[evlatests] Data Drops

[Rick Perley]

    This note is simply to record some characteristics before I forget them.

    The full polarization data from last night reveal some old, and some 
new and not subtle phenomena dealing with amplitude drops. 

    Old:

    A small fraction of the visibilities (less than 1%) are identically 
zero, with zero phase.  All channels are zero at the same time.  The 
drops are not antenna-based, and are independent in each of the four 
correlations.  This has been seen before, and is fairly easy to remove 
(through CLIP).

    New:

    A significant fraction (perhaps 5 -- 10%) of the visibilities have 
amplitudes low by a few tens of percent.  The phase is completely 
unaffected.  The drops are on single records, and are spaced 10 seconds 
apart -- always on seconds ending in 4 or 5 (like, 14, 24, 34, ...).  
These drops are independent on all correlations, and are not antenna 
based -- they cannot be calibrated out.  They were not present in the 
single-polarization data taken three nights ago.  They are difficult to 
handle, due to their varying amplitude  (i.e.  CLIP can't get them all).