PSF - or  Point-Spread Function Fitting is a photometry process and performed on stars within a frame and is used to optimize the picture post processing.

Image Post-processing SW like PixIsight is able to work with PSF parameters and will issue warnings in case the frames do not include them:

** Warning: PSF signal estimates are not available in the image metadata and are being calculated from possibly non-raw or uncalibrated data. Image weights can be wrong or inaccurate.

See articles:

• https://photutils.readthedocs.io/en/stable/psf.html
• Microsoft PowerPoint - Lecture6_imaging_IV.ppt [Compatibility Mode] (ufl.edu)

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