Understanding Roman WFI Sensitivity, Saturation, and Exposure Time
This article explains how sensitivity and saturation behave in Roman Wide Field Instrument (WFI) observations and how these effects should inform exposure-time decisions. Because the WFI uses near-infrared detectors with nondestructive up-the-ramp readout, exposure time is accumulated over a sequence of reads whose timing is specified by a MultiAccum (MA) Table.
Using representative imaging and spectroscopic MA tables, this page demonstrates how to read the associated sensitivity and saturation plots and how to use them to evaluate trade-offs between depth, dynamic range, and saturation when planning observations. This article assumes familiarity with the structure of Roman WFI MultiAccum Tables and the concepts of reads, resultants, and effective exposure time, as described in the MultiAccum (MA) tables article.
MA Tables and Their Impact on Sensitivity and Saturation
The structure of a Roman WFI MultiAccum (MA) table determines how a detector ramp is sampled and therefore directly affects both the achievable depth of the observations and the saturation behavior. When selecting an MA table, users should consider how ramp sampling interacts with source brightness, signal-to-noise requirements, and detector characteristics.
- Source Brightness and Dynamic Range
Shorter MA tables sample the detector ramp with shorter effective exposure times in the early resultants, making them better suited for bright sources that would otherwise saturate. This comes at the cost of reduced total integration time and lower sensitivity to faint sources. Longer MA tables extend the ramp to later times, improving depth and dynamic range for faint targets but increasing susceptibility to cosmic-ray impacts and saturation of bright sources.
- Signal-to-Noise Regime
Science cases that require reaching the background-limited regime or achieving signal-to-noise ratios of order SNR ~ 10 for faint sources may benefit from longer-duration MA tables. For brighter targets or shallow surveys, selecting a shorter MA table or an MA table with shorter effective exposure times in the early resultants may be more appropriate. For additional guidance on how backgrounds impact observations of faint sources, see the article on Assessing Background Levels for WFI Observations.
Saturation Behavior Along the Ramp
Saturation limits depend on where along the detector ramp a pixel exceeds the detector’s linear regime. For any selected MA table, saturation can occur in early or late resultants depending on the ramp sampling and source brightness. MA tables with shorter effective exposure times in the early resultants help avoid saturation for bright sources.
Detector-to-Detector Variations
Programs with stringent uniformity requirements should account for detector-to-detector variations in sensitivity when selecting an MA table. Small performance differences across the 18 WFI detectors may influence the optimal MA table choice needed to achieve uniform depth or signal-to-noise across the focal plane.
The sensitivity and saturation interactive figures on this page summarize how signal-to-noise performance and detector saturation behavior vary with wavelength, observing mode, and detector for a given MultiAccum (MA) table. Sensitivity reflects the faintest flux that can be measured at a specified signal-to-noise level over the full detector ramp, while saturation indicates the source brightness at which pixels exceed the detector’s linear response during the ramp.
The figures shown here are computed using representative MA tables — IM_193_11 for imaging and SP_300_16 for spectroscopy — which are provided as illustrative examples of how sensitivity and saturation evolve along a detector ramp for commonly used observing configurations. The sections below describe how to interpret these figures in detail: the Sensitivity Curves section explains the assumptions used to compute the reported sensitivities and how to read the wavelength-dependent performance, while the Saturation Limits section defines full and partial saturation for WFI detectors and explains how these limits should be interpreted when planning observations.
While IM_193_11 and SP_300_16 serve as useful reference configurations, observers are encouraged to use the Roman Exposure Time Calculator (ETC) to explore their specific science cases across a range of MA tables and ramp configurations and to evaluate trade-offs between depth, dynamic range, saturation, and cosmic-ray resilience.
WFI Sensitivity and Saturation Setup
The sensitivity and saturation data presented on this page are generated using the Roman WFI Exposure Time Calculator (ETC) with the following inputs:
Pandeia version:
R2025.9Reference data version:
R2025.9
The interactive figures below allow users to explore how sensitivity and saturation vary with wavelength, observing mode, and detector for representative imaging and spectroscopic MultiAccum (MA) tables.
