This article provides quick information about the capabilities of the Wide Field Instrument (WFI) and places these in context of other major astronomical projects. 

Key Information for Using Roman's WFI 

The Wide Field Instrument (WFI) is a one-of-a-kind instrument and is a key component to the survey capabilities of Roman. The Table of Key Parameters for Roman's WFI provides quick at-a-glance information about Roman, the WFI, and its capabilities. Links in the Table of Key Parameters for Roman's WFI provide direct paths to resources that may be valuable. For information on the data from the WFI and how it will be made available to the community, please see the Data Handbook.

Observatory Capabilities 

The Roman spacecraft will be maintained in a Sun-Earth L2 orbit. Observing zones are defined such that the solar panels can provide maximum power, and for avoidance of the bright Earth and Moon. The Field of Regard (FOR) provides access to 59% of the sky each day, and the full sky is available over the 5-year primary mission. The spacecraft slew and settle rates allow for 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 of 54 degrees to 126 degrees from the Sun line (a reference direction pointing from the telescope to the Sun), 360 degrees of yaw around the Sun line, and plus or minus 15 degrees about the line-of-sight of the max power roll angle. Keep-Out Zones are those areas of pitch less than 54 degrees from the Sun line and greater than 126 degrees from the Sun line. There are also minor sporadic constraints on pointing due to the Earth and Moon line-of-sight avoidance areas. This results in the Roman spacecraft being able to access 59% of the sky each day (a 72 degree swath with 360 degree rotation; J. Kruk 2022, presentation). 

More detailed information on target visibility and available roll angles can be found in the Roman Space Telescope Technical Information Repository v1.1 (Jan 30, 2025), in particular in /data/Observatory/Visibility that contains information on the Field of Regard and the /data/Observatory/RollAngles that has tables of roll angles for specific applications. 

Slew and Settle Times

As shown in the Figure of the Field of Regard and Slew + Settle Times for Roman, the observatory is designed for quick moves for efficient sky mapping. Moves of 5 degrees or less can be completed in under 3 minutes. Small slews, like those in a 5-point dither used to fill gaps, would be completed in a few seconds with a 10 second settle time. 

More detailed information on the slew and settle times can be found in the Roman Space Telescope Technical Information Repository v1.1 (Jan 30, 2025), in particular /data/Observatory/SlewSettle contains tables of representative slew and settle times.

How to Find more Detailed Information

Roman mission partners maintain the Roman Space Telescope Technical Information Repository v1.1 (Jan 30, 2025) where the /data/Observatory/ folder contains subfolders for Visibility, RollAngles, SlewSettle, and Orbit . This repository is routinely updated with the most current information. The Coordinate Systems article in the Data Handbook also provides descriptions of how to translate between commonly used coordinate systems. The PySIAF for Roman article in the Simulation Tools Handbook provides software access to relevant coordinate transformations. 

Capabilities of WFI on Roman 

The Wide Field Instrument (WFI) of the Roman Space Telescope has both imaging and slitless spectroscopy capabilities, and a large field of view. The Table of Wide Field Instrument Capabilities summarizes helpful reference information about the WFI. 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.

How to Find more Detailed Information

The WFI Design article provides a detailed description of the full instrument and its key subsystems. The WFI Detectors section provides an overview of the detector technology, testing campaigns, and performance.

WFI Optical Elements and Sensitivity

The WFI provides 8 filters, including 7 filters isolating various broad bands (corresponding to the ground-based filters of R, z, Y, J, H, H/K, Ks), and one wide filter (F146). The Table of WFI Imaging Filters & Sensitivity provides the wavelength ranges, the 5-sigma sensitivity for a 1-hour exposure time, and the half-light radius for the PSF. WFI will have no narrow band filters. Two dispersers can be used for slitless multi-object spectroscopy and these are the prism (P127) and grism (G150). The Table of WFI Slitless Spectroscopy & Sensitivity summarizes the wavelength ranges, resolution, and the 5-sigma continuum sensitivity for a 1-hour exposure time. The Figures Contextualizing the Survey Grasp and Sensitivity of Roman place the capabilities of the WFI into the context of other wide-field observatories (Rubin and Euclid) as well as comparing against other large-scale surveys in the near-infrared. 

How to Find more Detailed Information

The Optical Elements article in the WFI Imaging Mode User Guide 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 response functions using python.

The Roman Space Telescope Technical Information Repository v1.1 (Jan 30, 2025) contains additional information. For the imaging mode, /data/WideFieldInstrument/Imaging/Filters has the overall filter parameters, /data/WideFieldInstrument/Imaging/Sensitivity has sensitivity estimates, /data/WideFieldInstrument/Imaging/EffectiveAreas provides effective area tables for the position of each detector in the focal plane for each optical element. For the spectroscopic mode, /data/WideFieldInstrument/Spectroscopy/Elements has the overall parameters, /data/WideFieldInstrument/Spectroscopy/Sensitivity has sensitivity estimates, /data/WideFieldInstrument/Spectroscopy/EffectiveAreas provides effective area tables for the position of each detector in the focal plane for each 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 Detectors section of the WFI Imaging Mode User Guide 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 below.
Focal Plane Properties
Focal Plane Area	300 megapixels	See the WFI Focal Plane In Context below.
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 at STScI.

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 v1.1 (Jan 30, 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