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I found this image on space.com and it can also be found in NASA's JWST blog. This is one step in the process of aligning the 18 mirrors on JWST. A single relatively-isolated star has been selected. At this point, the orientation of each mirror has been adjusted so that the image it forms falls at a position on the Near Infrared Camera (NIRCam) sensor that corresponds directly to the position of the mirror in the array.

image from JWST during the alignment process

This early Webb alignment image, with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, is called an “image array.” Credit: NASA/STScI/J. DePasquale

This question is a follow up to the earlier question about the mirrors' actuators. Apparently the position and orientation of each mirror segment can be adjusted in all six degrees of freedom using a hexapod actuator. In addition, one more actuator is available to make small changes to the spherical curvature component (focal-length).

The picture above is the second in a sequence of three pictures. This first shows the 18 images wildly scattered. The third picture shows the same array but with each of the 18 images of the star now almost perfectly focussed.

I have two questions:

  1. Several of the 18 images show a lot of astigmatism (ellipses rather than circles) which would require cylindrical correction. But there is no obvious means of providing this. How is this achieved?
  2. Some of the 18 images show marked diffraction fringes - how do these arise?
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    $\begingroup$ I’m voting to close this question because this is better suited to space.SE $\endgroup$
    – Carl Witthoft
    Mar 11 at 13:47
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    $\begingroup$ Diffraction occurs any time you have an aperture - be it positive or negative (see "Spot of Arago"). $\endgroup$
    – Carl Witthoft
    Mar 11 at 13:47
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    $\begingroup$ @CarlWitthoft Is space.SE a better place to ask questions about telescopes? $\endgroup$
    – Roger Wood
    Mar 11 at 19:30
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    $\begingroup$ @CarlWitthoft Suppose the question was instead asked about how a multisegment ground-based telescope such as the Thirty Meter Telescope will be brought into alignment and focus. Such a question would be off-topic at space.SE but on-topic here. This is essentially the same question applied to a telescope in space. Questions about astronomical equipment, whether on Earth or in space, are on-topic at this site. This question is on topic at this site. $\endgroup$
    – David Hammen
    Mar 12 at 14:10
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    $\begingroup$ @RogerWood if you'd like to go down a rabbit hole there's this JWST PSF toolbox but in the mean time I think I can try to simulate those patterns. $\endgroup$
    – uhoh
    Mar 13 at 7:05

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