Hovering over points or curves in the figures reveals detailed information, including:
Observing mode: Imaging or spectroscopy
Filter or disperser:
Imaging: F062, F087, F106, F129, F146, F158, F184, F213
Spectroscopy: Prism (P120) or Grism (G150)
Wavelength (μm): For imaging, the effective wavelength of the bandpass
Flux density: In Jansky (Jy)
AB magnitude
Users can control which detectors are displayed in the figures by toggling the checkboxes for WFI01 through WFI18 located in the upper-right corner of the plot.
Sensitivity Curves
The sensitivity curves shown in the Figure of Sensitivity Curves report the faintest flux that can be measured at a signal-to-noise ratio of SNR = 10 for a fixed observing configuration. The reported sensitivities correspond to the cumulative signal-to-noise achieved over the full detector ramp defined by the selected MultiAccum (MA) table.
For imaging modes, sensitivities are computed using the IM_193_11 MA table and assume a total exposure composed of 51 integrations, corresponding to an effective exposure time of approximately 10,000 seconds. For spectroscopic modes, sensitivities are computed using the SP_300_16 MA table with 33 integrations; reported values are given per wavelength pixel.
All sensitivities shown here assume the hlwas-medium_field1_medium background (see the Table of Pre-Determined Backgrounds in the Assessing Background Levels for WFI Observations article). Since background levels directly affect the achievable signal-to-noise, observations taken under different background conditions will reach different depths for the same MA table and exposure configuration.
The sensitivity values shown here should therefore be interpreted as illustrative performance benchmarks, not prescriptive exposure recommendations. Different MA tables, background conditions, or signal-to-noise requirements will lead to different achievable depths.
The interactive figure displays flux density as a function of wavelength for each WFI detector. Imaging filters are shown as discrete points at their effective wavelengths, while spectroscopic modes are shown as continuous curves. Hovering over a point or curve reveals the corresponding AB magnitude for the selected detector, filter, or spectral element.
Bokeh plot interactivity is not supported in the PDF version of this handbook.
Figure of Sensitivity Curves
Saturation Limits
For the Roman WFI, pixel saturation occurs when a detector pixel deviates from linear response by more than a defined threshold, as determined through calibration measurements. Users unfamiliar with detector linearity and saturation behavior in H4RG detectors are encouraged to review the Linearity and Saturation section of the Sources of Pixel to Pixel Variation article.
When planning WFI observations, saturation must be evaluated in the context of the detector ramp defined by the selected MultiAccum (MA) table. Two saturation regimes are relevant:
Full Saturation - A pixel exceeds its saturation threshold in the second resultant of the MultiAccum Table.
Partial Saturation - A pixel exceeds its saturation threshold in the third or a later resultant of the ramp.
Saturation behavior depends on source brightness, ramp sampling, and detector characteristics. For any selected MA table, saturation may occur in early or late resultants depending on how rapidly signal accumulates along the ramp. If a source reaches full well in the second resultant (or earlier), subsequent reads become unreliable or unusable for scientific analysis due to non-linear detector response or charge full well effects. In such cases, only the unsaturated portion of the ramp can be used, significantly limiting the achievable signal-to-noise.
The Figure of WFI Saturation Limits provides a quick-look estimate of the saturation limits for two representative WFI MultiAccum Tables: IM_193_11 for imaging and SP_300_16 for spectroscopy. Users can consult this figure to identify the approximate flux levels or magnitudes at which saturation begins during an integration. For sources with fluxes close to these limits, observers may need to select an MA table with shorter effective exposure times in the early resultants to keep the target within the detector’s linear regime. In such cases, the Roman Exposure Time Calculator (ETC), and in particular the WFI ETC Web Interface, can be used to evaluate saturation behavior at the pixel level and explore trade-offs among the available WFI MultiAccum (MA) Tables.
Bokeh plot interactivity is not supported in the PDF version of this handbook.
Figure of WFI Saturation Limits
For additional questions not answered in this article, please contact the Roman Help Desk.