The first photons of starlight passed through the James Webb Space Telescope (JWST) and were detected by the Near Infrared Camera (NIRCam) instrument.
This is a significant milestone, but not yet the expected images once the alignment and commissioning of the observatory and its payloads is complete. It marks the transition of the JWST from a technological tour de force to a real telescope.
The first images expected from the JWST will likely be blurry as engineers gradually refine the telescope, matching expectations based on repeated rehearsals on Earth.
There are seven steps before the first crowd-pleasing photos can be taken. After deployment is 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 “big” mirror segment positioning errors are corrected. Next, image stacking occurs to ensure that each segment image falls in exactly the same place to create a unified image.
Coarse phasing and fine phasing come next. “The segments,” NASA explained, “must be aligned with each other with an accuracy of less than the wavelength of light.” This is done three times, with a round of Fine Phasing after each round of Coarse Phasing.
The alignment is then extended to the rest of the instruments before a final correction is made (although a few iterations of all the steps may be needed before completion) and the other instruments can be put into service.
“To put that into perspective, if the main Webb mirror was the size of the United States, each segment would be the size of Texas, and the team would need to line up the height of those Texas-sized segments with each other. the others for an accuracy of about 1.5 inches,” NASA said.
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The whole process is expected to take about 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 for for years. ®