NASA's Nancy Grace Roman Space Telescope has completed an important series of tests showing that it can keep its gaze extremely steady during observations. From Sept. 15 to 21, engineers tested Roman's fine-guidance system, which helps the observatory remain precisely locked onto its targets. Then, on Sept. 22, the telescope's Coronagraph Instrument received cosmic light for the first time.
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). 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.
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