[daip] Regarding an issue in using 'spectral index' image with AIPS task IMAGR {External} {External}

[egreisen]

On 2023-05-26 13:03, Subhashis Roy via Daip wrote:
> Hi Eric,
> 
> Thanks for your effort and time to check it out.
> I now agree that using Spectral index option through In3name and
> Imagrprm(17) does significantly improves the outcome for broadband 
> data.
> The model that I mailed could give me a factor of 3 improvement while
> using Spectral index image made from multichannel UV data.
> 
> However, now I believe to have been able to isolate the issue that
> causes no improvement despite Imagrprm(17) and In3name I reported
> earlier.  I believe you had switched off Imagrprm(8) [filter
> components] during the imaging that you tried.
> 
> After forcing the above option near the end, I could see the appearance
> of significant diffraction rings around the sources with high magnitude
> of spectral index, which did not go away with Clean later. When I
> switched it off, the problem mostly goes away.
> Are filtered CCs are not getting scaled by the spectral index for
> different frequency channels in this case ?
> 
> I have been setting IMAGRPRM(8) to ~3-5*rms, and use it manually couple 
> of times
> when deep clean has been done on extended emission. It does make the 
> final
> emission significantly smoother.

I think you have found it.  In a normal Clean major step, the model 
subtraction
routine uses the residual file as input.  But after a filter, it uses a 
copy of
the original data as input, making a new copy of the residual.  If that 
input
copy object lacks the spectral index triggering parameters, the spectral 
index
will not be honored.  I probably cannot fix this immediately but will 
look at
it next week.

i did say that you are using way more of IMAGR than I usually do.

Thanks

Eric Greisen