Introduction to WebbPSF for WFIRST
WebbPSF-WFIRST provides a customizable interface to perform point-spread function (PSF) simulations and calculations for the Wide-Field Instrument (WFI) planned for WFIRST. A simulated PSF provides a useful tool for predicting the observatory's performance at a particular combination of wavelength (or bandpass), field position, and pixel scale. PSFs are also an important input to simulate astronomical scenes. For example, WebbPSF-WFIRST PSFs are used by the Pandeia and STIPS toolkits developed at the Space Telescope Science Institute (STScI).
PSFs are simulated in the far-field limit (Fraunhofer domain) using the same optical simulation library as the existing WebbPSF software. WebbPSF was developed to simulate the James Webb Space Telescope (JWST) instrument PSFs, and its accuracy has been checked against ground test data for JWST (see the references listed below). The WebbPSF-WFIRST WFI model is based on the Cycle 6 WFIRST design revision, and incorporates information from GSFC Instrument Reference Information files.
WebbPSF-WFIRST allows users to calculate PSFs both in the bandpasses defined in the GSFC WFIRST instrument reference data and for the monochromatic case (which can be useful as an input to other calculations). For broadband PSFs, WebbPSF allows the user to select an input spectrum (e.g., a stellar spectral type or a galaxy spectral energy distribution) which then weights the individual monochromatic components of the PSF appropriately.
The software provides several built-in analysis tools to compute a radial profile, an encircled energy curve, or the difference image of two PSFs. All of these analysis tools work with standard FITS files with appropriate header keywords, and the PSF calculation results from WebbPSF-WFIRST can be written directly to FITS files for export to other tools.
Compared to the current WebbPSF instrument models for JWST, WebbPSF-WFIRST adds support for field-dependent PSF aberrations both within a single detector and among the 18 detectors in the WFI focal plane. These are modeled as Zernike coefficients, using the instrument reference data from the Cycle 6 design revision.
Getting Started with WebbPSF-WFIRST
The WebbPSF package includes both JWST and WFIRST instrument models and is available for download and installation. The instructions for installation for WebbPSF are the same whether you are using the JWST or WFIRST functionality, so users can refer to the WebbPSF installation documentation for installation options.
To aid users interested in getting started with STScI’s simulation tools for WFIRST, including WebbPSF, we have written tutorial Jupyter notebooks, available from GitHub. This repository also includes alternative instructions for installing STScI’s simulation tools WebbPSF and Pandeia in a self-contained installation using Docker. This is the suggested option for users who are less familiar with Python.
For those following the local installation instructions, after WebbPSF is installed, users of a scientific Python distribution like Ureka or Anaconda can either clone the above repository or simply download the notebook file to a folder and launch “IPython notebook” or “Jupyter notebook” in that folder from the command line. Users who select the Docker installation option will have the notebooks installed within the Docker container.
A technical report (WFIRST-STScI-TR1703: WebbPSF for WFIRST) by Long and Perrin discusses the assumptions underlying WebbPSF-WFIRST, and compares its model PSFs to those calculated with alternative tools. Users are also encouraged to cite one of the following publications covering WebbPSF's JWST functionality:
Perrin et al., 2014. Proc. SPIE. Vol. 9143.
Perrin et al., 2012. Proc. SPIE. Vol. 8442.
Assumptions and Caveats
The WFIRST functionality of WebbPSF is still in continued development. The present implementation is available to the community as a beta version.
The WFIRST mission is not yet in development, and observatory designs continue to evolve. Hence, WebbPSF-WFIRST simulations may not accurately reflect the actual future observatory.
At this stage, the model for the WFI makes several approximations beyond the basic assumption of far-field diffraction-based PSF calculation:
- The filter transmission for broadband PSF calculations is assumed to be flat across the filter band pass.
- No attempt is made to model detector effects such as inter-pixel capacitance or noise characteristics. These issues can be assessed instead with the Pandeia software.
- Aberrations are represented as Zernike polynomials from Z4 to Z22. To this we add higher spatial frequency wavefront error content that is based on the Optical Path Difference (OPD) map across the Hubble Space Telescope primary mirror. This is used as a proxy for higher-order aberrations beyond Z22, until measurements of the wavefront error for the actual WFIRST primary mirror become available.
- The WFI model selects the pupil shape appropriately based on the following statement from the final report of the WFIRST Science Definition Team: "The H158, F184 and W149 filters and the grism are mounted with proximate cold pupil masks." For custom bandpasses or monochromatic calculations, the unmasked pupil is used unless the maximum wavelength requested for the calculation is greater than 1.454 µm.
- WebbPSF-WFIRST does not yet model distortions of the apparent pupil shape as seen from the detector.
We are planning many additions and improvements for future WebbPSF-WFIRST versions, including the following:
- Improvements to the graphical notebook interface and additional documentation.
- Modeling of the distortion of the apparent pupil shape as seen from different points in the field of view.
- Better filter transmission curves and high spatial frequency wavefront error maps, once these have been measured and released by the project.
- Other fixes and features based on community feedback.
Credits, Acknowledgements, and Feedback
WebbPSF is developed by Marshall Perrin, Robel Geda, and collaborators in support of STScI's involvement in both the JWST and WFIRST missions. Software development takes place on GitHub. Contributions of bug reports or code are welcome!
Users are encouraged to address questions, suggestions, and bug reports to firstname.lastname@example.org with "WebbPSF-WFIRST question" in the subject line, where they will be directed to the appropriate members of the WebbPSF-WFIRST team at STScI.