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.

A turntable from a stereo system is the ideal device on which to spin the liquid. Be sure to remove the phonograph cartridge. The record player does not have to be of the highest quality- we have found that minor variations in turntable speed do not seriously affect the mirror. Those who have switched to compact discs should be able to find a secondhand turntable without too much trouble or expense.

The container to hold the spinning epoxy can be anything, even a common dinner plate, but we like to use food storage containers made of polyethylene or polypropylene. Such plastic containers are practical because the epoxy does not stick to the surface very well. The spun mirror pops out easily after the epoxy has cured. The container must have enough depth so the liquid does not spill over the sides during rotation. It can be any diameter so long as it can spin freely on the turntable. Be aware, however, that a majority of turntables operate only when the tonearm is cued over the edge of the platter. To provide clearance for the tonearm, you may need to choose a container that has a diameter slightly less than that of the platter.

The best epoxy is one that has the lowest viscosity and longest setting time. (Viscosity is the thickness or runniness of a liquid.) The epoxy should flow at least as well as warm maple syrup. The epoxies we tried were type CRS-107 from Custom Resin Systems in Carver, Mass., and Stycast 1266 clear epoxy from Emerson & Cuming in Canton, Mass. These two varieties are by no means the best or only choices. The number of epoxies available is truly astounding, so check the local hardware store or look under "epoxy" or "resins" in the yellow pages for a suitable one. You will probably need no more than about 0.1 liter (several fluid ounces) for each mirror.

As for setting time, we recommend choosing epoxies that have a one- or two-day cure period if you plan to spin mirrors larger than about 30 centimeters in diameter. Smaller mirrors should be made from epoxies that cure in no less than four to six hours. Do not use the quick-drying, "five-minute" epoxy.

Epoxy usually comes in two separate containers: one for the resin and the other for the hardener. They must be thoroughly mixed so that the resulting liquid is uniform. Follow the package instructions to determine the right proportions.

Epoxy is generally safe to use, but some caution should be exercised. The chemicals involved are rather nasty; gloves and good ventilation are required. Also, you may feel some warmth coming from the epoxy because the chemicals give off heat as they cure. If you intend to spin several mirrors and want to prepare large amounts, mix the epoxy in batches of no more than a liter at a time and keep the batches separate. It is possible that larger batches could smoke or even catch fire. (The fast-drying epoxies are the most susceptible types, one of the reasons to avoid them.)

To determine exactly how much epoxy to use, spin some water in the container first. Pour in just enough so that it wets the container but does not spill out during rotation. Make sure the container is centered on the platter. If necessary, adjust the tilt of the turntable so that the water spreads uniformly across the surface. Measure the amount of water used and mix that much epoxy. Be sure to dry the inside of the container completely before pouring in the epoxy, because moisture prevents many epoxies from curing properly.

The turntable must spin until the epoxy has cured. It is important to use a "windscreen," that is, a plastic wrap over the spinning liquid. The breeze generated by the rotation can cause ripples to form on the surface. If you do find defects in the dried epoxy, just repeat the procedure with another coat. In fact, we have found that two thin coats produce a smoother surface than does one thick coat of the same volume

The cured epoxy has the stiffness of a hard piece of plastic, like an acrylic The focal length will be proportional to the square of the rotation period. A turntable speed of 45 revolutions per minute (rpm) will yield a focal length of about 22.1 centimeters. A speed of 33 rpm gives 41.1 centimeters. A heavy container may make the turntable speed somewhat inconsistent and affect the focal length.

The surface can be made reflective through chemical silvering, a technique popularized by amateur telescope makers. You can ship your cured epoxy to a silver-plating company. The prices such shops charge vary greatly, ranging from a few dollars to about $70 per square foot. Alternatively, you can do it your self. Peacock Laboratories in Philadelphia offers the chemicals and detailed instructions, although the kits may be more appropriate to group projects. Each kit sells for just under $300 and provides enough material to coat 300 to 500 square feet of mirror. Like epoxy, these chemicals are somewhat noxious, so gloves and good ventilation are again recommended. The single most important requirement in obtaining a good silvering is a clean surface. Follow the cleaning instructions given in the kit; you will need distilled and demineralized water for the final rinse.

Note that some epoxies are easier to silverplate than others. If at first you don't succeed, try another epoxy. Of the two epoxies we mentioned, the Stycast was the most amenable. If you do not wish to take the trouble of silvering, you can try aluminized tape, which is quite reflective and is easy to apply.

Your silvered epoxy dish can serve as the primary mirror in a Newtonian reflector telescope. You will need a small, flat secondary mirror to angle the light rays out of the tube. The images formed by the spun mirror are reasonably good, although an amateur telescope maker would find them barely acceptable. The spun mirror works better as an infrared light collector. With a moderately sized spun mirror pointed at the sun, paper or wood can be ignited instantly at the focal point. You can record infrared images by coupling the system to an infrared camera [see "Seeing Infrared," by Donald G. Mooney, "The Amateur Scientist"; SCIENTIFIC AMERICAN, March 1992] or to

an ordinary camera with infrared film. The spun-mirror technique lends itself to several variations. You can create an "off-axis" parabolic mirror by placing the container slightly off the center of the turntable platter. The result is a mirror that is sensitive to microwave radiation. Materials other than epoxy, including urethanes, acrylics and silicone compounds, can yield good parabolic surfaces as well. To vary the focal length, slow down the turntable or experiment with other types of rotational devices.

Bibliography

AMATEUR TELESCOPE MAKING. Fourth edition. Albert G. Ingalls. Scientific American Publishing Company, 1935.

PROCEDURES IN EXPERIMENTAL PHYSICS. John D. Strong. Prentice-Hall, 1938.

OPTICAL SHOP TESTING. Edited by D. Malacara. John Wiley & 80ns, 1992.

LARGE OFF-AXIS EPOXY PARABOLOIDS FOR METRIC TELESCOPES AND OPTICAL LIGHT COLLECTORS. Donald L. Alvarez, Mark Dragovan and Giles Novak in Review of Scientific Instruments, Vol. 64, No. 1, pages 261-262; January 1993.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.