[evlatests] WIDAR baselines, and time offsets

[Rick Perley]

    As noted by Eric, images can be rotated by 180 degrees either by 
inverting the phase convention, or by inverting the baselines.  To check 
if the latter is the case, I printed the (u,v,w) coordinates for 
carefully selected pairs of antennas from the data taken the night of 
July 4. 

    To review:

    We ran the WIDAR and VLA correlators in parallel, on the same 
source, at the same bands, with the same script.   The WIDAR correlator 
had the 10 'WIDAR' antennas, the VLA correlator got the rest.  So 
although the baselines are not identical, they are similar. 

    I picked a specific time, and printed the corresponding visibilities 
from baseline pairs oriented the same way and with the same relative 
numbering (smaller x larger).  So, for example, at 06:23:40 I viewed 
baseline 1 x 24 ( W16 - W14) from the WIDAR dataset, and baseline 4 x 15 
(W10 - W6) from the VLA dataset.  I did this for similarly selected 
pairs on all three arms.

    Result:

    All coordinates have the same signs between the two arrays.   So, 
unless I am seriously misunderstanding something, it appears that we are 
inverting the signs of the visibilities (i.e., taking the complex 
conjugate). 

    ****************************

    While making the comparisons noted above, I noticed that the two 
databases are not precisely identical in their time axes.  Michael R. 
noted some time delay between the arrays while the data were taken.  I 
can confirm this, based on the index files of the two (supposedly) 
parallel sets:  The VLA dataset is 32 seconds behind the WIDAR dataset.  
In other words, all scans in the VLA dataset start and stop 32 seconds 
later than those in the WIDAR dataset.