How James Webb will align its 18 primary mirror segments

With the James Webb Space Telescope at its final destination in orbit around the sun, now the telescope has to go through a series of alignment processes to get it ready for collecting science data. The process of aligning all of its mirrors has already begun, and concurrently with this, its instruments have been turned on for the first time since it left Earth as well.

The big step this week is that Webb’s primary camera, the Near Infrared Camera (NIRCam) instrument, has detected its first photons of starlight. This is far from being a full image of space, like those taken by Hubble, however, and the impressive images aren’t expected to start arriving until this summer. For now, the data collected by NIRCam will be used in the process of aligning the mirrors.

“With deployment of the mirror segments now complete, and the instruments turned on, the team has begun the numerous steps required to prepare and calibrate the telescope to do its job,” Scott Acton and Chanda Walker of Ball Aerospace, plus Lee Feinberg of NASA Goddard, wrote in an update. “The telescope commissioning process will take much longer than previous space telescopes because Webb’s primary mirror consists of 18 individual mirror segments that need to work together as a single high-precision optical surface.”

They also laid out the steps for the commissioning process, which are:

• Segment Image Identification
• Segment Alignment
• Image Stacking
• Coarse Phasing
• Fine Phasing
• Telescope Alignment Over Instrument Fields of View
• Iterate Alignment for Final Correction

The first step involves pointing the telescope at a very bright star, isolated on its own, called HD 84406. The telescope captures images of this, which will look like 18 dots of light because of the 18 segments of mirror. These images can then be used to adjust the mirror sections to bring the dots into focus, and then stack them into a single point. Once that is done, all the light is in one place, but the segments of the mirror still need to be further adjusted to act as one large mirror rather than 18 smaller mirrors.

The entire process of alignment is expected to take around three months, after which the instruments will be commissioned.

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