[evlatests] Troubles with WIDAR-0

[Rick Perley]

    I made images of both the noise field, and the two calibrators, from 
the WIDAR-0 5-hour database taken last night.  The issues found 
yesterday in the northern source are evident in the maps of the two 
calibrators from last night.  However, the images of the noise field 
look fine. 

    To better clarify things, I extracted six databases for each of the 
three sources (two calibrators, one noise field).  The 6 databases  have 
differing bandwidth, spaced by factors of four:  0.125, 0.5, 2.0, 8.0, 
32.0 and 128 MHz. 

    The histograms of nearly all databases look normal, in both real and 
imaginary parts, with the expected relationship between noise and BW.  
However, there are excess counts in the tails of the histograms for the 
calibrators (to be explained in a moment).

     The images tell a very different story.  Images of the noise field 
were correct, and declined as expected until the widest bandwidths, 
where edge effects can be expected.
    Images of the calibrator are uniformly terrible, at all bandwidths, 
with the background dominated by 'lumps and bumps' which are certainly 
not real.  The rms noise has no relationship with BW, until the widest 
bandwidth, where it is twice the value of any other image! 
    These effects are due to large 'closure' errors, which are quite 
evident in the visibility plots of the strong calibrator. 

    So clearly something is wrong.  But whatever it is, it affects only 
the correlated signal, and was not present in the PTC. 

    Ken reports that Martin has a theory, which can be tested next 
week.  I'll be in Pasadena, so somebody will have to step forwards to 
work with this.