A disk of glass or other suitable material which is to be turned into a mirror. It may be cast as a disk or cut from a sheet. The ratio of diameter to thickness is usually at least 6 : 1 so that the mirror will not deform. Low-expansion glass such as Pyrex is advisable, to prevent distortion of the finished mirror's precise curve as a result of temperature changes.
Subjects: Astronomy and Astrophysics.