[daip] IMAGR channel binning problem

[Eric Greisen]

Eva Schinnerer wrote:
> 
> Hi Eric,
> 
> Thank you very much for looking into this! I am cc'ing Linda Watson on 
> this message as well as she has been doing all the hard work. I just 
> want to check that I understood you correctly: Due to the strong 
> continuum source off the field the continuum subtraction was non-perfect 
> and this is causing the effect we see in the averaged channels. Is that 
> a correct summary? I am only puzzled that we did not encounter this 
> effect when using the averaging option in SPLIT.

Your summary is correct except that the channel averaging has nothing to 
do with it and SPLIT and IMAGR averaging produce very near identical 
results.  I do not know why you THOUGHT that you did not see the effect 
after a SPLIT, I certainly did and I also saw it with no channel averaging.

I have thought of another possibility.  When there is strong continuum 
and a narrow total bandwidth, the interim VLA produced a bad aliasing 
only on EVLA-EVLA baselines.  This was an aliasing of the continuum from 
"negative" frequencies (to the left of the band) that caused the 
continuum to come in strongly at low channel numbers with the aliasing 
dropping off as channel number increased.

Unfortunately, I deleted your data set already since it was large - 
otherwise I would look for this.  Use POSSM on the uv data before the 
continuum subtraction.  There is a proc FXALIAS which helps with this 
problem but it is not a magic bullet.  See the AIPSLetter for June 30, 
2008 and the NRAO VLA observing pages.

ERic Greisen