Webb mirror segments repositioned as alignment proceeds

Scientists and engineers at the Space Telescope Science Institute are methodically adjusting the orientations of the 18 segments making up the James Webb Space Telescope’s primary mirror, re-arranging them in a hexagonal “image array” before more adjustments to focus the reflections from each one.

The team is now working through phase two of the alignment process, correcting large orientation errors and updating the alignment of Webb’s secondary mirror to help tighten up the focus of starlight reflected from each segment. Once this “global alignment” is complete, the segments will be fine tuned to merge the reflections into a single sharply focused beam directed to Webb’s four science instruments.

The top frame shows initial “first light” reflections of a star from the James Webb Space Telescope’s 18 primary mirror segments. The bottom view shows how the segments have been repositioned prior to “image stacking” when each segment’s orientation will be tweaked to move and merge the reflections into a single beam. Images: NASA (top); NASA/STScI/J. DePasquale (bottom)

“We steer the segment dots into this array so that they have the same relative locations as the physical mirrors,” said Matthew Lallo, systems scientist and Telescopes Branch manager at the Space Telescope Science Institute.

“During global alignment and image stacking, this familiar arrangement gives the wavefront team an intuitive and natural way of visualising changes in the segment spots in the context of the entire primary mirror. We can now actually watch the primary mirror slowly form into its precise, intended shape.”

Two frames from a NASA animation illustrate Webb’s initial mirror segment alignment and the eventual alignment when all are precisely aimed to achieve a razor-sharp focus. Image: Astronomy Now/NASA

Since Webb’s Christmas Day launch two months ago, work to bring the $10 billion telescope to life has proceeded almost exactly as planned.

Its power, guidance and computer systems are working as expected and the telescope’s optics and instruments, in the shade of Webb’s tennis court-size sunshade, are cooling down to less than 50 degrees above absolute zero as required.

Another three to four months will be needed to complete mirror alignment and to carry out extensive tests and checkout of Webb’s science instrument. The team remains on track to release the first science images in the June timeframe.