This article provides a basic python example on how to use STPSF . Users can find a more extensive list of tutorials and examples in the form of Python Jupyter notebooks on the Roman Research Nexus (RRN). Additional information is available from the tool's online documentation.

This documentation is written for  STPSF version 2.2.0 (released in December 2025).

In addition to the examples in the RNN, the GitHub repository of STPSF offers many tutorials, including one specific to Roman, to help users get started. The foremost example is a jupyter notebook with a general demonstration of how to simulate, visualize, analyze, and export a point spread function (PSF) for the Roman Wide Field Instrument (WFI). To run the tutorials on your system, please follow the installation instructions from the readthedocs documentation. 

The readthedocs dedicated page on the Roman Instrument Model also contains several more detailed examples alongside narrative text explaining how all of the functionality comes together. Readers with previous experience using the JWST modes of  STPSF  should note the differences highlighted for Roman simulations.

Below is an example of visualizing the PSF with default parameters. This is one of many STPSF examples that are in Roman Research Nexus (RRN). In the dedicated STPSF notebook, users will find: 

• Quick PSF Generation: use the .calc_psf() method to compute a WFI PSF using mostly default settings.
• PSF Profile and Encircled Energy: for a given PSF, measure its profile, FWHM, and encircled energy.
• Bulk PSF Generation: generate several individual PSFs in a grid.

Example: PSF with Default Parameters

First, we import the relevant packages:

import stpsf
import matplotlib.pyplot as plt

plt.rcParams['image.interpolation'] = 'nearest' # Don't blur/smooth image plots

wfi = stpsf.roman.WFI()

default_psf = wfi.calc_psf(display=True)

plt.show()

We can then calculate the default PSF, which is applicable to the F062 optical element. Since we will use the built-in function to plot the results, first we setup a few matplotlib parameters:

plt.rcParams['figure.figsize'] = (8, 10) # Make the default figure size larger
plt.rcParams['image.interpolation'] = 'nearest'

wfi = stpsf.roman.WFI() # Use Roman models

default_psf = wfi.calc_psf(display=True)
plt.show()

Which will then show: 

Finally, we can display the simulated PSF in detector pixels and save it in FITS format using the following:

plt.rcParams['figure.figsize'] = (8, 6) # Make the default figure size larger
stpsf.display_psf(default_psf, ext='DET_SAMP')
plt.show()

default_psf.writeto('./test_PSF.fits', overwrite=True)

The previous code block displays the figure below.

References

• STPSF on readthedocs : https://stpsf.readthedocs.io/en/latest/
• Roman Research Nexus: https://roman.science.stsci.edu/hub/
• STPSF on Github: https://github.com/spacetelescope/stpsf

For additional questions not answered in this article, please contact the Roman Help Desk. You may also open an issue at the GitHub repository.