Caveats of using WebbPSF for Roman
A description of known limitations and issues for the WebbPSF software with relevant advice for users. This documentation is written for WebbPSF version 1.2.1 (released on August 11, 2023).
Limitations
Here, we highlight some of the limitations of WebbPSF that may impact its use for some science applications.
Detector-Level Effects
WebbPSF and Poppy do not model any detector effects such as geometric distortion, pixel MTF (modulation transfer function), intrapixel sensitivity variations, interpixel capacitance, or any noise sources. Detector noise contributions should be added separately using independent detector models. These effects are taken into account in other tools like STIPS
Telescope Optical Path
As an approximation to the Roman primary mirror polishing errors, WebbPSF uses the default Hubble Space Telescope (HST) optical path difference map. The Roman model will be updated once the relevant data are available.
Dispersed PSFs
WebbPSF does not compute dispersed spectroscopic PSFs (e.g. for the Prism and Grism modes). The users are encouraged to compute monochromatic PSFs using WebbPSF and then combine them with an appropriate dispersion model.
Roman Coronograph
Regarding the WebbPSF Roman coronagraph model, the current functionality is limited to the Shaped Pupil Coronagraph (SPC) observing modes, and these modes are only simulated with static, unaberrated wavefronts, without relay optics and without DM control.
Aliasing in Large Fields of View
Simulated images of large fields of view (700-800 pixels) may contain aliasing artifacts. This could be mitigated by using a pupil image with a larger number pixels, although such a functionality is not yet supported.
PSF Sampling
WebbPSF does not output evenly-supersampled PSF models on a odd supersampled pixel grid, i.e., evenly-supersampled PSF models will not be centered at the center of a pixel, but their center will be placed at the corner point between 4 supersampled pixels. A current work around is to not supersample the PSF model itself but to change the pixel scale of the WFI.
Issues
Users interested in contributing to the ongoing development of WebbPSF may clone or fork from the main Github repository. Pull requests with code enhancements are welcomed.