The WFI Point Spread Function (PSF) can be simulated using STPSF . This tool is designed to simulate individual PSFs on a specified detector with high fidelity. STPSF simulates only one PSF at a time and allows for detailed characterization of the field-dependent WFI PSF.  

Understanding the impact of the PSF on the ability, for example, to distinguish astronomical sources or to measure galaxy shapes can be conducted using either  Pandeia or STIPS ; the best tool to use depends on the size and complexity of the astronomical scene. A scientific use case trying to understand the impact of the WFI PSF on an extended object will either need to: 

• Use Pandeia to produce a scene simulation at the scale of a few arcseconds, or
• Use Roman I-Sim to perform an astronomical scene simulation on detector scales, or
• Although both tools above rely on STPSF for PSF modeling, users may alternatively prefer to perform their own convolution of extended object (e.g., a Sersic profile, or any other model) with an appropriate STPSF output.

STIPS can take user-generated catalogs that contain a mix of objects (point sources and/or extended objects), from the level of a full Roman detector to the full 18 detector FOV, and uses STPSF  models to quickly simulate WFI images.  STIPS can also include some instrumental properties such as instrumental background and noise.

For similar high-fidelity scene simulations formatted to work with the Roman STScI pipelines (unlike STIPS which does not conform to Roman datamodels), Roman I-Sim can also be used, albeit with longer runtimes.

For users interested in exploring the benefits and impacts of dithered observations, we recommend using Roman I-Sim to produce products that can be processed by the Roman STScI pipeline for combining observations. 

STIPS uses  STPSF  models and  Pandeia background estimates to simulate L2 images for complex astronomical scenes. These images can be analyzed in order to estimate SNR over larger, more complex astronomical scenes. Note that  STIPS uses a grid of 9 PSFs for each detector, so if users want to understand field-dependent effects in greater detail, then they may want to consider using STPSF outputs and convolving with their source properties.

Pandeia offers the ability to estimate the exposure time for a target signal-to-noise ratio and for astronomical scenes of a few arc seconds across containing sources with user-specified properties. The ETC Web Interface also offers easy-to-use functionalities through a browser to facilitate the design of astronomical scenes and observations.

A complex scene could be many things, but largely we consider complex scenes to be larger than a few arcseconds and contain too many sources to input manually.  STIPS will use STPSF  models and  Pandeia background estimates to simulate L2 images for complex astronomical scenes that can be analyzed by the user to measure SNR. Users may first want to use the Roman ETC to get a sense of reasonable exposure times for their simulations.