The JWST has four mirrors. enter image description here

  • The large primary mirror made of 18 hexagonal mirrors
  • The secondary mirror
  • The fixed tertiary mirror
  • The flat fine steering mirror

I found very little information about the curvature of these mirrors. Obviously the primary mirror is concave, the secondary mirror seems to be convex.

Which of these mirrors are spherical, parabolic, hyperbolic, ellipsoidal, flat, aspheric, concave or convex?

  • $\begingroup$ There's some clues in this comment $\endgroup$
    – uhoh
    Jan 13 at 13:08
  • 1
    $\begingroup$ Wikipedia says the design is a three-mirror anastigmat with ellipsoidal primary & tertiary mirrors and a hyperbolic secondary. It cites these conference proceedings for this claim. Unfortunately, I don't have access to it or I would write up a definitive answer; anyone who does have access to the article should feel free to do so. $\endgroup$ Jan 13 at 16:21

Some information about the curvature is to be found here.

The JWST is a Korsch telescope.

A Korsch telescope is corrected for spherical aberration, coma, astigmatism, and field curvature and can have a wide field of view while ensuring that there is little stray light in the focal plane.

Korsch telescopes use a

  • concave ellipsoidal primary mirror
  • convex hyperbolic secondary mirror
  • concave ellipsoidal tertiary mirror

The fine steering mirror of the JWST is flat.

So all JWST mirrors are aspheric, none is parabolic. If the whole main mirror is ellipsoidal, each segment is ellipsoidal too.


To add a smidge to Uwe's answer, JWST is often described as a three-mirror anastigmat (TMA), and Korsch telescopes are a certain type of TMA. TMA designs are widely used in spaceborne imaging systems.


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