Why does the mirror surface have to be polished so accurately?
The surface of a telescope mirror must be polished to its precise paraboloidal shape within approximately 1/25 of the wavelength of light. For typical blue light, that means a surface accuracy of order 15-20 nanometers (less than 1.0 x 10-6 inch). Any small scale roughness (the lack of a good polish) will cause the light to be scattered and result in reduced contrast. Inaccuracies on larger scales, such as bending of the entire mirror, can result in an inability to focus the light into sharp images. The value of these large mirrors in telescopes is to collect lots of light from very faint astronomical objects and to focus it into very sharp images. • How are these very large and fragile pieces of glass moved? The castings are first lifted out of the oven with a lifting fixture that has 36 steel pads which are glued to the front surface of the mirror with RTV silicone rubber. An overhead crane in the casting lab sets the lifting fixture into the turning ring so the mirror can be rotated
