[alma-config] Re. Mark's comments on the Fidelity images

[Mark Holdaway]

> >
> > I am still lacking some understanding here.  Why do all of the images have
> > 66,000 on-source pixels?  It seems to me that you must be counting some
> > off-source pixels or something to get so many pixels with very low
> > fidelity.
> >
> > -Mark
> >
> 
> For generality all of the cases have been treated the same way when
> CLEANing, i.e. no CLEAN boxes, no default images. Thus when I am calculating
> the statistics for the images shown, they are derived from the whole image
> as posted, i.e. a 257x257 area (I intended 256x256, but it doesn't matter).
> So yes, there will be a lot of off-source pixels counted. However, as I have
> mentioned before, I can do CLEAN boxes and any other bespoke CLEAN parameter
> for that matter on a per model basis, if necessary.
> 
> Cheers,
>     Steven

I've probably missed some decision the alma-config group made while I
wasn't paying attention, but:

This is why there are so many low fidelity pixels in your results, because
you are basically dividing zero by some error number.  My understanding of
the fidelity is that it measures the on-source SNR.  When I calculate the
fidelity, I screen out off source pixels based on a lower limit on the
pixel values in the convolved model ... ie, when you convolve the model,
you will have Gaussian wings going off source, and the point of the cutoff
was to ignore these regions way off source.

I would recommend another procedure for studying the off-source errors,
rather than lump the on and off-source errors into a single measure;
they tend to be about an order of magnitude different.


	-Mark