How is the paraboloidal shape formed?
The rough paraboloidal shape is formed when the mirror blank is cast in the spinning furnace. By spinning the furnace at the proper speed while the glass is molten, the surface takes on a paraboloidal shape. By the time the cooling process is complete, this surface is accurate to a small fraction of an inch. By utilizing the spin-casting technique, we avoid the need to grind tons of glass in the sagitta (or center) of the mirror. This method saves many tons of glass and years of grinding. A precise paraboloidal shape is generated (ground with a spinning tool impregnated with diamond particles) with a numerically-controlled milling machine. This procedure improves the surface accuracy to about 50 microns (0.002 inch). The final shape of the surface is produced by polishing with a lap using a very fine polishing compound. This shape is carefully polished to an accuracy of better than 25 nanometers (1.0 x 10-6 inch). • Why does the mirror surface have to be polished so accurately? The sur
