Measured values of in-flight JWST NIRISS imaging point spread function (PSF) metrics are presented. Simulated PSFs in each filter 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. WebbPSF can also query the set of NIRCam observations of the primary mirror OPD as a function of time to use in calculation of the NIRISS predicted PSF. This is done using the load_wss_opd_by_date function in the WebbPSF NIRISS object. This is recommended to compare the predicted PSF with specific observations.

During commissioning, observed properties of the PSFs for NIRISS Imaging in each filter were measured, including the full width half maximum (FWHM) and radii for the encircled energy fraction at 50% and 80%. These measurements showed very good agreement with pre-flight WebbPSF predictions.

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) for the F380M filter and longer wavelength filters. Below these wavelengths, the PSF is undersampled. PSF sampling is improved by performing dithers between exposures, which is required for imaging and Wide Field Slitless Spectroscopy (WFSS) exposures.

Simulated NIRISS PSFs

The PSFs in Figure 1 and Figure 2 were made with a wavelength sampling of 20 wavelengths per filter and a pixel oversampling of 9 in order to achieve FWHM and encircled energy values to be accurate to 1%. Both the detector and oversampled PSFs are centered on the pixel (rather than on the pixel corner).

The PSFs with filters in the longer wavelength range, specifically filters F227W to 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 in the NIRISS Pupil and Filter Wheels article. 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 1 shows the detector-sampled PSF with each NIRISS filter on a log scale. Figure 2 zooms in to the cores of the PSFs. 

PSF FWHM

Figure 3 shows the FWHM measured from in-flight data during commissioning as a function of the filter’s pivot wavelength, compared with pre-launch WebbPSF models resampled onto the native NIRISS pixel array. The black dashed line shows the theoretical JWST diffraction limit and the horizontal dotted lines indicate the size of the NIRISS pixel (0.066"/pixel) in integer units. The measured FWHM values from in-flight data are also tabulated in Table 1 below. 

Table 1. FWHM values (in arcsec and pixels) for each in-flight measured PSF

Encircled energy

Figure 4 shows the measured encircled energy curves from commissioning data. An ASCII table with the encircled energy data is available here. Numerical values for 50% and 80% encircled energy (the fraction of light contained in a circular aperture) for each filter are reported in Table 2.

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