What Roman Simulation Tool Should I Use?
The Science Operations Center (SOC) at STScI develops and supports simulation tools for the Wide Field Instrument (WFI) Imaging Mode. These SOC simulation tools can help users understand the imaging capabilities of the WFI, based on the most up-to-date characterization of the WFI and the observatory available to date.
This article aims to assist users in navigating the Simulation Tools maintained by the SOC by providing a simplified set of potential use cases for the different simulation tools. The article is organized around high-level general tasks, such as simulating a PSF or calculating signal-to-noise ratios, which are then broken down into specific use cases in order to guide users toward the most appropriate workflows for their objectives. The optimal workflow may involve using multiple tools or tools not yet released to the public. Therefore, forward-looking statements are included and linked to the Future Development of Simulation Tools article.
Users interested in the capabilities of the WFI Spectroscopy Mode can find more information in the documentation maintained by the Science Support Center (SSC) at IPAC.
How is simulating Roman data different?
Notable differences between the Nancy Grace Roman Space Telescope and prior missions, such as HST and JWST, are its wide field of view (0.28 square degrees) and its focal plane array containing 18 individual detectors (see Introduction to the WFI). Given these characteristics, Roman is optimized for conducting astronomical surveys and, as a result, simulating certain datasets may be more complex and produce much larger data volumes compared to simulators for other missions. This article offers guidance on selecting the appropriate tools for basic investigations aimed at understanding the capabilities of the WFI.
I want to simulate an individual Point Spread Function ...
... for different detectors and filters.
The WFI Point Spread Function (PSF) can be simulated using WebbPSF . This tool is designed to simulate individual PSFs on a specified detector with high fidelity. WebbPSF simulates only one PSF at a time and allows for detailed characterization of the field-dependent WFI PSF.
I want to simulate an astronomical scene...
... with extended objects.
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 STIPS or Roman I-Sim to perform an astronomical scene simulation on detector scales, or
- Perform that calculation by taking the convolution of an appropriate WebbPSF output with the specifications of the extended object (e.g., a Sersic profile, or any other model).
Learn about for WebbPSF for Roman
... with many sources in a complex configuration.
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 use WebbPSF 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.
... with dithered exposures.
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.
I want to calculate the signal-to-noise ratio...
... for a simple astronomical scene.
Pandeia offers the ability to perform signal-to-noise ratio (SNR) calculations for an input number of exposures (of fixed length) with a set of properties for an astronomical source or a small astronomical scene with multiple input sources (a region with lengths of a few arcseconds). The ETC Web Interface also offers easy-to-use functionalities through a browser to facilitate the design of astronomical scenes and observations.
Note that effects related to the location of the source within a detector are currently not modeled in Pandeia . If sources need to include detector location effects, then the user is advised to generate PSFs with WebbPSF and then apply any additional detector effects to achieve the desired accuracy.
... for a complex scene.
STIPS uses WebbPSF 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 the user may want to consider using WebbPSF outputs and convolving with their source properties.
I want to estimate exposure times...
... for a simple astronomical scene.
Pandeia offers the ability to estimate the exposure time for a target signal-to-noise ratio and a set of properties for an astronomical source or a small astronomical scene with multiple input sources (a region with lengths of a few arcseconds). The ETC Web Interface also offers easy-to-use functionalities through a browser to facilitate the design of astronomical scenes and observations.
... for a complex scene.
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 WebbPSF 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. In the future, a web tool will offer streamlined functionality to do estimates of exposure times for simpler scenes which can help users plan STIPS simulations.
For additional questions not answered in this article, please contact the Roman Help Desk at STScI.