This article provides a quick reference to the capabilities of the Wide Field Instrument (WFI) and places them in the context of other major astronomical observatories. 

Roman WFI: Key Parameters and Characteristics

The Wide Field Instrument (WFI) of the Roman Space Telescope has both imaging and slitless spectroscopy capabilities, and a large field of view. The active area on the sky is 0.281 square degrees, roughly 100 times the area covered by HST/ACS or JWST/NIRCam, and 200 times that of WFC3/IR. The WFI has eighteen H4RG-10 (HgCdTe) 4096 pixel by 4096 pixel detectors for over 300 million active pixels with a plate scale of 0.11 arcsec/pix, similar to WFC3/IR. WFI is sensitive to wavelengths from 0.5 to 2.3 microns. The imaging capabilities of the Hubble Space Telescope, Roman Space Telescope, and JWST are compared in the Figure Comparing the Hubble, Roman, and James Webb Space Telescopes.

The Table of Key Parameters for Roman's WFI provides at-a-glance information about Roman, the WFI, and its capabilities. The table provides links to resources that may be valuable to users. For information on WFI data and their distribution to the community, please see the Data Handbook.

Roman Observatory: Key Parameters and Characteristics

The Roman spacecraft will operate in a Sun-Earth L2 orbit. Observing zones are defined to maximize solar power generation and avoid the bright Earth and Moon. The Field of Regard (FOR) provides access to 59% of the sky on any given day, with full-sky coverage achievable over the five-year primary mission. The spacecraft's slew and settle performance enables the efficient execution of large surveys.

Field of Regard

As shown in the Figure of the Field of Regard and Slew + Settle Times for Roman, the observing zone is described by a pitch angle of 54°-126° relative to the Sun line (the reference direction from the telescope to the Sun), 360° of yaw around the Sun line, and ±15° about the line-of-sight corresponding to the maximum-power roll angle. Keep-Out Zones defined as pitch angles less than 54° or greater than 126° from the Sun line. Additional, minor pointing constraints arise intermittently from Earth and Moon line-of-sight avoidance requirements. Together, these constraints allow the Roman spacecraft to access 59% of the sky on any given day (72° swath with 360° rotation; J. Kruk 2022, presentation). 

More detailed information on target visibility and available roll angles is provieded in the Roman Space Telescope Technical Information Repository v1.2 (July 2025). In particular the /data/Observatory/Visibility folder  contains Field of Regard information, and the /data/Observatory/RollAngles folder includes tables of roll angles for specific applications. 

Slew and Settle Times

As illustrated in the Figure of the Field of Regard and Slew + Settle Times for Roman, the observatory is designed to support rapid repositioning for efficient sky mapping. Slews of 5° or less can be completed in under 3 minutes. Small slews, such as those in a five-point dither pattern to fill detector gaps, are executed in a few seconds, followed by a nominal 10 second settle time. 

Additional details on slew and settle times are available in the Roman Space Telescope Technical Information Repository v1.2 (July 2025), specifically within the /data/Observatory/SlewSettle folder, which contains representative timing tables.

How to Find more Detailed Information

Roman mission partners maintain the Roman Space Telescope Technical Information Repository v1.2 (July 2025), which is routinely updated with the most current information. The /data/Observatory/ directory contains subfolders for Visibility, RollAngles, SlewSettle, and Orbit . Descriptions of coordinate systems and guidance on coordinate transformations are provided in the Coordinate Systems article in the Data Handbook. The pySIAF for Roman article in the Simulation Tools Handbook describes software for relevant coordinate transformations. 

