Figuring

Figuringis the process of finalpolishingof an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.

Polishing and figuring of theHubble Space Telescope's primary mirror

Types

An example of figuring is that used inreflecting telescopeprimary mirrorsin a process of converting the smooth spherical mirror produced by earlier stages into the aspherical orparabolicshapes needed to form the correct image. It is done by applying different polishing stroke lengths with different sized and shaped tools. Manual figuring is a very laborious process, since the heat produced by polishing has to be allowed to dissipate before the shape of the mirror can be measured again, and the places for later polishing selected. Testing of the figure is usually done by aFoucault knife-edge testorRonchi testinamateur telescope makingand with very sophisticatednull testerson research telescope optics.

For large mirrors,ion figuringis often used, in which a neutralatomic beamis used to remove material from the optics in a very controlled way.This is particularly useful in the manufacture ofsegmented mirrors,since the shape of the optics can be maintained correctly all the way to the edge of the aperture, whilst mechanical polishing techniques tend to have trouble with distortion of the polishing tool when it overhangs the edge. The first major use of ion figuring was in making the mirror segments for theKeck telescope.

The ultra-high precision requirements for optical surfaces forX-ray astronomyand deep-ultravioletlithographyoften require ion figuring.

Issues for ion beam figuring included re-sputtering and contamination.