[evlatests] C-Band Imaging comparisons: VLA vs. WIDAR

[Rick Perley]

    I've now been able process the C-band parallel data sets taken last 
July 4/5. 

    1) As noted before, all WIDAR images are rotated by 180 degrees 
about the phase center. 

    2) The calibrator was 2007+404, an excellent unresolved source.  The 
VLA data, after closure corrections (recall that both VLA and EVLA 
antennas are included here) provided a decent map with 28000:1 DR.  
(There's a weak background source, 2 mJy, located 30 arcsec to the NW). 
       The WIDAR data are expected to provide a slightly better Dynamic 
Range:  10 antennas with 512 MHz total BW, vs. 15 antennas with 200 MHz 
total BW, so the sensitivity should be slightly better.  To obtain this, 
I had to apply closure corrections to the WIDAR data as well -- the 
corrections are distressingly large:  typically 0.2% for subbands 1 and 
2, and up to 2% for subbands 3 and 4.  (These latter two sub-bands were 
placed outside the old polarizers' range, and four of the antennas in 
the WIDAR subarray have old polarizers.  The large corrections needed 
for subbands 3 and 4 should not be viewed with especially concern, as 
they are likely due to the polarizers.).  The 'final' DR was 40,000:1.  
The same weak secondary is easily seen -- reflected about the origin. 

    3) Cygnus A:  The VLA image, after standard self-calibration, 
provided a nice image with perhaps 1000:1 DR -- about what is expected 
for this very powerful and heavily resolved object with the old 
correlator. 
The WIDAR image provides about the same formal DR, but is notably poorer 
in its true fidelity.  I attribute this primarily to the poor (u,v) 
coverage -- we have a lack of intermediate spacings which are needed to 
better link the long and short spacings.  

    I'll now move to the very messy 20cm data.