[evlatests] Dynamic range of images from Widar.

[Vivek Dhawan]

A couple questions on dynamic range, looking for insight.

1. We usually run the closure-preserving calibration (FRING, BPASS,
    CALIB) on shortish timescales, reach a floor in the rms, then do
    non-closing corrections with BLCAL over the whole timerange.
Q: Are there antenna-based errors left over, that show up on longer
    integrations (higher SNR)?
A: No - long CALIB solutions do not make a dent in the corrections
    needed by the final BLCAL.


2. Whence the non-closing errors?

If BL errors originate in polarization leakage, then the correction
for a baseline should correlate with the magnitude of polarization
leakage (D-term) on that baseline.

Plots:
1. BL correction (re & im) vs. pol leakage - only weak correlation.
2. BL correction for 2 sources with no intrinsic polarization
    - strongly correlated.

The pol leakage RL/RR was quite stable between 3 unpolarized sources.
Also LR/LL, etc. all looked very similar and stable.  In summary, BL
solutions transfer between sources and times, but it is not clear that
D-terms are the cause.

I had reported verbally that the BL corrections were mostly real.
Upon closer examination, that is only weakly so - the im(bl) is about
2-3 times smaller than re(bl) on average.

I have the data saved  - any ideas on how else to slice it?

---------
Procedure:

a. Do self-cal type operations Fring, Bpass, Calib.

b. Extract, e.g. via POSSM, after step (a), the magnitude of the
    pol leakage (RL/RR etc.) for each baseline, averaged over subband.

c. Then Blcal and image. The image is good to ~200K:1. There is one
    time-independent BL factor (re+im) on each baseline/subband/pol.

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