[evlatests] Deep VLA integration test

[Rick Perley]

    I reported yesterday that a WIDAR L-band image of our northern point 
source showed no background sources, and had a disappointing dynamic 
range. 

    We decided to utilize a few hours of 'ESP' time last night to get a 
deep integration of the field around 0217+738 using the VLA correlator, 
to have a 'truth standard' to which we can compare any subsequent 
testing.  Observations were made at L, C, and X bands, in ideal weather. 

    Results:

    1) As expected, the 20cm (L-band) image is populated with dozens of 
background sources.  The total flux in background sources is 123 mJy.  
The peak is about 5.5 mJy.  (The calibrator source is 2500 mJy at this 
frequency).  So, we need a DR of only ~1000 to see the brightest 
background objects...   The VLA correlator image (2048 x 2048 to make 
sure I got it all) has a formal DR of 62500 -- about the best we can 
expect with the 50 MHz-wide continuum system without 'heroic efforts'. 

    2) At 6cm, the field is blank, except for a single background source 
(the same one as the peak background object at 20cm), with a peak of 1.3 
mJy.  This is indeed the same source that I have seen in the WIDAR map 
***** except that the position of the background source in the WIDAR map 
is rotated by 180 degrees from its true position shown in the VLA 
correlator map ***. 
The peak flux in the WIDAR map is also about half of the correct 
value.   At a minimum, we have the wrong signs of (u,v) for some, and 
perhaps all, of the baselines.  A (u,v) plot of the initial hour of the 
data from yesterday evening (taken at about the same time as the WIDAR 
test the previous day) shows the (u,v) coverage to be the same, so it 
looks to me that |u| and |v| are both correct -- i.e. the geometry is 
basically right.  But some, or all the signs are wrong. 

    3) At 3.6 cm, the field is completely blank other than the 
calibrator itself.  The formal DR is 75000. 

    I'll next do a more careful comparison of the (u,v,w) coordinates...