Liquid Mirrors; February 1994; Scientific American Magazine; by Borra; 6 Page(s)

For almost four centuries, reflecting telescopes have gathered the shimmering light from billions of stars and galaxies. The observations they made possible have revealed a universe vast and complex. In doing so, these instruments have rescaled our world and our imaginations. And yet, for all their success, classical reflector technology has some severe limitations. It is very expensive, and often nearly impossible, to grind and polish a massive glass surface into a perfect parabola, the ideal contour for a device that focuses parallel rays of light into a single point. Such mirrors can deform during temperature changes, and beyond a certain size they are prone to buckle under their own weight.

Astronomers and optical experts have therefore sometimes toyed with an old, even bizarre technological alternative: a mirror made from liquid. A liquid mirror could never sag and so conceivably could be made as large as purpose requires. Moreover, coaxing a liquid into the shape of a parabola is trivial. The pull of gravitational and centrifugal forces shapes the surface of a rotating mass of reflecting liquid, such as mercury, into a perfect parabola. This phenomenon, which also occurs when you stir your coffee, provides a superb optical surface that needs no polishing. As a result, liquid mirrors could be far less expensive than glass mirrors. The potential reduction in cost could render large optics affordable even for the amateur astronomer [see "Making a Mirror by Spinning a Liquid," by Mark Dragovan and Don Alvarez, "Amateur Scientist," page 116]. In addition, because optics are so central to many scientific measurements, liquid mirrors could prove useful in many research fields and engineering as well.