Hubble's Primary Mirror has the Wrong Shape

When the two principal mirrors of the Hubble Space Telescope were being ground, polished and coated a decade ago, great pains were taken to ensure that their surfaces not deviate from the desired hyperboloids of revolution by more than 100 angstroms. (See the article by C. R. O'Dell in PHYSICS TODAY, April, page 32.) In a sense, these exacting requirements have been met. The star images sent back by the orbiting observatory appear to confirm that the HST's 2.4‐meter concave primary mirror and the 30‐cm convex secondary are as close to being perfect, axially symmetric hyperboloids as one could wish.