[evlatests] VLA image -- and an interesting clue ...

[Vivek Dhawan]

I was wondering, based on evidence below, whether the scaling
of phase with frequency offset within a subband was correct -
perhaps I'm seeing evidence of the same thing Rick suspects.

Test data: Cyg A at C band, July 5, calibrated & imaged by Rick.

The intent was to check whether the data and model had the same
sign of phase offset and phase slope from the offset double source.
In short, they do on every baseline, so the phase flip is truly
global and not sneaking in on some baselines that then mess up
the image.

In the same data, it is clear that:

1. the model follows the data pretty well, but often misses by
tens of degrees near the many nulls from the double source.

2. The 4 IFs are spread over ~500 MHz, each IF (subband) selected
for imaging was 1.1MHz (9 channels wide) from the 128MHz.
The data show the change of structure with frequency, the model
also changes but still misses the date as above. The signal is
very strong so this is a many-sigma discrepancy.

3. The closure phases of model/data are quite discrepant, in some
cases by upto 180 deg. I speculated that for some reason, say mislead
by the header information, the scaling of model with frequency is not
done right. I was still working on this, but Rick & Barry's message may
be pointing in the same direction so I thought it worth putting out
before I have a conclusion.

Vivek.


On Sun, July 12, 2009 12:48 pm, Barry G. Clark wrote:
| This is all very strange.  Can this be duplicated in CASA?
| I suppose it is possible, though extremely unlikely, that
| something could be going wrong with the application of delay
| and bandpass corrections as channels are added together.  We
| are running short of straws to grasp at here.
|

|> From this, I'm guessing that the act of coherently summing over
|> the bandpass is actually destroying the signal (but not the noise).
|> Something is backwards?