  
Notable were ea04 (one instance), ea12 (one instance), ea21 (one 
instance).  Three antennas, ea23, ea25, and ea26 had a different kind of 
bad behavior -- essentially random gains within the block of seven 
observations (between referenced pointing scan), with amplitude 
fluctuations corresponding to ~10 arcsecond errors.

Rick
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