Teams at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has signed off on the most critical optical component of the Nancy Grace Roman Space Telescope. The 7.9-foot (2.4-meter) primary mirror, the surface responsible for gathering and concentrating light from objects across the cosmos, has passed its last hands-on inspection before launch.

The review took place on May 20 and 21. Engineers tilted the observatory onto its side and extended the deployable visor, sometimes called the “hood,” that folds away for launch and shields the optics from stray light. With the cover in place, the crew walked through a careful visual survey to verify that no particles had settled on the mirrors during earlier testing and that the optical path and alignment had not drifted.

“The Roman engineering team laid eyes on the telescope for the final time before it, in turn, becomes the eyes of humanity, revealing the wonders of the cosmos,” said J. Scott Smith, the Roman telescope manager at NASA Goddard in a NASA press release. “It is a profoundly humbling moment to witness the culmination of hard work from so many dedicated individuals, teams, and partner organizations, including L3Harris.”

To run the check, the team turned to a high-resolution camera fitted with a long-reach zoom lens, an approach that let them accomplish several inspection goals at once.

Bente Eegholm, optics lead for Roman’s Optical Telescope Assembly, said the mirror came through cleanly and kept the mission lined up for a launch as early as September. “We developed a method of using a high-resolution camera equipped with a very powerful zoom lens to do a multi-purpose inspection,” he said. “The mirror passed with flying colors, keeping the mission on track for an early September launch.”

The technicians traced the route light will travel toward the Wide Field Instrument’s detector array, confirming the alignment had held even after the observatory’s earlier shake test. Eegholm noted that every part of Roman has to be built to extreme tolerances in order to measure faint, far-flung objects, and that the primary mirror meets that bar.

A mirror tuned for infrared light

The surface carries a silver coating thinner than 400 nanometers, roughly 200 times finer than a strand of human hair. Engineers picked silver for its strong reflectivity in near-infrared wavelengths, the range Roman is built to study. That choice sets it apart from other flagship observatories: Hubble’s mirror uses aluminum with a magnesium fluoride layer tuned for visible and ultraviolet light, while the James Webb Space Telescope relies on a gold coating suited to its longer infrared wavelengths.

The polish is remarkably precise. The typical irregularity across the surface stands just 1.2 nanometers high, more than double the smoothness the mission actually called for. Scaled up to the size of Earth, those bumps would rise only about a quarter of an inch.

The mirror is also made from a specialty ultralow-expansion glass, chosen so it resists warping as temperatures swing from comfortable conditions on the ground to the extreme cold of space. Holding its shape matters because any distortion in the mirror would carry straight through into the telescope’s images.

“We’re really proud of the amazing optical system we’ve delivered for the Roman mission alongside our partners at L3Harris,” said Josh Abel, lead Optical Telescope Assembly systems engineer at NASA Goddard. “Now that it’s assembled, aligned, and all shined up, we’re ready to go.”

Next stop: Florida

In the weeks ahead, the Roman team plans to ship the observatory to its launch site at NASA’s Kennedy Space Center here on the Space Coast. NASA expects the telescope to begin sending back its first sweeping views of the universe within several months of liftoff.

The Nancy Grace Roman Space Telescope is managed at Goddard, with contributions from NASA’s Jet Propulsion Laboratory, Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and researchers at a range of institutions.

More on the mission is available at nasa.gov/roman.