NASA Checks Roman Guidance System, Takes First Coronagraph Observation

NASA's Nancy Grace Roman Space Telescope's fine-guidance system, which allows the observatory to lock very steadily onto its targets during observations, passed a spate of tests from Sept. 15 to 21. Then on Sept. 22, the mission's Coronagraph Instrument opened its eyes to cosmic light for the very first time.

The team assigned a small part of each of the 18 detectors in Roman's primary instrument, the Wide Field Instrument, to rapidly observe a separate guide star - stars with well-known positions used as reference points. When Roman's attitude control system gets the spacecraft pointed in the right direction, information from the fine-guidance system then helps hold it steady throughout each observation so it doesn't drift off target. Using only the attitude control system, Roman's images wouldn't be as crisp.

"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."

Roman's current stability is roughly equivalent to focusing a laser beam on a U.S. dime from about 150 miles (about 240 kilometers) away. The team aims to improve this even further by tuning the guidance system, boosting the distance in the analogy to about 230 miles (370 kilometers).

Roman also has a new type of guiding that hasn't been attempted 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."

Coronagraph captures first glimmer

Next, on Sept. 22 and 27, the team tested out the fine-guidance system with the Coronagraph Instrument, which will demonstrate cutting-edge technology to block starlight and view planets and dusty disks around 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's critical because even tiny vibrations or pointing errors could let starlight leak through, washing out the faint planets scientists are trying to see.

The coronagraph woke up on Sept. 1 and stretched its digital, electronic, and mechanical "limbs" mid-month. Once the team confirmed that the fine-guidance system was holding the Coronagraph Instrument steady, scientists adjusted the focus so it could take its first bleary-eyed look at space that will help scientists fine-tune the instrument's view.

"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."

For this step, the Coronagraph Instrument captured a faint star in a neighboring galaxy called the Large Magellanic Cloud, with "noise" seeping through because the detectors have been kept at a warmer temperature than they will ultimately operate. That helps to ensure contamination does not stick 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!"

To learn more about Roman's commissioning process, visit: