A mirror substrate is made of a material having a coefficient of mean linear thermal expansion of less than or equal to 1*10.sup.6/K. The mirror substrate includes at least one of: a ratio of a lateral dimension to a maximum thickness of at least 100,, a weight per unit area of 100 kg/m.sup.2 or less,, and a mirror surface with a roughness (R.sub.a) of at most 3.5 m.