[daip] JMFIT/IMEAN booby trap

[R. Craig Walker]

I just stumbled on a booby trap for the unwary (like me).  Nothing is
formally being done wrong, but in practice, people could be led badly
astray.  Consider the following sequence, which is very common for me:

IMEAN, full image, DOCAT=1
   Puts correct ACTNOISE in header.
IMEAN, tight window to measure flux density, DOCAT=1 because I'm not paying
   attention to it - never did until very recently.
   Replaces ACTNOISE with a much higher value
JMFIT.  Uses bad ACTNOISE to calculate errors.

I just did this.  Fortunately I noticed the statement about ACTNOISE being
used and knew I had likely measured a flux density with a tight box.  I
reran
IMEAN with a wide open box, replacing ACTNOISE.  Then JMFIT came up with
much smaller errors.  My worry is that people will blunder into the bad
case very easily.  I suggest protecting against it somehow.  Is there any
reason not to default to recalculating the noise with a histogram fit? 
Unfortunately RADIUS doesn't really have room for another option.

Cheers,

Craig




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    R. Craig Walker            Array Operations Center
    cwalker at nrao.edu           National Radio Astronomy Observatory
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