https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirror_making https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirro... "The mirrors start as a fl

https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirror_making https://en.wikipedia.org/wiki/Amateur_telescope_making#Mirro... "The mirrors start as a flat disk of glass, typically plate glass or borosilicate glass (Pyrex).[4] The disk is carefully ground, polished and figured to an extremely accurate shape, usually a paraboloid... The tools used to achieve this shape can be simple... Through a whole series of random strokes the mirror naturally tends to become spherical in shape. At that point, a variation in polishing strokes is typically used to create and perfect the desired paraboloidal shape.