I read that there are A, B, and C type mirror segments on Webb. Why are there three types, and why are they distributed around the way they are?

I assume the segments have to overall form exact sections of a parabola in their respective positions, so I naively thought each section would be more symmetrical, not having just three optical formulas.

The question boils down to, why are there B and C segments?

Here is the official NASA page on this. And below is a Flickr image from NASA that maps out the different segments.

The James Webb Space Telescope's Mirrors

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    $\begingroup$ There's a type for each distance from the optical axis. What's not symmetrical? $\endgroup$ Jan 1 at 3:52
  • $\begingroup$ @ChristopherJamesHuff I guess some kind of statement about translational/rotational invariance in the context of tiling a sphere statement might work, except that it's a parabola. $\endgroup$
    – uhoh
    Jan 1 at 5:02
  • $\begingroup$ So the primary is a parabola? How come a true Cassegrain rather than a Ritchie-Christian? $\endgroup$
    – Vince 49
    Jan 1 at 6:15
  • $\begingroup$ There's a nice piece about the JWST mirror system, including how they are synchronized, how the 21 (yes 21) mirrors work together, etc. at jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-telescope (p.s. jwst is a Korsch telescope) $\endgroup$ Jan 1 at 6:41

2 Answers 2


The James Webb Space Telescope is a three-mirror anastigmat featuring an ellipsoidal primary, hyperboloidal secondary, and ellipsoidal tertiary.

Contreras, James W.; Lightsey, Paul A. (22 October 2004). "Optical design and analysis of the James Webb Space Telescope: optical telescope element". Conference Proceedings of the SPIE. 5524. doi:10.1117/12.559871. It's paywalled there but available here

As @uhoh answered they vary by necessity from the main axis.

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    $\begingroup$ Welcome to Stack Exchange! I've added some links to thie item(s) you've mentioned. The only part of your answer post that currently addresses "Why does the optical design of the Webb mirror have three different types of panel?" is the pointer to another answer. Can you locate some information in your linked paper and quote it here? Otherwise it's either not an answer, or a link-only answer. Thanks! $\endgroup$
    – uhoh
    Jan 22 at 1:23

One might also ask Why do telescopes use hexagonal mirror pieces instead of pie slice shaped ones? since in that case all segments would be identical and perhaps allow for interesting alternatives in fairing-packing and unfolding pattern.

Of course it would pose other challenges.

I know this one because I spend all day every day thinking about hexagonal tilings. (1, 2, 3, 4)

1. Tiling the "sphere" and keeping uniform gaps between elements means the "hexagonal" shapes are not hexagonal and must differ

It turns out that you can not tile a sphere (or a parabola or ellipsoid or other primary mirror figure1) with hexagons. For a sphere you need at least regular 12 pentagons plus 0 or more regular hexagons.

1hat tip to @leftaroundabout for setting me straight

2. The shapes of the surfaces differ as well.

The primary of the JWST is not spherical (and it seems not even parabolic1), so mirrors at different distances from the axis will have different shapes.

The mirror has two hexagonal rings, the center or (0, 0) hexagon is completely missing. It could have been included with a hole through the center (Cassegrain) but that would have added a fourth kind of mirror so it was a lot more complexity for a small increase in light (and small reduction in the complexity of the point spread function due to diffraction by the hole).

However the 2nd ring has six corner and six side elements, which differ not only in their distance from the axis but in their orientation. For the corner mirrors the line from the mirror center to the optical axis passes through an edge, while for the "side" mirrors it passes through a corner.

So you have three different off-axis parabolic shapes of mirrors that have to be made:

  1. first ring (six)
  2. second ring corners (six)
  3. second ring sides (six)

From this answer to How would NASA confirm the James Webb Space Telescope is undamaged after the clamp release incident?:

Three kinds of off-axis parabolic hexagons (A, B, C):

From JWST.NASA.gov's Webb's Mirrors





  • $\begingroup$ They are still hexagons, just not perfect regular hexagons. $\endgroup$ Jan 1 at 11:59
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    $\begingroup$ @PaŭloEbermann It's a good point. I did write "For a sphere you need at least regular 12 pentagons plus 0 or more regular hexagons." I don't think there are any sentences that are wrong as written (please let me know if there are) but I don't know how to prove that the mirrors can't be the same shapes and have constant uniform gaps. $\endgroup$
    – uhoh
    Jan 1 at 16:33
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    $\begingroup$ “The primary of the JWST is parabolic not spherical” – actually it seems to be ellipsoidal. That would at least be the standard Korsch design. $\endgroup$ Jan 1 at 22:50
  • $\begingroup$ @leftaroundabout yikes! I did have a momentary worry when I wrote that, it seems I should have paid more attention. I'll make a quick patch to the answer now then go see if I can find a definitive source for the mirror's exact figure. Thanks! $\endgroup$
    – uhoh
    Jan 1 at 23:11
  • $\begingroup$ RE hexagonal shapes are not hexagonal and must differ – I'm intrigued, how do they actually differ? $\endgroup$
    – ymb1
    Jan 3 at 15:39

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