Photon fantastic: James Webb Space Telescope spies its first starlight

Three months of mirror alignment gets under way

The first photons of starlight have travelled through the James Webb Space Telescope (JWST) and been detected by the Near Infrared Camera (NIRCam) instrument.

This is a milestone, albeit not yet the imagery expected once the alignment and commissioning of the observatory and its payloads is complete. It marks the transition of the JWST from technological tour de force to an actual telescope.

The first images expected from the JWST will likely be blurry as engineers gradually fine tune the telescope, matching expectations based on repeated rehearsals on Earth.

There are seven steps before the first crowd-pleasing pictures can be taken. With deployment complete, the spacecraft's star trackers are used to point the spacecraft while engineers tilt the 18 mirror segments to create an "image array" using a bright, isolated star (HD 84406) as the target.

The next step is Segment Alignment, where most of the "large" positioning errors of the mirror segments are corrected. Then Image Stacking occurs to ensure each segment image falls in precisely the same place to create one unified image.

Coarse and Fine Phasing come next. "The segments," explained NASA, "need to be lined up with each other with an accuracy smaller than the wavelength of the light." This is conducted three times, with a round of Fine Phasing following each round of Coarse Phasing.

The alignment is subsequently extended to the rest of the instruments before a final correction is done (although a few iterations of all the steps might be needed before completion) and the other instruments can be commissioned.

"To put this in perspective, if the Webb primary mirror were the size of the United States, each segment would be the size of Texas, and the team would need to line the height of those Texas-sized segments up with each other to an accuracy of about 1.5 inches," NASA said.

Definitely one for The Register's Online Standards Converter.

The whole process is expected to take approximately three months, after which the JWST's 18 primary mirror segments should work together as a single mirror and deliver the data scientists have been waiting years for. ®

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