The James Webb space telescope first image showing the same star 18 times with the unaligned mirrors is great. How do they tell which image corresponds to which mirror?

see Photons Received: Webb Sees Its First Star – 18 Times

  • $\begingroup$ They probably don't know. They'll move a mirror and see which dot moves. $\endgroup$ Feb 12, 2022 at 21:09
  • $\begingroup$ They do know. The video shows the 18 spots, each labelled with a the mirror that produced it. $\endgroup$
    – James K
    Feb 12, 2022 at 21:51
  • 2
    $\begingroup$ You're both right! First they don't know, then they move mirrors by unique amounts and directions, then they take a second image, then they do know. $\endgroup$
    – uhoh
    Feb 12, 2022 at 22:41

1 Answer 1


How do they tell which image corresponds to which mirror?

The JWST team did this commanding each mirror segment to reorient itself by a bit, one segment at a time. The changes in the captured imagery provides the information needed to identify which mirror segment was responsible for which image.

The initial mosaic had eighteen different images of the target star. This was fortunate. Suppose it only had seventeen images. This could have happened for one of two reasons: Two of the mirrors happened to be aligned with one another, or one of the eighteen was so far out of alignment that its image was outside of the NIRCam field of view. The one segment at a time movement scheme would have quickly resolved the first possibility. (Note that this was close to the case with mirror segments A1 and B1.) The JWST team had arduous plans in the case that the missing image was due to a segment that was so out of alignment that that segment's image did not fall within the NIRCam field of view. The team did not have to go through this much longer process.

Now that each mirror segment has been identified, the next step is to bring all of the segments into alignment. This will be done slowly because the alignment process is a bit non-linear. After that has been accomplished, the steps after that will address focus, astigmatism, and phase. Despite the fortunate fact that all eighteen segments have already been identified, the entire alignment process will still take multiple months.

  • $\begingroup$ Even if the initial alignment algo is linear, solving roughly a 18X18 matrix by way of perturbation is not easy :-) $\endgroup$ Feb 14, 2022 at 16:10

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