This article provides quick at-a-glance 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 provide direct paths to resources that may be valuable. For information on the data from 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 roughly 40% 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, 360 degrees of yaw around the Sun line, and a +/- 15 degrees about the line of sight off the max power role 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. More detailed information on target visibility and available roll angles can be found at the GSFC Field, Slew, and Roll article, with a detailed roll angle table available for more precise 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. A more detailed Slew Table is provided by GSFC. Small slews, like those in a 5-point dither, would be of only a few seconds with a 10 second settle time. 

How to Find more Detailed Information

The Goddard Space Flight Center provides a Field, Slew, and Roll article, with a detailed roll angle table available for more precise applications and a very detailed machine readable Slew Table is provided. The pySIAF for Roman 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 the at-a-glance information. 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 18 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 WFI Design article provides a detailed description of the full instrument and its key subsystems. 

How to Find more Detailed Information

The WFI Design article provides a detailed description of the full instrument and its key subsystems. 

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 (P120) 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 direct response function tables, as currently characterized, are available with the Roman Space Telescope Reference Information that is maintained by NASA Goddard Space Flight Center.

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, 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

Table of Basic Detector Properties 

Characteristic	Quick Reference Values	Learn More
Individual Detector Properties
Detector type	18 individual Teledyne H4RG-10  Pixels are 10 x 10 microns	More Description is given in the WFI Design article.
Single Detector Pixel Dimensions	4096 x 4096 pixels total 4088 x 4088 pixels active
Single Detector on the Sky	450 x 450 arcsec	See the WFI Focal Plane In Context below.
Focal Plane Properties
Focal Plane Area	300 Mega Pixels	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/sec/pixel	Not yet Document on RDox
Quantum Efficiency	above 80% from 0.8 to 2.1 microns above 60% from 0.6 to 0.8 microns	Not yet Document on RDox
Noise	5 electrons correlated noise floor read noise below below 20 electrons	Not yet Document on RDox

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