Rotating furnace

A rotating furnace is a device for making solid objects which have concave surfaces that are segments of axially symmetrical paraboloids.

The furnace makes use of the fact, already known to Newton, that the centrifugal-force-induced shape of the top surface of a spinning liquid is a concave paraboloid, identical to the shape of a reflecting telescope's primary focusing mirror.

Paraboloids can be used in various ways, including (after being silvered) as primary mirrors in reflecting telescopes and solar cookers.

[1] The furnace includes a mechanism that rotates an open-topped container at constant speed around a vertical axis.

When the glass is solid, the rotation is stopped and the paraboloidal shape of its top surface is preserved.

The other surface is shaped by the container that holds the molten glass acting as a mold.

The focal length of the paraboloid is related to the angular speed at which the liquid is rotated by the equation:

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 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 rotation shapes them into paraboloids that are accurate enough to be used as primary reflectors in telescopes.