[evlatests] S-band PDif compression effects

[Rick Perley]

I have repeated at S-band the tests reported on last week showing that 
Cygnus A and Cassiopeia A caused a small effective compression of the 
switched power at L-band.

     Summary of L-band effect:  Cas and Cyg, which each approximately 
quadruple the system noise, cause an effective 4% compression in the 
'PDif' monitor.  Hence, applying this to the data will overestimate the 
flux densities of Cas and Cyg by this fraction.  The effect was barely 
visible for Taurus A, which doubles the system temperature.

     At S-band we expect a greater effect, since the PDif compression 
caused by Cygnus and Cas is manifestly much greater.  (Meaning, plotting 
PDif vs. time for all sources clearly shows the drop, for nearly all 
antennas, whenever we observe either source).

     And this is exactly what happens.

     I generated two databases from the S-band 'flux density' data, 
taken in C configuration.  One had only the RQ corrections applied, the 
other the 'PDif'.  These two databases were identically calibrated 
(including adding the shadow flags ... :), using 3C286 and 3C295 to 
establish the flux scale.

     Perusal of the visibilities, and imaging them, show that the 
deduced flux densities of the four strongest objects are higher with the 
PDif correction than the RQ correction by the following factors:

     Cassiopeia A         11.5%
     Cygnus A               10.7%
     Taurus A                  7.2%
     Virgo A                    3.1%

     This 'expansion' is not visible on the next strongest source, 
3C123, to the 0.5% level.

     Cass multiplies Tsys by a factor of about 4.  Cygnus by about 3.5, 
Taurus by about 3.   (These will be overestimates, as we must use the 
PDif values to determine these ratios).

     The RQ-only flux density value for Cygnus A is much closer to the 
standard Baars et al. value than the PDif corrected value.

     S-band is by far the most strongly affected by this phenomenon. 
(PDif compression is completely absent at P-band, and is only 
appreciable at the ~5% level at L-band for the strongest sources). 
Investigations into the origin should be focused on the S-band system.