[evlatests] Another 'PDif' Offset Test -- with surprising results ...

[Rick Perley]

    At my behest, Ken observed Cygnus A at L, S, C, and X bands, 
alternatively turning off and on the noise diodes.  The idea here is to 
compare the 'zero offsets', which have been seen to occur when the noise 
diodes are turned off, to those measured when we were pointing on cold 
sky.  Tests done earlier predict that the zero offsets when on Cyg A 
should be larger than when off by roughly the ratio of the system 
temperatures. 

    The predictions are wrong!  Indeed, we can't even reproduce the 
results at C-band, taken last week, which alternated on and off Cyg A, 
with the diodes switching between on and off. 

    In short:

    At L-band (where Cyg A quadruples the system temperature), no 
antennas show any PDif offsets when the diodes are off.   This agrees 
with the off-Cyg A results, but does not agree with data taken a few 
weeks ago, which showed a reduction in PDif by typically 10% when on Cyg 
A. 

    At S-band, there are a few antenna with small zero offsets, but 
these are generally *less* than those observed two days ago on cold sky. 

    At C-band, the zero offsets are present, but none got larger than 
when off Cygnus A, and most got smaller.  (Antenna 24, in RCP, remains 
the largest offset). 
   
    At X-band, there are no antennas showing any measurable offsets.