This is one of two nearly identical main mirrors built by Corning for the Hubble Space Telescope. The mirror installed on the Hubble was finished by Perkin-Elmer Corporation using computerized techniques. This backup mirror was finished by Eastman Kodak, which used conventional optical techniques to shape and polish the mirror.

Facts:

How big is it? This mirror measures 2.5 meters (98 inches) across and weighs 748 kilograms (1,650 pounds). The useable surface of the mirror in the Hubble was slightly smaller-about 2.4 meters (94 inches)-because the mirror mounting covered the outer edge.
Why doesn't it look like a mirror? This mirror was never used, so it never received a reflective coating. The mirror in the Hubble was coated with a thin layer of aluminum and also overcoated with magnesium fluoride, so it could better reflect ultraviolet light.
What is it made of? The mirror is made of Corning ultra-low expansion glass. The front and back surfaces are fused to a lattice core and to the inner and outer bands, creating a sturdy but lightweight structure.