[evlatests] Baseline labels are OK

[Rick Perley]

     Some more trials on the data at hand show:

    1) I compared the deep 0217 C-band image taken with the full array 
(via the old correlator) with that taken with the 10 WIDAR antennas.  
Both show a single background source.  I confirm that the WIDAR position 
of this is *exactly* opposite (to within one cell on the map, and it is 
312 arcseconds from the calibrator). 

    2) I attempted imaging this field with various subsets of the WIDAR 
dataset:  The 5 'flipper' antennas, the 5 'unflipper' antennas, the 
baselines between these two groups, antenna 1 vs. the others, in an 
attempt to see if any of these subsets makes a substantially different 
image.  All images give the same result.  The amplitude of the 
background source in the WIDAR dataset is about half of that for the VLA 
-- but this is nearly certainly due to bandwidth smearing, as I used a 
110 MHz-wide single channel formed from the spectral line data.  (I'll 
confirm this with the fundamental data later). 

    3) I returned to the short observation of 3C273 taken at X-band.  I 
have an ancient plot of the visibility function, from which I must 
conclude that my earlier conclusion about the visibilities being 
assigned to the wrong baselines cannot be supported -- the baseline 
coverage and observation duration are not sufficient to deduce with 
certainty the correct visibility envelope.  These data do not have a 
point-source calibrator, so there is no easy way to calibrate the data.  
I attempted self-calibration, but it will not converge on a suitable 
timescale.  (Interpretation:  It will be faster to take some more data, 
with a suitable calibrator.) 

    Thus, I conclude:

    1) There is good evidence we have a phase convention backwards. 
    2) It seems less likely that we have scrambled the visibilities 
amongst the baselines. 
   
    I need a cleaner and more extensive dataset to confirm the above.  I 
suggest at least a couple hours on 3C273, with regular observation of a 
nearby calibrator, at X-band. 

   

Michael Rupen wrote:
> Ken and I observed a strong source with WIDAR this morning at C band,
> using our standard WIDAR-0 setup, but strongly attenuating the signal
> from each WIDAR antenna in turn, for one minute each (i.e., 1 minute with
> ant 1 dead; 1 min with ant 2 dead; etc.).
>
> I checked for this attenuation by (1) lagfan/lagcomp on the lag frames,
> and (2) VPLOTting the data (amp vs. time for each baseline) as passed
> through to AIPS, averaging over the central two channels of IF 1 = subband 1.
> Both the lag frames (blf) and the data passed through to AIPS accurately
> reflect the changing attenuation on all baselines.  Thus the labelling of
> the antennas/baselines is correct.
>
> Rick previously found that the uvw's are also correct; and his C band images
> show that we can correctly image a double at C band, at least to within a
> 180-degree flip about the origin.
>
> So why are the L band maps from WIDAR data so clearly wrong?
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>