Rotating furnace

A rotating furnace is a device used to create lenses or primary mirrors for optical telescopes. It was observed that the centrifugal-force-induced shape of a spinning liquid is approximately the same as the shape of a telescope's primary focusing mirror. So glass is placed in the furnace, the furnace is spun up and heated, the top surface of the melted glass takes on a concave shape, and the glass is cooled to solidification while still spinning. If the shape does not exactly represent a telescope mirror, it is then corrected by computer-controlled grinding machines. The net amount of grinding done, and the net mass of glass material used, is less than would have been required without spinning. Once solidified, the concave top surface can be silvered and used as a focussing mirror. (If centrifugal force & gravity are the main shaping forces, this would tend to create a concave top-surface of the lens, and the bottom surface would be shaped by the liquid's container much like a mold; only the top surface would be shaped by non-contact forces.)

The paraboloid of revolution is the natural concave shape of a spun-up liquid.
Reflective telescopes use primary mirrors which are parabolids, to focus collimated light from the far field onto a nearby point.

Very large-diameter focusing mirrors are also made this way with the permanently-liquid elemental metal mercury. The rotation is continuous and the metal remains liquid; once all waves and imbalances die out, the telescope is then used.

See also .