Simulated JWST MIRI imaging point spread functions (PSFs) in each filter and predicted values for the full width at half maximum (FWHM), radial profiles, and encircled energy curves can be found using WebbPSF.

Predicted point spread functions (PSFs) for every JWST instrument can be simulated using WebbPSF. Given a wavefront error budget for the mirror alignment, WebbPSF can create multiple realizations of the PSF with a given filter. Each of these realizations assumes various contributions to the wavefront error from several optical components, which cause the PSF shape and orientation to vary slightly.

WebbPSF allows the user to select between a "predicted" optical path difference (OPD) map and a slightly more conservative "requirements" OPD map. The following figures show one realization for MIRI imaging in each filter for the "requirements" OPD map. Below is some introductory information used to calculate the following PSFs.

At wavelengths λ > 2 μm, JWST obtains diffraction-limited imaging with a Strehl ratio = 0.8 and PSF full width at half maximum (FWHM) of ~λ/D radians (JWST's D = 6.5 m mirror). The MIRI detector achieves Nyquist sampling or better (FWHM > 2 pixels) above ~7 µm. Below these wavelengths, the PSF is undersampled. PSF sampling may be improved by performing dithers between exposures.

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Simulated MIRI PSFs

The PSFs were made with a wavelength sampling of 10 wavelengths per filter and a pixel oversampling of 5. Both the detector and oversampled PSFs are centered on the pixel (rather than on the pixel corner).

Figure 1 shows the detector-sampled PSF with each MIRI filter on a log scale. Figure 2 zooms in to the cores of the PSFs.

Figure 1. One realization of simulated MIRI PSFs

Figure 2. One realization of simulated MIRI PSF cores

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PSF FWHM

Figure 3 shows the FWHM as a function of the filter’s average wavelength for each simulated PSF. Numerical values for each FWHM, in units of arcsec and pixel, are reported in Table 1. These values were calculated with WebbPSF v0.7.0 using the PSF parameters described in the previous section. Note: due to undersampling below ~7 µm, the oversampled PSFs were used to calculate the FWHM.

Figure 3. PSF FWHM in each filter, as measured in one realization of simulated PSFs

Table 1. FWHM values for each PSF, from Figure 3

Filter	Wavelength (μm)	PSF FWHM (arcsec)	PSF FWHM (pixel)
F560W	5.589	0.182	1.636
F770W	7.528	0.243	2.187
F1000W	9.883	0.321	2.888
F1130W	11.298	0.368	3.318
F1280W	12.712	0.412	3.713
F1500W	14.932	0.483	4.354
F1800W	17.875	0.580	5.224
F2100W	20.563	0.665	5.989
F2550W	25.147	0.812	7.312

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Radial profiles

Figure 4 shows the radial profiles for each simulated, oversampled PSF. The radial profiles have been normalized to the value of the peak pixels.

Figure 4. PSF radial profiles

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Encircled energy

Figure 5 shows the encircled energy curves for each simulated, oversampled PSF. Numerical values for 50% and 80% encircled energy (the fraction of light contained in a circular aperture) can be found both in Figure 5 and Table 2.

Figure 5. PSF encircled energy curves

Table 2.  Radius (in arcsec and pixels) at which the encircled energy (EE) is 50% and 80%

Filter	radius(EE=50%) (arcsec)	radius(EE=50%) (pixel)	radius(EE=80%) (arcsec)	radius(EE=80%) (pixel)
F560W	0.131	1.177	0.422	3.805
F770W	0.168	1.510	0.519	4.672
F1000W	0.209	1.882	0.636	5.726
F1130W	0.236	2.124	0.712	6.416
F1280W	0.266	2.397	0.801	7.218
F1500W	0.307	2.765	0.932	8.395
F1800W	0.367	3.311	1.110	10.001
F2100W	0.420	3.783	1.276	11.497
F2550W	0.510	4.591	1.545	13.919

Figures and tables on this page were generated using the 0.9.1 development version of WebbPSF.