Figuring

[1] An example of figuring is that used in reflecting telescope primary mirrors in a process of converting the smooth spherical mirror produced by earlier stages into the aspherical or parabolic shapes needed to form the correct image.

It is done by applying different polishing stroke lengths with different sized and shaped tools.

For large mirrors, ion figuring is often used, in which a neutral atomic beam is used to remove material from the optics in a very controlled way.

[2] 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.