NIRISS Point Spread Functions
The simulated JWST NIRISS 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 of PSFs for NIRISS imaging in each filter assuming the "requirements" OPD map. Below is some introductory information used to calculate these 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 NIRISS detector achieves Nyquist sampling or better (FWHM > 2 pixels) above ~4 µm. Below these wavelengths, the PSF is under-sampled. PSF sampling may be improved by performing subpixel dithers between exposures.
Simulated NIRISS PSFs
The PSFs were made with a wavelength sampling of 20 wavelengths per filter and a pixel over-sampling of 9 in order to achieve FWHM and encircled energy values to be accurate to 1%. Both the detector and over-sampled PSFs are centered on the pixel (rather than on the pixel corner).
The PSFs with filters in the longer wavelength range, specifically filters F227W - F480M, use the CLEARP aperture which has an occultation in the form of a pupil alignment reference (PAR). The shape of this PAR can be seen in Figures 1 through 3 on the NIRISS Pupil and Filter Wheels page. The reduced transmission from the CLEARP aperture has been compensated for by normalizing the PSFs to the OPD's exit pupil (rather than the entrance pupil). However, the presence of the PAR does cause morphological differences in the PSFs created with these longer wavelength filters.
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 pixels, 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 severe under-sampling below ~4 µm, the over-sampled PSFs were used to calculate the FWHM.
Table 1. FWHM values (in arcsec and pixels) 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) for each simulated PSF, extracted from the center of the pixel, are shown in Figure 5.
The curves are normalized to the exit pupil in order to account for the CLEARP aperture blocking some of the light on longer wavelength filters.
Table 2 lists the average encircled energy radii, calculated from a grid of WebbPSF (v. 0.7.0) models. The grid has 121 positions within the pixel, with 0.1 pixel spacing from the edge to the center of the pixel, and 10 realizations per position per filter.
Table 2. Radius (in arcsec and pixels) at which the encircled energy (EE) is 50% and 80%