Figuring

Figuring is the process of final polishing of an optical surface to achieve the shape required for a given application. The standard example in amateur telescope making is the process of converting the smooth spherical mirror produced by earlier stages into the parabolic mirror required for a Newtonian telescope.

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.

For large mirrors, ion figuring is often used, in which a beam of neutral atoms is used to remove material from the optics in a very controlled way. [http://www.ssd.itt.com/capabilities/ion-figuring.shtml] This is particularly useful in the manufacture of segmented 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 the Keck telescope.

The ultra-high precision requirements for optical surfaces for X-ray astronomy and deep-ultraviolet lithography often require ion figuring.