[AstroPy] A Mystery: DAOStarFinder Not Finding an Obvious Star in Just One Image

[Robert Carr]

I am hoping for help with a mystery!

I am mystified by a problem with DAOStarFinder.  DAOStarFinder has seemed
to be working flawlessly.  But a couple days ago I was surprised to
discover that it refused to find a target in one of my images.  The target
is bright: 15.3 magnitude in g with a peak flux of about 350.  The star's
image is quite round.  DAOStarFinder has no problem finding other stars of
comparable luminosity, as well as a great number of brighter and dimmer
stars, all the way down to about 38.  DAOStarFinder finds the star in
other better and worse frames.  But it refuses to find my target in just
this one image!  Why?!  This is completely maddening.  And worrying: what
else might it be missing?

The relevant code is very simple.  I have set few options for DAOStarFinder:
>>from photutils import DAOStarFinder
>>fwhm = hdulist[0].header['FWHM']*2.5
>>source_snr = 1.5
>>bkg_sigma = mad_std(data)
>>daofind = DAOStarFinder(fwhm=fwhm, threshold=source_snr*bkg_sigma)
>>sources = daofind(data)

The numbers above work out to be:
- fwhm = 4.76 * 2.5 = 11.9
- bkg_sigma = 8.9
- threshold = 13.3

I have tried playing with the above settings with no effect.  I have
rounded up the usual suspects but the mystery remains.  It is like the star
has a cloak of invisibility when it comes to DAOStarFinder for just this
image!

Clues and leads on solving my mystery are very much appreciated.  Thank you!

Bob Carr
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