MIRI Point Spread Functions
The 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 assuming 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 under-sampled. PSF sampling may be improved by performing dithers between exposures.
Simulated MIRI PSFs
The PSFs were made with a wavelength sampling of 10 wavelengths per filter and a pixel over-sampling of 5. Both the detector and over-sampled PSFs are centered on the pixel (rather than on the pixel corner).
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 v 0.7.0 using the PSF parameters described in the previous section. Note: due to under-sampling below ~7 µm, the over-sampled PSFs were used to calculate the FWHM.
Table 1. FWHM values for each PSF, from Figure 3
|Filter||Wavelength (μm)||PSF FWHM (arcsec)||PSF FWHM (pixel)|
Figure 4 shows the radial profiles for each simulated, over-sampled PSF. The radial profiles have been normalized to the value of the peak pixels.
Figure 5 shows the encircled energy curves for each simulated, over-sampled 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.
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)|