The Mid-Infrared Instrument (MIRI), on-board the James Webb Space Telescope (JWST), was designed to produce a diffraction-limited Point Spread Function (PSF) at the detector image plane in the 5 to 28 micron wavelength range. For the MIRI Medium-Resolution Spectrometer (MRS), a PSF broadening of 60% down to 10% is observed in the 5 to 28 micron range. Additionally, 20% of the light is scattered into the wings as an extended component on the detector. The same PSF systematics manifest in the MIRI Imager and Low-Resolution Spectrometer (LRS) data. We use physical optics propagation to propagate a uniform wavefront from the JWST pupil to the MIRI Imager detector plane. The camera F-number and variation of incidence angle across the detector allow us to reproduce the detailed features of the cruciform, as well as an observed bending in the cruciform arms that changes across the detector. This presents a significant leap for PSF-weighted photometry. The model can be extended to the LRS and potentially to the MRS, although the optical path of the latter is much more complex to model.
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