Simulation Tools help users understand the technical capabilities of the Wide Field Instrument (WFI) on the Nancy Grace Roman Space Telescope. These tools provide simulated outputs based on the current best characterization of the observatory and its instruments. The Science Operations Center (SOC) at STScI supports tools for the WFI Imaging Mode and users interested in the capabilities of the WFI Spectroscopy Mode can find more information in the documentation organized by the Science Support Center (SSC) at IPAC.

This article aims to help users navigate the WFI Simulation Tools using a simplified set of possible use cases that can be selected from the Table of Contents below. The article is organized by high-level general tasks, e.g., simulate a PSF, calculate signal-to-noise ratios, etc,  that are then broken into specific use cases that guide the user to the best workflow for their goals. The best workflow may require using several tools or require tools not yet released to the public. Thus, forward looking statements are included and linked out to the Future Development of Simulation Tools article.

I want to simulate the Point Spread Function (PSF) for ... 

The WFI PSF can be simulated by WebbPSF . Some use-case examples are given below. Please note that using WebbPSF might not be the best starting point for all simulation purposes.

... a small number of astronomical point sources in simple configurations in a single exposure. 

WebbPSF can take a source characterization (e.g., an observed or predicted spectrum) to produce a reliable PSF simulation at a single position for a single source on any one of the 18 WFI detectors. Individual  WebbPSF outputs can be combined to produce realistic simulations of small numbers of objects in simple configurations (e.g., neighboring point sources or small clustered sources).  

  Learn about for WebbPSF for Roman.

I want to simulate the impact of the PSF in an astronomical scene that contains ...

WebbPSF provides a PSF model at a specific source location on any of the 18 Roman detectors and this approximates how astronomical point sources would appear in Roman images. Understanding the impact of the PSF on the ability, for example, to distinguish astronomical sources or to measure galaxy shapes can be conducted using  STIPS .

... any number of extended objects. 

 A scientific use case trying to understand the impact of the WFI PSF on an extended object will either need to: 

• • 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) or 
  • use STIPS to perform an astronomical scene simulation. 

  Learn about STIPS.
Learn about for WebbPSF for Roman. 

... a modest to large number of astronomical sources in more complex configurations in a single exposure. 

STIPS can take user-generated catalogs that contain a mix of objects (point sources and/or extended objects) at the level of a full Roman detector (or combinations of detectors) and use WebbPSF models to simulate WFI images. STIPS can also include some instrumental properties.

  Learn about STIPS.

... astronomical source(s) in a set of dithered exposures that are combined. 

Combinations of STIPS outputs can be used to generate realistic co-added Roman images to see the impacts of combined observation patterns on the PSF of a source or sources of interest.

  Learn about STIPS.

I want to calculate the signal-to-noise ratio for ... 

... a simple astronomical scene observed at a given exposure time. 

Pandeia offers the ability to perform signal-to-noise calculations for a given exposure time 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). Currently, Pandeia computations for Roman are only available by installing the Pandeia package. In the future, a web tool will offer streamlined functionality for quick use. 

  Learn about Pandeia for Roman.

... for a complex scene observed at a given exposure time. 

STIPS will use  WebbPSF models and  Pandeia background estimates to simulate L2 images for complex astronomical scenes.

  Learn about STIPS.

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). Currently, Pandeia computations for Roman are only available by installing the Pandeia package. In the future, a web tool will offer streamlined functionality for quick use. 

  Learn about Pandeia for Roman.

... for a complex scene. 

STIPS will use  WebbPSF models and  Pandeia background estimates to simulate L2 images for complex astronomical scenes. Users may first want to use  Pandeia to get a sense of reasonable starting points for their simulations; in the future, a web tool will offer streamlined functionality for this purpose. 

  Learn about STIPS.
  Learn about Pandeia for Roman.

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