MIRI Point Spread Functions
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.
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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.
On-sky MIRI imager PSFs
This section presents the MIRI imager PSFs as measured during the JWST commissioning. These measurements were obtained as part of the MIRI imager PSF characterization commissioning activity, PID 1028 (P. I. P. Guillard). For most MIRI filters, the data were obtained using the Cycling dither pattern (SMALL, 4 points). For the F560W filter, that is slightly under-sampled, data were obtained using a custom engineering micro-scanning dither to generate a super-sampled PSF.
Figure 1 shows the measured MIRI PSFs (see also Rigby J., et al. 2022).
MIRI PSF stamps from flat-fielded (stage 2 "cal") images. These are log scale, 15 arcsec cropped images of the different sources used to compute PSF properties. The sources are 2 MIRI spectrophotometric standard stars, J1743045 and BD+60-1753, as well as the planetary nebula SMP-LMC-58, which has strong emission at long wavelengths. This latter object has not been observed at 21 μm, thus BD+60 was used, which has a much lower signal to noise. (Figure Credit: P. Guillard)
PSF FWHM
Figure 2 shows the FWHM as a function of the filter’s average wavelength for each PSF made with commissioning data. Numerical values for each FWHM, in units of arcsec and pixel, are reported in Table 1. Note: due to undersampling below ~7 µm, an oversampled PSF for the F560W filter was used to calculate the FWHM.
Table 1. FWHM values for each PSF, from Figure 3
| Filter | Wavelength (μm) | PSF FWHM (arcsec) | PSF FWHM (pixel) |
|---|---|---|---|
| F560W | 5.589 | 0.207 | 1.882 |
| F770W | 7.528 | 0.269 | 2.445 |
| F1000W | 9.883 | 0.328 | 2.982 |
| F1130W | 11.298 | 0.375 | 3.409 |
| F1280W | 12.712 | 0.420 | 3.818 |
| F1500W | 14.932 | 0.488 | 4.436 |
| F1800W | 17.875 | 0.591 | 5.373 |
| F2100W | 20.563 | 0.674 | 6.127 |
| F2550W | 25.147 | 0.803 | 7.300 |
Radial and encircled energy profiles
Figure 3 shows the radial (left column) and encircled energy profiles (right column) of the PSF in different MIRI bands. The radial profiles are normalized to the maximum in the center. The integrated encircled energy profiles are normalized at a radius of 5" (~45 pixels), except for the F2100W filter, which does not have sufficient signal to noise. For F2100W, the normalization is done at a radius of 3" (~27 pixels).
PSF metrics
Table 2 summarizes basic measurements of PSF metrics made with in-flight data and WebbPSF models (note the models for the 5.6 and 7.7 profiles do not include a proper model of the MIRI cruciform pattern). The WebbPSF models are computed with the OPDs (optical path differences) of May 24th 2022, the closest date to that of PID 1028, the program that observed the MIRI standard for the PSF analysis. The results show that the objective of “>56% of the total PSF flux (normalized at a radius of 5 arcsec) contained within the first dark Airy ring” is met for all filters. The only exception is position one for the F560W filter (~54.5%). This is due to the presence of a bad pixel close to the PSF center, not flagged at the time of this analysis.
Table 2. Basic PSF metrics for all MIRI bands using in-flight data (left) and WebbPSF models (center)
The comparison between in-flight data and WebbPSF is presented in the right columns.
| In-Flight Data (Encircled Energy normalized to 5 arcsec) | WebbPSF (Encircled Energy normalized to 5 arcsec) | Data / WebbPSF | ||||
|---|---|---|---|---|---|---|
| Filter | FWHM (arcsec) | EE within the first Airy ring | FWHM (arcsec) | EE within the first Airy ring | FWHM (ratio) | EE (ratio) |
| F560W | 0.207 | 55 - 63 % | 0.182 | 60.9% | 1.14 | 0.9-1.03 |
| F770W | 0.269 | 62 % | 0.260 | 65.3% | 1.03 | 0.95 |
| F1000W | 0.328 | 71 % | 0.321 | 70.9% | 1.02 | 1 |
| F1130W | 0.375 | 73% | 0.368 | 72.7 % | 1.02 | 1 |
| F1280W | 0.420 | 65% | 0.412 | 72.8 % | 1.02 | 0.89 |
| F1500W | 0.488 | 77% | 0.483 | 73.5 % | 1.01 | 1.05 |
| F1800W | 0.591 | 74% | 0.580 | 74.4 % | 1.02 | 0.99 |
| F2100W | 0.674 | 72% | 0.665 | 75.1 % | 1.01 | 0.96 |
| F2550W | 0.803 | 68% | 0.812 | 76.1 % | 0.98 | 0.89 |
References
Rigby, J., et al. 2022, eprint arXiv:2207.05632
Characterization of JWST science performance from commissioning
Guillard, P., et al. 2022, in preparation