The Amateur Scientist; February 1994; Scientific American Magazine; by Dragovan, Alvarez; 2 Page(s)

When a liquid is spun in a container, the surface of the liquid does not remain flat but takes on a concave, parabolic shape. The parabola, of course, is the basis for the primary mirrors in optical telescopes because the shape will focus parallel light rays to a point. Investigators have exploited the rotational effect in fluids to make large telescope mirrors from liquid metals, primarily mercury. The parabolic shape remains as long as the liquid is kept spinning. Liquid mirrors made from mercury have been shown to have an optical quality near their ground-glass cousins, while having the advantage of being lighter and less expensive [see "Liquid Mirrors, " by Ermanno F. Borra, page 68].

A simpler variation of this procedure, which can be done at home, is to use a liquid that hardens, so that the parabolic surface is preserved after the spinning has ceased. Epoxy fits the bill. Consisting of a mixture of a resin and a hardener, epoxy slowly cures to a solid with little change in shape. The resulting mirror cannot compete against aluminized glass mirrors for image quality; it is perhaps best suited as a collector of light at the infrared and microwave end of the spectrum. We have relied on this method to produce an inexpensive mirror the size of a satellite dish to be used for observations of the cosmic microwave background radiation.