[evlatests] WIDAR bronze dataset - C band dual pol.

[Vivek Dhawan]

           Re-weighting the data offers no improvement.
           --------------------------------------------

I have not treated data weights with any respect so far. Now, given:

1. I am stuck at 2 (or 5) times the theoretical rms, for 5 MHz BW
   (or 105 MHz), and

2. The image artifacts resemble the dirty beam,

I thought it might be somehow related to data weights being imperfect
over many baselines i.e., distributed over the UV coverage.  This is
examined here.

The short answer is that re-adjusting weights does not improve the
final images. The problem is intrinsic, or has been locked in at an
earlier stage of processing.

-------

Procedure: Start with previous best stage: data flagged + delay fit
+ self-cal 1sec solint A&P (5.5Jy clean-comp model model, plenty SNR)
+ BPASS every 3min using same model + averaged to 21 chan of 5MHz each.

The weights in these data look realistic, poor antennas have low weight.
This is a result of default behaviour of CALIB etc. working on data with
intrinsic spread in SNR. This (I think) is better than using the  weights
coming in from CASA, all 1 - no Tsys or corr-coeff normalization has been
imposed.

There are 2 ways to re-weight:
1. WTMOD - reset all weights to same - make a new dataset.
2. FIXWT - recompute weights from amp rms over a solint (3 minutes),
           make another new dataset.

And 2 ways to image:
1. Image the new datasets directly.
2. Rerun CALIB on the new datasets, using the best previous model,
   then image.

I tried the 4 combinations above, averaging over all channels in the
images but keeping the RR, LL, IF1, IF2 separate. As expected, the
option 1,1 (equal weights and no calib) is worst, increasing rms
by upto 2 on IF2 LL, which had the poorest data quality. None of the
options improved the off-source rms - at best they were harmless.
The various options did stir around the close-in sidelobes but I
could not distill any insight from that.

Onward - other ideas??



|
| The Super-Short Summary:
|
| __ NO dropouts. No symmetric artifacts.
|
| __ LCP raw data has wider range of uncalibrated amplitudes across
|      antennas, but steady in time and calibrates out OK, finally
|      only 10-20% worse rms than RCP. (Ant 3,IF2,LCP notably bad)
|
| __ Using 5 MHz channels, the rms of the central few channels is a
|      factor 2 worse than theoretical=0.2mJy. Edge channels worsen
|      gradually by another factor ~2. This is true of R & L.
|
| __ V image cubes (5 MHz) are a bit better, compared to theoretical
|      they are a factor 1.5 worse in middle, factor 2 at edge chans.
|
| __ Averaging up to 105 MHz, the image rms should improve by a factor
|      4.6. It improves by less than 2. The artifacts look more-or-less
|      like the dirty beam.
|