NASA’s Roman telescope just opened its planet-hunting eye

Roman's primary science instrument, the Wide Field Instrument, contains 18 detectors. Engineers designated a small section of each detector to rapidly monitor a different guide star -- stars whose positions are already known with high precision.

Roman's attitude control system first points the spacecraft toward the correct area of the sky. The fine-guidance system then continuously tracks those reference stars and supplies information that helps the observatory correct tiny movements during an exposure. Without those additional adjustments, Roman would not remain steady enough to produce its sharpest possible images.

"Every Roman observation relies on its ability to stay precisely pointed at the correct region of space long enough to collect an image, which can take from minutes to hours for a deep exposure," said Begoña Vila, Roman's guiding instrument systems lead at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The fine-guidance system reports the positions of the guide stars about four times each second to the attitude control system, which can move the observatory a tiny amount to counter any drift as needed. Our tests confirmed that we are able to keep the observatory very stable for science operations: better than 1/100,000 of a degree for half an hour at a time for Wide Field Instrument observations or for eight hours at a time for Coronagraph Instrument observations, which take much longer. This was a very exciting moment for the team."

The level of stability Roman has already demonstrated is roughly comparable to keeping a laser focused on a U.S. dime from about 150 miles (about 240 kilometers) away. Engineers expect to improve that performance even further as they continue tuning the guidance system. If they reach their goal, the same comparison would stretch to about 230 miles (370 kilometers).

A New Way to Guide a Space Telescope

Roman will also test a form of guidance that has never been used this way before.

"Roman doesn't have a separate guider instrument, like other space telescopes do," Vila said. "Instead of tracking only a star's point-like appearance, it will guide on detailed wavelength patterns called spectra. Because Roman is already equipped to measure spectra for science, it can use that same information to precisely position the telescope. We are looking forward to validating this spectral guiding mode in the coming weeks."

Spectra contain information about how an object's light is distributed across different wavelengths. Roman is already designed to collect this information for scientific observations, allowing engineers to use the same capability to help determine and maintain the telescope's precise position.

Roman's Coronagraph Sees Cosmic Light

On Sept. 22 and 27, the team went a step further by testing the fine-guidance system together with Roman's Coronagraph Instrument. The coronagraph is designed to demonstrate advanced technologies that suppress the brilliant light from stars so scientists can study much fainter planets and dusty disks orbiting nearby stars.

"Roman's coronagraph also has its own internal stability process, making it much more stable even than the Wide Field Instrument," Vila said.

That additional stability is essential for coronagraph observations. Even extremely small vibrations or pointing errors could allow unwanted starlight to leak into an image and overwhelm the much dimmer planets scientists hope to detect.

The coronagraph woke up on Sept. 1 and stretched its digital, electronic, and mechanical "limbs" mid-month. After confirming that Roman's fine-guidance system could keep the instrument stable, the team adjusted its focus and allowed the coronagraph to take its first preliminary look at space. Those early observations will help scientists continue refining the instrument before more demanding tests begin.

"This observation confirms that the instrument can produce a focused image," said Vanessa Bailey, a Roman Coronagraph Instrument scientist at NASA's Jet Propulsion Laboratory in Southern California. "It's a very limited test that kicks off a methodical process of increasingly complex tasks that help us prepare for the instrument's future observations."

First Stars Appear in the Large Magellanic Cloud

During the first test, Roman's Coronagraph Instrument observed a faint star in the Large Magellanic Cloud, a neighboring galaxy. The image contained extra "noise" because the instrument's detectors were intentionally being kept warmer than their eventual operating temperature. Keeping them warmer at this stage helps prevent contamination from sticking to the detectors.

"We were kicking the tires, making sure light goes through the system," Bailey said. "The second step, on Sunday, was an observation that confirmed our pointing. The team cooled the detectors down for better sensitivity, and we observed a new location in the Large Magellanic Cloud where we expected to see many stars in a single image. And we did! We're breathing a sigh of relief!"

That second observation provided another important confirmation that Roman was pointing where scientists expected and that the coronagraph could detect multiple stars once its detectors were cooled for greater sensitivity. The successful tests mark another step in the instrument's commissioning process as NASA prepares Roman for more sophisticated observations in the future.