Webb primary mirror alignment goes smoothly – Astronomy Now

Slowly but surely, the 18 hexagonal segments that make up the James Webb Space Telescope’s 6.5-meter (21.3-foot) segmented main mirror are aligned to bring starlight to razor-sharp clarity.

After Webb arrived in space on Christmas Day, the segments were only roughly aligned. But using the observatory’s near-infrared camera, NIRCam, scientists and engineers map this alignment and gradually adjust the orientation of each segment using actuators on the back of each.

First, the team pointed Webb at a star in Ursa Major and adjusted the segments so that the light reflected from each reflected their physical positions. Then, further 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 were changed to fine-tune the reflections of a star. Image: NASA/STSci

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

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

In the fourth phase of segment alignment, known as “coarse phasing”, spectra will be collected from 20 distinct segment pairings, showing minute height differences between each. As these differences are adjusted, the single stacked image will become progressively sharper.

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

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