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

[Mark Holdaway]

> Thanks for the comments Mark.
> 
> Regarding the technical points raised regarding the creation of the results:
> 
> 1) Negative fidelities- these do indeed arise from negative values in the
> model brightness distribution (or in this case the brightness distribution
> of the model dowsampled to the appropriate pixel scale and convolved with
> the specified CLEAN beam). Doesthe consensus say redo the results using the
> absolute value of the smoothed model for the division, or use the absolute
> value of the model from the word go i.e. redo the imaging and CLEANing as
> well, prior to differencing etc.

I would actually say "redo the simulations".  The model brightness
distribution should have no negatives, unless you are explicitly
modeling spectral line absorption against a background source or
something like the Sunyaev-Zel'dovich effect.  The regridding operation
will make errors, and some of those errors will show up as negative pixel
values.  Clip them off to zero.  We don't really care about the errors
in regridding (after clipping), because even though it is not consistent
with the initial model, it is still a valid model to begin the simulation
process with.

> 
> 2) I believe the quoted mean fidelities are correct, with the log scale of
> the histogram perhaps being deceptive.
> Considering the  CCMAR_4-23B7 case (spiral, 4 hour track, imaging Mars, -23
> d) with a quoted mean of 11,  the attached text file shows the full output
> from running IMEAN on the fidelity image, with the same task used to
> generate the histogram. The histogram is indeed approximately flat out to a
> fidelity of 50, with the important exception of the first bar of fidelities
> between plus and minus 0.19 which contains 44,000 of the 66,000 pixels,
> implying a ball park mean of approximately (25*22/66)=8.3, with the actual
> figure plausibly raised to 11 by the slight peak in the histogram between 37
> and 45.

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