# Curvature of the JWST mirrors

The JWST has four mirrors.

• 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?

• There's some clues in this comment
– uhoh
Commented Jan 13, 2022 at 13:08
• 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. Commented Jan 13, 2022 at 16:21

To also add to Uwe's answer, JWST's primary mirror is very close to being parabolic with an eccentricity of ~0.99835. The primary mirror is also designed so that the Radius of Curvature is 15.88m at its operational temperature.

Source:

• Stahl, H. Philip. "Testing the James Webb Space Telescope Primary Mirror," March 5, 2014 (NTRS ID: 20140008581)
• That's helpful! Strictly, I'm not sure 'radius of curvature' is quite the right term for a paraboloid. I suppose it refers to the curvature at the center of the mirror? "Focal length" might be a better descriptor although it's not a perfect parabola? That would be the 7.94 m which is presumably a bit more than the distance from primary to secondary mirrors. Commented Mar 14, 2022 at 23:22

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.

• Korsch's patent can be found e.g. here. Commented May 3, 2022 at 15:39
• Any idea whether the ellipsoid is only chosen as an approximation of the (presumably) more difficult to produce paraboloid, or is it indeed optimal, in conjunction with the other two mirrors? Commented May 3, 2022 at 15:43

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.