Caveats of using Pandeia for Roman
The Roman module of Pandeia is still under active development. Some features may not accurately reflect the observatory’s final capabilities, and there are known issues with the software that are tracked and updated regularly. Both the limitations and the main known software issues are described below.
This documentation is written for Pandeia version 2025.9 (released on September 15, 2025).
Current Limitations in Pandeia for Roman
The WFI detectors are currently undergoing active testing and characterization. As a result, some of the limitations below as well as detector-specific inputs may change in the future (see this Appendix for more information).
Simplifying Assumptions for Point Spread Functions
Notional Reference Point
The Point Spread Functions (PSFs) used in Pandeia are obtained using the Roman module of STPSF . More information on STPSF can be found in the STPSF for Roman articles in the Simulation Tools Handbook.
For the astronomical scene, the PSF is computed at a single notional point within the WFI field of view. Thus, the field dependence of the PSF (both within a detector or from one detector to another) is not currently accounted for in these scene simulations. We suggest the user consider using STIPS , the Space Telescope Imaging Product Simulator software, or Roman I-Sim - The Roman Image Simulator for field-dependent PSF models.
Limited Area
Pandeia is primarily designed for scene calculations under 25 arcseconds on a side and that have an overall small number of objects. The PSF is pre-calculated as a finite stamp image that is 3 arcsec by 3 arcsec. While simulating larger PSFs (e.g. to account for diffraction spike contamination from bright stars) is in principle possible, it is not recommended with Pandeia . Once again, we suggest the user consider using STIPS , the Space Telescope Imaging Product Simulator, or Roman I-Sim - The Roman Image Simulator software for more flexibility in the PSF size.
No explicit support for dithering
The Roman WFI ETC does not support dithering. Users are recommended to use the number of exposures to estimate the effect of dithering.
Ramp-fitting algorithm
The ramp-fitting algorithm used in Pandeia is based on Casertano, STScI Technical Dcoument, 2022 whereas romancal , the Roman Data Pipeline, offers two different ways to fit ramps (e.g. Casertano 2022 and Brandt 2024). In general, the different algorithms’ results differ by at most a few percent in the computed signal-to-noise (SNR) ratio (e.g., 0-2% between Casertano’s and Brandt’s algorithms). See the Ramp Fitting page on the romancal readthedocs for more details.PRISM/GRISM extraction direction
The dispersion direction of the Prism and Grism in the ETC is flipped with respect to flight and is a representative only. Caution is needed for the crowded field studies when determining the position angle constraints to avoid overlaps. The effect of the difference in the dispersion direction on the SNR is 1-3% for isolated point sources. The dispersion directions will be updated in a future Pandeia release.
2-D Pixel Inaccuracy
Incorrect pixel scale on the web ETC's display
The axis labels on the Images display of 2D results are slightly inaccurate in both Pandeia and the web ETC. The pixel (and axis) scale used in the plots is effectively for a scene one pixel larger than the actual. The effect is more prominent for smaller scenes, and for sources farther from the center of the field of view. This issue will be addressed in a future Pandeia release.
Incorrect coordinate grid math of the engine
For circular apertures, Pandeia is using an incorrect grid math that resulted in the offset in the extraction aperture location. This offset affects the predicted SNR and the engine currently is estimating an SNR that is ~3% higher. This issue will be addressed in a future Pandeia release.
Open Issues in the Pandeia Software
A complete list of known issues for Pandeia can be found on the Documentation for Known Issues that is continuously updated by the Pandeia and STScI ETC team. This documentation is written for Pandeia version 2025.9 (released on September 15, 2025).
For additional questions not answered in this article, please contact the Roman Help Desk.