NASA’s Roman telescope takes first coronagraph test image after guidance checks

NASA’s Nancy Grace Roman Space Telescope captured its first coronagraph test image on September 22, observing a faint star in the Large Magellanic Cloud after guidance checks earlier in the month. In an update published September 30, NASA said the image showed the instrument could produce a focused view, an early commissioning step for a telescope intended to study planets and dusty disks around other stars.

The result is narrower than the coronagraph’s eventual science goal. The September 22 target was a star, and NASA described the observation as a limited test. A second observation on September 27 checked that the telescope could point at another field in the Large Magellanic Cloud and record the many stars the team expected to see there.

What Roman’s first coronagraph image showed

Before the first observation, the coronagraph had powered on September 1 and gone through electronic and mechanical checks in mid-September, according to NASA. The team then adjusted its focus. Vanessa Bailey, a Coronagraph Instrument scientist at NASA’s Jet Propulsion Laboratory, said the first observation confirmed that the instrument could produce a focused image. She called it a limited test at the start of a sequence of increasingly complex tasks.

The first image contained detector noise. NASA said the detectors were kept warmer than their eventual operating temperature to help prevent contamination from sticking to them. For the September 27 observation, the team cooled the detectors to improve sensitivity, then aimed at a different part of the Large Magellanic Cloud. Bailey said the image contained the expected stars and confirmed the team’s pointing.

How Roman keeps its observations steady

NASA said Roman’s fine-guidance system passed tests conducted from September 15 to 21. Small portions of each of the Wide Field Instrument’s 18 detectors observe guide stars whose positions are well known. The guidance system reports those positions to the spacecraft’s attitude-control system about four times a second, allowing it to correct drift while an observation is under way.

Begoña Vila, Roman’s guiding instrument systems lead at NASA’s Goddard Space Flight Center, said the tests showed pointing stability better than one hundred-thousandth of a degree. NASA reported that performance over half an hour for Wide Field Instrument observations and over eight hours for Coronagraph Instrument observations. The two instruments need different observation durations, making sustained pointing a central part of the checkout.

NASA likened the current stability to keeping a laser aimed at a US dime from about 150 miles, or 240 kilometres, away. The mission team aims to improve it through further tuning to the equivalent of about 230 miles, or 370 kilometres. Those distances are NASA’s illustrations of pointing precision; the longer one is a goal, not a result the team says it has reached.

Roman also plans to guide using spectra, the detailed wavelength patterns it can already measure for science, instead of relying on a separate guider instrument of the kind used by other space telescopes. Vila said the team expected to validate this spectral-guiding mode in the coming weeks. NASA did not say in its September 30 update that this additional mode had been validated.

Why the coronagraph matters for planet studies

Roman launched on August 30, according to NASA’s mission page. Its planned work spans exoplanets, dark energy and other astrophysics questions. The coronagraph is a technology demonstration intended to block the overwhelming light from a star so that much fainter planets or dusty disks around nearby stars can come into view.

In reporting published before these observations, Space.com explained how Roman’s coronagraph is designed to correct optical distortions with adjustable mirrors. It described that active control as a way to improve the suppression of starlight needed for direct views of exoplanets. Those were descriptions of intended capability, made before the September checkout, rather than evidence that planet imaging has already succeeded.

The sequence now reported by NASA establishes several smaller steps: the spacecraft held its pointing during guidance tests, the coronagraph produced a focused test image, and a later image showed stars in an expected field after the detectors were cooled. NASA says increasingly complex commissioning tasks remain. Its update gives no date for the end of commissioning or the coronagraph’s first science observation.