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

[Stephane Guilloteau]

-----Original Message-----
From: Mark Holdaway <mholdawa at nrao.edu>
To: Steven <heddle at totalise.co.uk>
Cc: alma-config at nrao.edu <alma-config at nrao.edu>
Date: Wednesday, December 20, 2000 6:40 PM
Subject: Re: [alma-config] Re. Mark's comments on the Fidelity images


>
>> >
>> > 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.
    Yes, but there is an arbitrary decision here. What criterium do you use
?
>
>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.
>
    We have two proposals there:
- John Conway suggestions of using 3 different thresholds to define the
"on-source"
  region to compute the mean fidelity (e.g. 10 %, 1%, 0.1 % of the peak
intensity)

- some variations of what I mentionned in my previous E-Mail, i.e. getting
the slope
 of the Error/Intensity scatter plot. That plot in itself contains the whole
information
 on fidelity, and you could just get the previous result by fitting the
slope for all points
 above 10 %, 1% or 0.1 % of the peak intensity...

I personally don't like defining the thesholds in terms of Peak Source
fraction, but rather
in terms of Mean Residual value. In other words, I would prefer to have the
number of pixels
with fidelity higher than some value, rather than the mean fidelity value of
some arbitrary
number of pixels. This was the idea of the Fidelity plot.

    Stephane