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Mirror alignment is measured using a laser reflected off the mirror segments into an interferometer. https://www.nasa.gov/feature/goddard/2017/aligning-the-pri

• nomel 5y ago
  It appears that a star is used, while in space, with the laser being used as an artificial star. I only see this directly mentioned in the video (1:24), and indirectly in the text, "thus acting like a source of artificial starlight". I guess this makes sense, as it's the only way to do it "end-to-end" while in space.
  • simonh 5y ago
    There are actually two stages of calibration in space. The first stage is aligning the mirrors, using direct measurement of the mirror alignment with the interferometer, after they are unfolded from their unaligned stowed positions. > To measure the shape of the Webb telescope’s primary mirror, engineers use a test device called an interferometer, which shines a laser down onto the mirror. The next phase you’re referring to also uses a laser, but this time it’s shone off the mirrors into the actual scientific observation sensors, hence they describe it as being used as a source of artificial starlight.
    • deleted 5y ago
      [deleted]
    • nomel 5y ago
      I don't see anything that suggests a laser is being used in space. At 1:24 in the video, it says starlight is used. Only stars are mentioned in the press releases: https://www.universetoday.com/154623/heres-exactly-how-engineers-are-aligning-jwsts-segmented-mirror https://www.universetoday.com/154623/heres-exactly-how-engin... On the ground, you can easily place a laser in an optically identical position as a star (near infinity). In space, I don't think this would be possible, since it would require that the laser, and the optics that would be required, would all be within a fraction of a wavelength of the true optical path. In the end, you would still need to align to a star anyways. Interferometry works just fine with starlight.
      • simonh 5y ago
        You may be right.