The Roman Interactive Sensitivity Tool (RIST) is a simulation tool designed to help users quickly explore the feasibility of point source observations with the Wide-Field Instrument (WFI). RIST is available on GitHub as a Jupyter Notebook.

RIST Overview

RIST is a simplified and interactive version of Pandeia (the Exposure Time Calculator, or the ETC) and uses a pre-computed grid of Pandeia results that samples a set of exposure parameters and observing setups. RIST provides users an estimate of signal-to-noise ratio (SNR) in their observation based on the selected source brightness, spectral energy distribution (SED) type, multi-accumulation (MA) table, and number of resultants. To do this, RIST interpolates within a grid of pre-computed results, assuming a single on-axis point source.  Because the results are pre-computed, RIST offers less flexibility in the design of scenes (e.g. point sources only) and observations, but is useful when needing quick SNRe estimates of point sources.

The figure below shows the different elements that can be specified in Pandeia calculations. Additional information on Pandeia can be found in the Pandeia for Roman article.

RIST and Other Roman Tools

RIST is designed to provide users a quick estimate of SNR for the WFI observations with a few user-specified parameters. For users interested in higher-fidelity calculations of SNR, we recommend using the WebETC or Pandeia . These tools allow for the fine-tuning of input sources and observatory setups, and can be used in parallel with APT. 

More information on these other tools can be found in the guide to selecting Roman Simulation Tools.

RIST Interface

RIST is designed to work within a Jupyter notebook cell (hosted on GitHub here). The RIST interface can be initialized by importing the plot_rist module. The figure, figure of RIST interface, shows the initialized RIST interface. Users can control the parameters from the left and can get the estimated SNR from the plot on the right.

Each dot in the interactive plot represents the SNR for each of the eight WFI filters. Hovering the mouse over the points will print the value of the SNR rounded to the nearest integer and the name of the filter. The names are also shown in the legend on the right. 

Fixed Parameters of RIST

By design, RIST uses a simple source and observatory setup for the benefit of much faster SNR calculations. Here, we explain in detail the default setup used to generate the grid of pre-computed Pandeia results used by RIST. 

• The source is set to
  • A single point source
  • No redshift, no extinction, and no emission and absorption lines.
• The instrument and the detector configurations are set to
  • The imaging mode of the WFI
  • On-axis source at the center of the detector
  • Single exposure 
  • Compute SNR for all eight Roman filters
• The background is set to "minzodi_benchmark"
  • The background is pre-calculated using the JWST Backgrounds Tool. As shown in the Figure of the elements of Pandeia ETC, setting the background requires both the location and a date for a dated background or a level for a dateless background.
  • The "minzodi" background option refers to a background at 17^h26^m49^s & -73°20'03'' (J2000). At this location, the zodiacal emission is 20% higher than the celestial minimum.
  • The "benchmark" refers to the background at the minzodi location on June 19, 2020.
• The extraction strategy is set to have
  • The aperture size of 0.2’’ for the source
  • The sky annulus of 0.4’’ and 0.6’’, for the inner and outer radius, respectively, for the background subtraction

Tunable Parameters of RIST

RIST offers different options for four parameters either through drop-down menus or a slider. 

1. Magnitude system: Two options are available between the AB magnitude and the Vega magnitude
2. SED types: Four options are available from flat, A0V, G2V, and M5V
3. Source magnitude: the source magnitudes are available from 29^th magnitude up to 16^th magnitude in each of the magnitude system.
4. MA tables: the MA tables describe how the up-the-ramp sampling is performed using the sequence of individual reads on the WFI detectors. RIST offers nine MA table options. Please refer to the MA Table page for more information on the MA tables.
   1. One for the Microlensing Survey (C1_IMG_MICROLENS)
   2. Eight for the High Latitude Wide Area Survey (C2A_IMG_HLWAS through C2H_IMG_HLWAS)
5. Resultants: The resultants are groups of reads that are averaged (using an unweighted arithmetic mean) and downlinked to the ground system as science data. The available options for the number of resultants depend on the choice of the MA table. 

The figure below highlights the default and configurable elements of RIST, as described above. The elements in yellow represent the ‘Default’ setup of RIST and the elements in red represent the ‘Parameters’ of RIST with which users can explore the feasibility of their science with the WFI.

RIST is designed using the  bokeh Python package. Its interactive plot tools offer additional features that can be enabled and disabled by clicking on the icon, respectively.

• The Bokeh website () opens the Bokeh website
• The Pan () tool allows users to pan the plot without changing the plot scale.
• The Box Zoom () tool allows users to draw a box to zoom in on a particular part of the plot
• The Reset () tool allows users to revert the plot back to its original scale
• The Save () tool saves the current plot as a PNG format and downloads the image to the Downloads folders
• The Crosshair () tool draws a crosshair annotation over the plot
• The Hover () tool shows the annotations when a mouse cursor is over the points

Limitations of RIST

RIST inherently has the same limitations as  Pandeia . Please see the Caveats for Pandeia article for the details on these limitations for Roman.

The current version does not handle the following features and users need to use Pandeia for:

• Extended sources -- all SNR computations are performed for a point source.
• The spectroscopic observing mode -- future versions of RIST will support the spectrocopic mode.

Software Release

RIST will be updated every time Pandeia and its reference files are updated, and a new grid of pre-computed results will be generated. The current grid is generated using the versions of  Pandeia and its reference data:

• Pandeia version 2024.12
• Pandeia reference data version 2024.12

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