Webb primary mirror alignment proceeding smoothly

Slowly but surely, the 18 hexagonal segments making up the James Webb Space Telescope’s segmented 6.5-metre (21.3-foot) primary mirror are being brought into alignment to bring starlight to a razor-sharp focus.

After Webb made it into space on Christmas Day, the segments were only roughly aligned. But using the observatory’s Near Infrared Camera, of NIRCam, scientists and engineers are mapping that alignment and incrementally adjusting the orientation of each segment using actuators on the back of each one.

First, the team aimed Webb at a star in Ursa Major and adjusted the segments so the light reflected from each one mirrored their physical positions. Then, additional adjustments brought each reflection into focus in a process known as segment alignment:

Before-and-after images showing how the orientations of Webb’s 18 primary mirror segments have been tweaked to sharpen reflections from a star. Image: NASA/STSci

Now, all the segments have been aimed so the reflections are “stacked,” or merged into a single beam. Although the image stacking process puts all the light from the segments in one place inside NIRCam, more adjustments are needed to ensure all 18 combine to form a single 6.5-metre mirror.

Reflections from all 18 mirror segments have now been “stacked” so they fall on Webb’s optical axis as a single beam. Engineers are now adjusting the height of each segment to further focus the merged images. Image: NASA/STScI

In the fourth phase of segment alignment, known as “coarse phasing,” spectra will be collected from 20 separate segment pairings, showing tiny height differences between each one. As those differences are adjusted, the single stacked image will get progressively sharper.

“We still have work to do, but we are increasingly pleased with the results we’re seeing,” said Lee Feinberg, optical telescope element manager for Webb at NASA’s Goddard Space Flight Center. “Years of planning and testing are paying dividends, and the team could not be more excited to see what the next few weeks and months bring.”