Mirror and mirror substrate with high aspect ratio, and method and means for producing such a mirror substrate

An assembly comprising:

• a mirror substrate that has a material having a coefficient of mean linear thermal expansion of less than or equal to 1*10−6/K and at least one feature selected from a group consisting of:
• a ratio of a lateral dimension to a maximum thickness of at least 100, a ratio of the lateral dimension to the maximum thickness of at least 150, a ratio of the lateral dimension to the maximum thickness of at least 200, a ratio of the lateral dimension to the maximum thickness of at least 300, a weight per unit area of 100 kg/m2 or less, a weight per unit area of 50 kg/m2 or less, a weight per unit area of 30 kg/m2 or less, a weight per unit area of 15 kg/m2 or less, a mirror surface with a roughness (Ra) of at most 3.5 μm, and a mirror surface with a roughness (Ra) of less than 1.2 μm;
• and
• a support that supports the mirror substrate during processing, transport, or both processing and transport and comprises a material having a coefficient of mean linear thermal expansion of less than or equal to 1*10−6/K,
• wherein a surface of the support and a surface of the mirror substrate which is bearing on the surface of the support have opposite complementary curvatures,
• wherein the support has a surface on which the mirror substrate is bearing, the surface having a curved shape and being an envelope having a deviation of the support from the envelope that is not more than 0.5 mm when the mirror substrate has a lateral dimension of at least 4000 mm.