Measuring the average FWHM value of a FITS

Siril can measure the average FWHM value of a FITS from the average PSF parameters of its stars. This is very useful.
However I compared this FWHM value to those obtained with other software (Moffat and Prism) and found quite different results.
I obtained with Siril much lower values.

  • with Siril : FWHMx = 2.09” FWHMy = 1.92"
  • with Moffat4 : FWHMx = 2.74”
  • with Prism : FWHMx = 2.81”
    Is Siril taking into account the correct focal length ?
    I uploaded the FITS (M_81_Light_001.fits) on
    pasteImage1620207572580 pasteImage1620207572580

I just copy a sentence from the PixInsight documentation. But this is the same for siril:

  • Avoid comparing FWHM and other fitted star parameters with the results obtained in other applications, unless you know what you are doing and have enough information on the fitting functions and algorithms used by the other application(s). Each application implements different methods in very different ways and the results are in general not compatible.

Different approach can be used.

Thanks CYRIL !!!

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By the way, in a minor bug was found and value of fwhm can be less than excepted.