NIRCam Point Spread Functions
Simulated JWST NIRCam point spread functions (PSFs) in each filter have predicted FWHM between 0.03" and 0.16" assuming nominal mirror wavefront errors.
At wavelengths λ > 2 μm, JWST obtains diffraction-limited imaging with a Strehl ratio = 0.8 and point spread function (PSF) FWHM of ~λ/D radians (JWST's D = 6.5 m mirror). The NIRCam detectors achieve Nyquist sampling or better (FWHM > 2 pixels) above 2 µm in the short wavelength (SW) channel (0.6–2.3 µm) and above 4 µm in the long wavelength (LW) channel (2.4–5.0 µm). Below these wavelengths, the PSF is undersampled. PSF sampling may be improved by performing subpixel dithers between exposures.
Simulated NIRCam PSFs
Expected JWST PSFs may be simulated with WebbPSF. Given a wavefront error budget for the mirror alignment, WebbPSF can create multiple realizations of the PSF in a given NIRCam 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 "requirements" OPD map. The figures below show one realization for NIRCam imaging in each filter assuming the (slightly more conservative) "requirements" OPD map.
These are a single realization of NIRCam PSF FITS images for all filters simulated by WebbPSF using the "requirements" OPD map. SW (LW) PSFs are centered in the A1 (A5) detector. Each PSF is assumed to be centered within a pixel. Each FITS file contains both an oversampled PSF (oversample = 4) and a version rebinned down onto the detector pixel scale (0.031" and 0.063" in the short and long wavelength channels, respectively). Source spectrum is a G5V star spectrum from the PHOENIX model library.
NOTE: Diffraction spikes are oriented relative to the telescope (V3 Position Angle), not NIRCam (see Figure 2 in the NIRCam Field of View page).
Figures 1 and 2 show the PSF in each filter on a log scale. Figures 3 and 4 zoom in to the PSF cores.
Figure 5 shows the FWHM as a function of the filter pivot wavelength for each simulated PSF. Numerical values for each FWHM, in units of arcsec and pixel, are reported in Table 1. Note: due to severe undersampling below 2 µm (4 µm) in the SW (LW) channel, the oversampled PSFs were used to calculate the FWHM.
Table 1. FWHM values for each PSF, from Figure 5
|PSF FWHM |
PSF FWHM (pixel)
Figures 6 and 7 show the radial profiles for each simulated PSF. The radial profiles have been normalized to the value of the peak pixels.
Encircled and ensquared energy
Figures 8 and 9 show the encircled energy curves for each simulated PSF. Numerical values for the encircled energy (the fraction of light contained in a circular aperture) and ensquared energy (the fraction of energy falling within a certain number of pixels) for each simulated PSF are provided inn these ACII tables:
radius (EE = 50%)
radius (EE = 80%)
Perrin, M., et al. 2014, Proc SPIE 9143
Updated point spread function simulations for JWST with WebbPSF