altOptical Surfaces (Surrey, UK) offers standard and custom optical flats measuring up to 600 mm in diameter and manufactured of materials such as glass, Zerodur, and silica. The optical flats can be used for measuring surface flatness of polished areas by determining the variations between work surfaces and the surface of the optical flat. They are suitable for applications in astronomy, laser beam steering, inspecting gauge blocks for wear and accuracy, and interferometric flatness testing of prisms, filters, and optical windows. Surface accuracy of up to lambda/20 p.v. can be achieved, as well as surface roughness of 10 A rms on individual flats. Optics up to 450 mm in diameter are provided with a Fizeau interferometric test report; larger flats are quality assured using the Ritchey-Common test procedure.