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CNN's Webb telescope's first test images include an unexpected 'selfie' says only:

The mirror selfie was captured by a special lens inside NIRCam that can image the primary mirror rather than what Webb sees in space. The lens mainly exists for engineering and alignment, so it was a bonus image that made the Webb team say "holy cow," said Lee Feinberg, Webb optical telescope element manager at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Question(s):

  1. How exactly did JWST take a "selfie" of its own primary mirror?
  2. What is the real purpose of this capability?
  3. "bonus points" Why is one mirror so much brighter?

This "selfie" shows the primary mirror of the James Webb Space Telescope as it orbits about a million miles from Earth (CNN, cropped)

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  • $\begingroup$ potentially some help here: blogs.nasa.gov/webb/2022/02/11/… $\endgroup$
    – uhoh
    Commented Feb 11, 2022 at 22:51
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    $\begingroup$ I also saw that and thought "how are they doing that!?" I think it's the Pupil Imaging Lens mentioned in the NIRcam optics page that's doing it - not sure how yet (suspect there is an SPIE paper(s) with more details) $\endgroup$
    – astrosnapper
    Commented Feb 11, 2022 at 23:30
  • $\begingroup$ I think the quote answers the first two, but I have no idea why one lens would be brighter $\endgroup$
    – Rory Alsop
    Commented Feb 12, 2022 at 16:12
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    $\begingroup$ @RoryAlsop The third question is answered in the image caption in that blog article linked in uhoh's comment: "... the bright segment was pointed at a bright star, while the others aren’t currently in the same alignment." $\endgroup$
    – uUnwY
    Commented Feb 12, 2022 at 22:14

1 Answer 1

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Let us just ask Lee Feinberg, the Webb Optical Telescope Element Manager:

How exactly did JWST take a "selfie" of its own primary mirror?

  1. In this video at 2:09:

There's actually a special lens in the Near-Infrared camera that you can put in and it allows you to take a picture of the primary mirror itself [...].

What is the real purpose of this capability?

  1. In the same video at 2:31:

[...] and we can see how well the primary mirror and the telescope is aligned to the instrument and that gives us some initial confidence that the alignment looks good. [...] We have now gotten some data looking through focus and we've been able to see that we don't see any surprises in the shapes of the mirrors that we are looking at.

I suppose that the purpose of the camera is to check the overall state of the telescope, whether some of the mirrors underwent major deformations.

Why is one mirror so much brighter?

  1. At 2:16:

[...] and in this particular case, one of the segments is pointing at a star. So that is the segment that lights up, [...].

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