Uses
Generally, a spin-cast paraboloid is not sufficiently accurate to permit its immediate use as a telescope mirror or lens, so it is corrected by computer-controlled grinding machines. The amount of grinding done, and the mass of glass material wasted, are much less than would have been required without spinning.
Spin casting can also be used, often with materials other than glass, to produce prototype paraboloids, such as spotlight reflectors or solar-energy concentrators, which do not need to be as exactly paraboloidal as telescope mirrors. Spin casting every paraboloid that is made would be too slow and costly, so the prototype is simply copied relatively quickly and cheaply and with adequate accuracy.
Liquid mirror telescopes have rotating mirrors that consist of a liquid metal such as mercury or a low-melting alloy of gallium. The mirrors do not solidify, but are used while liquid and rotating. The rotation shapes them into paraboloids that are accurate enough to be used as primary reflectors in telescopes. No correction of the shape is necessary. Spin-cast glass mirrors need correction because of distortions that arise during and after solidification.
Read more about this topic: Rotating Furnace