[evlatests] Some nice C-band imaging results

[Rick Perley]

I spent some time playing around with the C-band Cygnus A observations 
made by Michael last week.  Specifically, I made an image of the 
calibrator field, with excellent results.   Michael used four subbands, 
all four polarizations, with 500 kHz channel resolution -- a total of 
512 MHz bandwidth in each polarization. 

    The calibrator was 2007+404, about 3.5 Jy, and about 1.60 degrees 
away from the very strong source Cygnus A.  It was observed 9 times 
during this short run, each observation about 90 seconds long.  Other 
than some odd behavior at the beginning of the run, the data were of 
outstanding quality.  There are no 'zeros' that I could find in this 
dataset.  I removed the delay and delay rocking, using FRING, determined 
the bandpass for each short observation, then calibrated the gains in 
the usual way.  A BLCAL was run to remove any steady component of the 
cross-polarization leaking into the parallel-hand gains.  A CLIP for 
discrepant points found 0.05% of the data to lie beyond my limits -- I'm 
sure that essentially of these are at the band edges. 

    Histograms of single channels showed beautiful Gaussian noise. 

    The image, using the central 150 channels from all four subbands, in 
Stokes ' I, gave a 50,000:1 dynamic range -- about a factor of two short 
of theoretical (based on the single channel gaussian widths).  The image 
rms is dominated by a single sinusoid component -- aligned with Cygnus 
A, and probably due to that source.  A 2nd field placed on Cyg A found 
the hospots, but not a plausible image of the lobes -- not surprising 
given that the source is in the ~tenth sidelobe, where I would expect 
the antenna phases to be all quite different. 

    Comparison of the 9 BPASS solutions shows excellent bandpass 
stability (better than 0.1% in amplitude) for most (but not all) 
antennas and times.  A vector-averaged bandpass plot shows variations of 
about 1 part in 4000 across the inner 80% of each of the four subbands.  
There are curious edge effects that I cannot easily explain.