This section briefly summarizes the scientific capabilities of the Nancy Grace Roman Space Telescope. 

Note: The content of the Call represents the known ground characterization of the observatory performance through October 2025. It is anticipated that improved and/or additional technical performance specifications will be available over the next few months, pending analysis of data obtained during integration and testing of the Roman hardware. For the latest information, please refer to the Late Breaking News.

Introduction

The Nancy Grace Roman Space Telescope is a NASA space telescope that observes at visible and infrared wavelengths and is designed to tackle questions in the areas of dark energy, exoplanets, and general astrophysics. Roman has a primary mirror that is 2.4 meters in diameter (the same size as Hubble's) and has two instruments, the Wide Field Instrument and the Coronagraph Instrument. The Wide Field Instrument is the main survey instrument having a field of view 200 times larger than Hubble's infrared view. Roman is slated to launch no later than May 2027, with the team working toward a potential early launch as soon as September 2026.

Science Payload

The Roman Space Telescope science payload has two instruments, the Wide Field Instrument and the Coronagraph Instrument, integrated with the telescope via the instrument carrier. The Coronagraph Instrument is a technology demonstration for an exoplanet imaging instrument on a future space-based observatory, and is thus not part of the General Investigator program in Cycle 1 and is not discussed any further in this document.

The telescope has a three mirror anastigmat design, providing good optical performance over a wide field of view. 

The Wide Field Instrument (WFI) is a 300.8-megapixel camera providing multiband visible to near-infrared (0.48 to 2.30 um) imaging using eight filters. A HgCdTe-based focal-plane array captures a 0.28 square degree field of view with a pixel size of 0.11 arcseconds x 0.11 arcseconds. The detector array is composed of eighteen 4096x4096 pixel detectors.  It also carries both high-dispersion grism (1.00 to 1.91 um) and low-dispersion prism (0.76 to 1.80 um) assemblies for wide-field slitless spectroscopy.

Additional technical details about the science instrument are found on the Instrument section of RDox.

Observatory

Derived positional accuracies are expected to be better than 1 milliarcsecond for well-detected sources. The observatory was designed to be stiff and thermally stable to provide excellent pointing stability better than 0.008 arcsec, with jitter remaining below 0.014 arcsec. The fact that the observatory is both light and stiff means that the observatory is particularly well-suited to executing short slews needed to map out large regions of the sky, settling within 10 seconds following a slew. 

Orbit / Sky Visibility

The Roman Space Telescope will be in a quasi-halo orbit around the second Sun–Earth Lagrange point (L2). In this orbit, the telescope is in a benign thermal environment. Moreover, this choice of orbit substantially reduces the projection of the Sun–Earth–Moon avoidance zones on the sky, yielding high astronomical observing efficiencies.

The telescope's instantaneous visibility region is a 72-degree wide annulus, extending from 54° to 126° in solar elongation, and encompassing all ecliptic latitudes. The observatory must point more than 54° away from the sun and antisun directions. About 59% of the entire sky is accessible to Roman at any given time.

The amount of time that any particular target is visible to Roman is a function of ecliptic latitude. Objects located more than 54 degrees away from the ecliptic plane are within the Continuous Viewing Zone. Additional information on target visibility and available roll angles can be found in the WFI Quick Reference. 

Observing Modes

In Cycle 1, Roman Space Telescope WFI observations will be executed with any of ten distinct optical elements (one of the imaging passband filters, the grism, or the prism). Observers specify their observations through the Roman APT. These specifications for a given target form a sequence of observations called a segment. Multiple optical elements can be used in a segment, together with various mosaicing and dither/subpixel dither pattern combinations and integration times and repeats. The segments and passes can be combined to form a survey.

The Roman observing modes are listed below. Details about these observing modes and the available choice of APT parameters are provided on RDox. The two observing modes available for Cycle 1 observational programs are:

• WFI Imaging
• WFI Spectroscopy

Because the processing of WFI slitless spectroscopic data requires images of the same field, Cycle 1 requests for grism or prism observations must include imaging in at least one Roman filter; two or more filters and roll angles are highly recommended for optimal results. Observers requesting grism or prism observations of the Core Community Surveys (CCS) planned to be executed in the first cycle and with no or limited spectroscopic data planned as part of the survey (the High Latitude Time Domain or Galactic Plane Survey) do not need to include additional imaging for spectroscopic extraction purposes, providing the imaging data of the CCS meets the technical and scientific needs of the proposal. See the Roman Observations section for more details. Recommendations for combined imaging and spectroscopic observation requests can be found on the Science Data Pipelines section of RDox.

The project will be verifying observatory capabilities and pointing stabilities and performance during commissioning, including the ability of WFI to guide in spectroscopic mode when observing crowded fields.

Science Operations

An integrated team of personnel from the Roman Project Science Office (PSO) at NASA Goddard Space Flight Center (GSFC), the Science Support Center (SSC) at Caltech/IPAC, and the Science Operations Center (SOC) at Space Telescope Science Institute (STScI) conduct science operations for the Roman project. Mission operations activities are conducted out of the Mission Operations Center at GSFC. The SSC solicits analysis and observational research programs through Calls for Proposals, organizes the peer review and proposal selection process, and administers NASA research funds for investigations selected by the Roman Selecting Official from PSO. The SOC schedules all Roman observations, calibrates the WFI data (the level of the calibration depends on the mode and core community survey), and archives and distributes all the Roman Space Telescope data through the Mikulski Archive for Space Telescopes (MAST). The SSC is responsible for high-level processing and calibration of all WFI spectroscopy mode data, along with high-level data processing of the data from the Galactic Bulge Time Domain Survey. The SOC also provides a cloud-based science platform, the Roman Research Nexus, that gives the community access to a rich computing environment, while allowing a low barrier access to data, compute, and software resources.