The Roman WFI Exposure Time Calculator (ETC) enables users to estimate the expected signal-to-noise ratio for different astronomical scenes and observing strategies.

The following series of articles provides an overview of the different ways to interact with the ETC, outlines each tool’s specific capabilities and limitations, and offers example use cases and tutorials.

Introduction to the Roman WFI Exposure Time Calculator

The Wide Field Instrument (WFI) Exposure Time Calculator (ETC) enables users to evaluate the feasibility of proposed observations by simulating astronomical scenes and providing accurate estimates of signal-to-noise ratios and other key performance metrics.

The ETC can be accessed in several ways:

1. Pandeia is the core Python-based engine that performs the exposure time calculations.
2. The WFI ETC Web Interface runs the Pandeia backend within a browser, providing an easy-to-use graphical interface and sharing capabilities.
3. RIST uses a pre-computed grid of Pandeia outputs to enable rapid estimates of the signal-to-noise ratio for WFI observations.

Together, these tools comprise the Roman WFI ETC and they are described in more detail below and in related articles:

• Pandeia for Roman
  • Overview of Pandeia
  • Pandeia Tutorials
  • Caveats of using Pandeia for Roman
• WFI ETC Web Interface
  • Roman ETC Interface Help
• ETC Appendix: Inputs and Calculations
• Roman Interactive Sensitivity Tool

Pandeia

Pandeia is the ETC system originally developed for JWST. It is based on a Python engine that calculates three-dimensional data cubes from user-specified spatial and spectral properties of one or more sources. The data cubes are projected to a detector plane given an instrument configuration and two-dimensional pixel-by-pixel signal and noise properties are extracted.   Pandeia appropriately handles realistic point spread functions (as calculated by STPSF ), MULTIACCUM (MA) detector readouts, correlated detector readnoise, and multiple photometric and spectroscopic extraction strategies.  Pandeia also includes support for a variety of observing modes and has a highly modular, data-driven design that allows for easy extensibility to other instruments and telescopes.

Although  Pandeia provides the most flexibility and widest range of options for users, it can be more complex to setup and run correctly. We provide detailed documentation, tutorials, and other resources to those who wish to explore and benefit from the full power of the Pandeia engine.

Please see the Pandeia for Roman articles for more information.

WFI ETC Web Interface

The WFI ETC Web Interface runs on the Pandeia engine and can be accessed through a browser. The interface allows for easy scene and observation setup while still remaining flexible and allowing for complex designs. In addition, when a user is logged in, the ETC scenes and outputs are saved and can easily be shared with collaborators, all within the ETC Web Interface. This makes the Web Interface an excellent tool for users who (1) require detailed scene and observations setups while not necessarily wanting to code it all up in Python and (2) want an easy way to share their results with collaborators.

Please see the WFI ETC Web Interface articles for more information.

Roman Interactive Sensitivity Tool

RIST is software designed for quick, straightforward checks of the signal-to-noise ratio for WFI point-source observations. The software uses a grid of pre-computed Pandeia results spanning a limited set of exposure parameters and observing setup. Users will want to use RIST when they need a quick result that does not require the full capacity of the  Pandeia engine or the ETC Web Interface.

Please see the Roman Interactive Sensitivity Tool article for more information.

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