A fine layer of aluminum is deposited on the glass to create a reflective surface. A technique that reduces some of the time needed to grind a mirror to shape was developed in the 1990s. The glass is spun into the desired shape while it is still molten. Rotational forces move some of the glass toward the edge of the spinning container and into a shape called a paraboloid. After it cools, a spin-cast mirror does not require laborious grinding to remove excess glass.
Astronomers seek ever-larger mirrors to increase the power and efficiency of telescopes.
However, huge mirrors are expensive and difficult to make, and they are challenging to move while tracking celestial targets. One particularly daunting problem is that a solid glass mirror is heavy. Hale telescope on California's Mount Palomar weighs 14 tons. Next Page Return to Cosmiverse Reference Library Microsoft Corporation. All Rights Reserved.