Rotating furnace

A rotating furnace is a special type of furnace used to make lenses or mirrors for telescopes. Glass is placed in the furnace which is then spun up and heated. The top surface of the melted glass takes on a concave shape. The glass is cooled while still spinning. The spinning makes the glass the same shape as the main mirror used in a telescope. Computer controlled grinding machines can correct the shape of the glass. The concave top surface can be coated with silver and be used as a focusing mirror. The shape of bottom surface is made by the container the glass was melted in. The technology is being used for the Giant Magellan Telescope's seven mirrors. 
Large focusing mirrors can also be made using liquid mercury, such as the Large Zenith Telescope. The rotation is continuous and the metal remains liquid; once all waves and imbalances die out, the telescope is then used.