NASA Roman Team Checks Out Main Instrument, Sharpens Telescope's Focus

NASA's Nancy Grace Roman Space Telescope team completed a marathon of activities to fine-tune the observatory's vision and prepare the primary instrument for routine use.

"All of Roman's sweeping surveys will rely on the Wide Field Instrument," said Dave Hickey, a Wide Field Instrument systems engineer at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Many of the mission's primary goals, like studying dark matter and dark energy, wouldn't be possible without it, so it's both exciting and a huge relief to see that it's working well!"

Since the Wide Field Instrument was activated in September, the Roman team has been running it through a series of tests to exercise its components both individually and together. They started by checking out the 18 sticky note-sized detectors and all of their electronics before moving to the element wheel, a mechanism containing filters and other tools to pass specific wavelengths of light to the detectors and disperse light into its individual colors.

Then engineers turned on all of the internal LEDs in the calibration system, which are like precisely dimmable lights of various colors engineers control from the ground. These components are designed to measure how the detectors respond to different light levels and correct for any variations in their sensitivity.

The team tested the alignment mechanism, then deployed it to its predicted best focus position from the out-of-focus position it was stowed in for launch. Finally, engineers put the thermal system through its paces, ensuring it will keep the instrument's various components at the proper temperatures.

"Calibrating an instrument is like tuning a piano or guitar before a concert," said Maxime Rizzo, a Wide Field Instrument systems engineer at NASA Goddard. "You adjust everything until it's just how you want it and then leave the settings alone so it performs consistently while you use it."

Meanwhile, another team worked on sharpening the telescope's focus.

"The telescope's performance was excellent even before we began adjusting the alignment of all the mirrors and other optics, thanks to exceptional preparatory work done on the ground prior to launch," said Matthew Bolcar, Roman's optical systems lead at NASA Goddard. "All of the testing and modeling work done at Goddard and by contractors at L3Harris and BAE Space and Mission Systems left only small corrections for us to make to bring everything into near-perfect alignment."

That precision is significant because it's difficult to anticipate exactly how things will shift during launch and in space. Roman launched on Aug. 30.

"Our ground testing couldn't exactly reproduce the environment Roman would experience in space, such as the absence of gravity," Bolcar said. "And there are also slight differences in temperature variations and moisture content. Despite those challenges, the recent assessments showed that our ground testing was excellent, with both instruments either matching or exceeding the performance we had predicted."

When cosmic light enters the telescope, it bounces around between mirrors and other optics in a carefully choreographed way. Everything must be extremely precisely aligned to focus the light and direct it to Roman's two instruments.

Since the optics were already arranged near perfectly, the team only had to make miniscule adjustments.

"We moved the secondary mirror by about one-sixteenth of the diameter of a human hair," said Alden Jurling, a senior optical engineer working on Roman at NASA Goddard. "This adjustment nudged the height of the secondary mirror above the primary. If we scale that height up to the height of the empire state building, the change in height is a little bit less than you would get by stacking a single U.S. penny on top of the tip of the tower."

This microscopic modification could have a big impact on the mission's science.

Initially, the Coronagraph Instrument's images were slightly out of focus, though not to the degree the team had anticipated, which was relatively simple to correct. Now the instrument's performance meets the team's high expectations. The Coronagraph Instrument team will continue to calibrate its camera settings and determine the best settings for its deformable mirrors before they attempt to demonstrate its light-blocking capabilities in the coming weeks.

After fine-tuning the Coronagraph Instrument's focus, the team moved on to the Wide Field Instrument to make another round of adjustments.

Now, the Wide Field Instrument's performance is nearly 30% better than the Roman team predicted. The requirement for image quality was set at about 90 nanometers RMS (Root Mean Square), which means the optics only deviate from absolute perfection by 90 nanometers - that's around 1,000 times thinner than a human hair.

The team did even better: After applying corrections, the Wide Field Instrument's optics have an error of just 63 nanometers RMS.

"All of the images from Roman's cosmic surveys will be even sharper than we expected," Jurling said. "It's a great outcome from years of careful modeling, precision metrology, and testing."

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

More from Nancy Grace Roman Space Telescope