[evlatests] Deep Imaging!

[George Moellenbrock]

Rick, et al.,

>    The BLCAL solutions were generated for the entire 6-hour duration --
> one solution per baseline.  It would seem that this is all that is
> needed to reach thermal noise at this level.

A bit dishonestly, though it is gratifying that a constant correction
was effective.  Alas, this is not a general solution.

So, what is the origin?

D-terms.  For finite D-terms that differ among the antennas, dynamic range 
will be limited to something like 1/(<D*D>*sqrt(N)) where <D*D> is the 
mean square of the typical residual D-term (that part not partially 
absorbed by the gain calibration), and N is the number of baselines.  For 
D-terms of a few percent and 12 antennas, a few 10000s:1 may not be 
surprising at all.  (Rick just stopped by and noted that the C-band 
polarizers are often worse than a few percent.  At X-band, where imaging 
works better, the Ds are known to be better than at C-band.)

Rick has also pointed out that the sources he images are weakly polarized. 
Thus the D-term contribution to the closure errors will be ~constant 
(especially if the Ds are large), consistent with constant BLCAL 
solutions.

We have 3C48/3C84 data in hand to test the hypothesis that a polarized 
source (3C48) requires variable BLCAL.  I went on vacation and haven't
reduced it yet.  (Steve has such evidence from 3C286, in fact.)

The upshot is that the newly available sensitivity and the fact of 
bandpass-calibrated continuum (and no variable ripples, etc.) now expose a 
non-closing feature that has always been there.  Alas, there's no free 
lunch.

-George










>
>    Two baselines for 0555+398 were seen to have high residual
> amplitudes or phases (probably oscillatory).  I'll be looking into these
> now.
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