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

[Eric Jensen]

Hi Bob, 

You might take a look at the individual pixel values for that star in that particular image - I wonder if it might have a cosmic ray hit or some other anomaly in the pixel values that might cause it to be rejected, for example by exceeding the “peakmax” setting, or by throwing off the roundness or sharpness values. 

Just a thought - 

Eric


> On Feb 26, 2022, at 2:56 PM, Robert Carr <rspencercarr at gmail.com> wrote:
> 
> 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