The Roman module of Pandeia is still under active development. Therefore, some features might not accurately reflect the final capabilities of the observatory and these limitations are described below. There are also some known open issues with the software itself that are maintained by the developers and these are referenced here. This documentation is written for  Pandeia version 3.1 (released on December 11, 2023).

Current Limitations in Pandeia for Roman

Simplifying Assumptions for Point Spread Functions 

Notional Reference Point

The Point Spread Functions (PSFs) used in  Pandeia are obtained using the using the Roman module of  WebbPSF . More information on WebbPSF can be found in the WebbPSF 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 a 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, that will query WebbPSF for PSF models closer to the simulation point in a single detector or across the WFI field-of-view.

Limited Area

Pandeia is primarily designed for small-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 spikes contamination from bright stars) is in principle possible, it is not recommended with Pandeia . We suggest the user consider using  STIPS , the Space Telescope Imaging Product Simulator software.

Simplifying Assumptions for Detectors

Quantum Efficiency

Pandeia currently uses a single Quantum Efficiency curve representative of the set of 18 flight detectors. Note that the throughput can vary between 5% and 10% between the detectors. 

Simplifying Assumptions for WFI

Filter Transmission

The filter transmission curves for PSF and Pandeia calculations are assumed to be flat across the filter bandpass. More information on the actual filter throughput is available in the WFI Optical Elements article of the WFI Imaging Mode User Guide.

Instrument Modes

The instrument modes, including details such as available readout patterns and subarray configurations, are modeled after the JWST's NIRCam instrument (including the NIRCam imaging and grism modes). The implementation of Roman-specific readout patterns in the form of Multi-Accumulation tables is currently under development and will be available in the next release of Pandeia . More information on the Multi Accumulation tables that are currently accessible in planning tools are given int the WFI MultiAccum Tables article of the Roman APT User's Guide.

Simplifying Assumptions for Astronomical Inputs

Noise Models

Pandeia assumes noise and background models identical to those of JWST, with the exception of the observatory self-emission background, which has been updated to accurately reflect the contribution from Roman. These estimates will be updated as more characterization data is acquired before and after the launch of the mission.

Open Issues in the Pandeia Software 

A complete list of known issues for  Pandeia can be found on the Developer Documentation that is continuously updated by the  Pandeia and STScI ETC team. This documentation is written for  Pandeia version 3.1 (released on December 11, 2023).

For additional questions not answered in this article, please contact the Roman Help Desk at STScI.