# Roman Space Telescope surveys

## High-Latitude Wide-Area Survey (HLWAS)

Roman’s broadest survey sweeps thousands of square degrees of sky away from the Milky Way’s dusty plane, imaging and taking spectra of galaxies across most of cosmic history.

By measuring the shapes of distant galaxies, astronomers can detect the faint distortion caused by dark matter along the line of sight — weak gravitational lensing. Mapping where galaxies sit in three dimensions also traces the imprint of sound waves from the early universe. Together these tell us how fast the universe has expanded and how structure has grown, the two strongest clues to what dark energy is.

The survey is split into tiers of different depth. Current plans cover roughly 2,400 square degrees in several filters plus about 2,700 more in a single filter, using the Y, J, H and F184 filters and the grism for spectroscopy, over about two years of observing time.

Its footprint is designed to overlap with Rubin Observatory’s ground-based survey and ESA’s Euclid, so the three datasets can calibrate one another.

- **Area:** ≈ 2,400 deg² multi-filter + ≈ 2,700 deg² single-filter
- **Filters:** F106, F129, F158, F184 + grism
- **Observing time:** ≈ 2 years
- **Science:** Dark energy, dark matter, galaxy evolution

## High-Latitude Time-Domain Survey (HLTDS)

Roman will revisit small patches of sky every few days to catch Type Ia supernovae — standard candles whose brightness reveals how far away they are — out to great distances.

Comparing a supernova’s distance with how much its light has been stretched by the expansion of space shows how the expansion rate has changed over billions of years. Roman’s wide field and infrared vision let it find these explosions far more efficiently than Hubble.

The adopted design includes wide and deep imaging tiers of roughly 18 and 6.5 square degrees, with slitless spectroscopy over about 4.5 and 0.5 square degrees, and cadences as short as about five days. Repeated visits will also catch other transients, from tidal disruption events to variable active galaxies.

- **Imaging tiers:** ≈ 18 deg² (wide) and ≈ 6.5 deg² (deep)
- **Cadence:** As short as ≈ 5 days
- **Duration:** ≈ 2 years mid-mission
- **Science:** Type Ia supernovae, transients

## Galactic Bulge Time-Domain Survey (GBTDS)

Roman will stare toward the crowded heart of the Milky Way, photographing roughly 100 million stars every few minutes to catch the brief brightening when a foreground star — or a lone planet — passes in front of a background star.

This effect, gravitational microlensing, can reveal planets in cold, distant orbits and even free-floating planets with no star at all — worlds that other methods struggle to see. The same data will also record many transiting planets and huge numbers of variable stars.

The adopted plan images five fields in the Galactic bulge and one field on the Galactic Center roughly every 12 minutes, across six observing seasons — three early and three late in the mission — covering about two square degrees.

- **Fields:** 5 bulge fields + 1 Galactic Center field
- **Cadence:** ≈ 12 minutes
- **Seasons:** 6 (3 early, 3 late in the mission)
- **Science:** Microlensing planets, free-floating planets, stellar variability

## Galactic Plane Survey (GPS)

Early in the mission Roman will image several hundred square degrees along the plane of our galaxy, peering through dust that hides much of the Milky Way at visible wavelengths.

Planned at roughly 700 square degrees over about a month of observing, the survey targets star-forming regions, star clusters and the structure of the galactic disk, and covers a much larger area of the bulge than the time-domain survey.

It was defined through the same open, community-led process as the core surveys and is scheduled for the first part of the mission.

- **Area:** ≈ 700 deg²
- **Duration:** ≈ 1 month of observing
- **When:** Early in the mission
- **Science:** Star formation, clusters, galactic structure

## Coronagraph Instrument (CGI)

Roman’s second instrument blocks a star’s glare so the faint reflected light of nearby giant planets and dusty disks can be seen directly.

It carries deformable mirrors and advanced masks that actively correct starlight, aiming for contrast far beyond any coronagraph flown before. It works in visible light, roughly 0.4 to 1.0 microns, with imaging, spectroscopic and polarimetric modes.

Its goal is to show these technologies work in space — a stepping stone to NASA’s proposed Habitable Worlds Observatory, which would try to image Earth-like planets around Sun-like stars.

- **Type:** Technology demonstration
- **Wavelengths:** ≈ 0.4–1.0 µm
- **Modes:** Imaging, spectroscopy, polarimetry
- **Built by:** NASA JPL