Glass ceramic having a low thermal expansion

A method for producing an optical component comprising the steps of:

• providing a substrate for an optical element which comprises a glass ceramic which has a coefficient of thermal expansion CTE of from −0.5×10−6/K to +0.5×10−6/K in a temperature range of from 0 to 50° C.,
• superpolishing the surface of the substrate to a surface roughness HSFR of at most 0.5 nm rms, and
• processing the superpolished surface by a beam processing method to a fine surface figure error of at most 0.5 nm rms and a surface roughness MSFR of at most 0.5 nm rms, the surface roughness HSFR being substantially undeteriorated even after said processing has ended so as to still be at most 0.5 nm rms;
• wherein said glass ceramic is free of BaO, free of arsenic, and has a composition, in percent by weight based on oxide content, comprising:
• SiO2
• 50–70
• Al2O3
• 17–32
• P2O5
• 3–12
• Li2O
• 2.5–5
• Na2O
• 0–2
• K2O
• 0–2
• MgO
• 0–2
• CaO
• 0.1–4
• SrO
• 0–2
• ZnO
• 0–4
• TiO2
• 1.5–5
• ZrO2
• 0–2.5.

A glass ceramic having a coefficient of thermal expansion CTE of from −0.5×10−6/K to +0.5×10−6/K in a temperature range of from 0 to 50° C., and which is free of BaO, free of arsenic and which has a composition, in % by weight based on oxide content, comprising:

• SiO2
• 50–70
• Al2O3
• 17–32
• P2O5
• 3–12
• Li2O
• 2.5–5
• Na2O
• 0–2
• K2O
• 0–2
• MgO
• 0–2
• CaO
• 0.1–4
• SrO
• 0–2
• ZnO
• 0–4
• TiO2
• 1.5–5
• ZrO2
• 0–2.5.