WFI Optical Elements and Sensitivity

The WFI is equipped with eight imaging filters: seven broad-band filters corresponding approximately to the ground-based R, z, Y, J, H, H/K, and Ks bands, and one wide filter (F146). The Table of WFI Imaging Filters & Sensitivity summarizes the wavelength coverage, the 5σ point-source sensitivity for a one-hour exposure, and the PSF half-light radius for each filter. The WFI does not include any narrow-band filters. In addition to imaging, the WFI supports slitless multi-object spectroscopy using two dispersers: a prism (P127) and a grism (G150). The Table of WFI Slitless Spectroscopy & Sensitivity provides the wavelength ranges, spectral resolution, and the 5σ continuum sensitivity for a one-hour exposure. The Figures Contextualizing the Survey Grasp and Sensitivity of Roman place the WFI sensitivity in context with other wide-field observatories, including Rubin and Euclid, and compare it with other large-scale near-infrared surveys.

How to Find more Detailed Information

The Optical Elements article in the WFI Handbook provides significantly more detail on the wavelength coverage and throughput of the optical elements. In the Simulation Tools Handbook, the Synphot for Roman article describes how to access the optical elements' response functions using python.

The Roman Space Telescope Technical Information Repository v1.2 (July 2025) contains additional information.

For the imaging mode:

•  /data/WideFieldInstrument/Imaging/FiltersSummary - filter parameters
• /data/WideFieldInstrument/Imaging/Sensitivity - sensitivity estimates
• /data/WideFieldInstrument/Imaging/EffectiveAreas - effective area tables for each detector position in the focal plane for each optical element

For the spectroscopic mode:

• /data/WideFieldInstrument/Spectroscopy/PrismGrismSummary - overall parameters
• /data/WideFieldInstrument/Spectroscopy/Sensitivity - sensitivity estimates
• /data/WideFieldInstrument/Spectroscopy/EffectiveAreas - effective area tables for each detector position in the focal plane for each optical element

WFI Detectors 

The WFI mosaic plate assembly provides both fine guiding and science data readout. The array of 18 detectors (Teledyne H4RG-10; also known as Sensor Chip Assemblies or SCAs) covers 0.281 square degrees on the sky with 300 megapixels. To manage data volume, the WFI employs both data compression and multi-accumulation (MA) readouts. The Table of Basic Detector Properties provides at-a-glance information about the Roman detectors. 

How to Find more Detailed Information

The WFI Performance section of the WFI Handbook provides additional information on the WFI detectors and how they are characterized.

Table of Basic Detector Properties 

Characteristic	Quick Reference Values	Learn More
Individual Detector Properties
Detector type	18 individual Teledyne H4RG-10  Pixels are 10 microns by 10 microns	More Description is given in the WFI Design article and the WFI Detectors section.
Single Detector Pixel Dimensions	4096 pixels  by 4096 pixels total 4088 pixels by 4088 pixels active
Single Detector on the Sky	450 arcsec by 450 arcsec	See the WFI Focal Plane In Context.
Focal Plane Properties
Focal Plane Area	300 megapixels	See the WFI Focal Plane In Context.
Focal Plane Array Area on Sky	0.320 square degrees (total) 0.281 square degrees (effective)
Detector Performance
Dark current	below 0.1 electrons per second per pixel	The WFI Detectors section provides more information on the characterization of the detectors.
Quantum Efficiency	$\gtrsim 80\%$ (0.8–2.1 microns)  ≳ 60% (0.6–0.8 microns)
Noise	Correlated noise floor: 5 electrons Read noise: ≲ 20 electrons

Release Tag

Detector performance values are based on the performance benchmarks (J. Schielder 2022, presentation) and results from performance testing (Mosby et al. 2020); more information on ground testing can be found and the WFI Detectors section. This information will be updated in the Roman Space Telescope Technical Information Repository in a future release. 

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

References

Journal Papers or Reports:

• "Properties and characteristics of the WFIRST H4RG-10 detectors", Mosby et al. 2020

Other Material:

• Roman Space Telescope Technical Information Repository v1.2 (July 2025)
• "Wide Field Instrument Status" presentation at the Roman Community Forum on September 14, 2022 by J. Schlieder
• "Mission Status Update" presentation at the Roman Community Forum on September 14, 2022 by J. Kruk