Liquid Mirror Telescopes at Work

A mirror, three meters in diameter, made recently by the National Aeronautics and Space Administration (bottom left) will search for space debris. The diagram (top) shows how the mirror works. The mirror and bearing rest on a three-point mount; the axis of rotation is aligned vertically using two adjustable screws. A synchronous electric motor, connected by pulleys and a belt, drives the turntable under the mirror's container. An AC power supply, stabilized with a crystal oscillator, controls the motor.

A solid parabola made from a polyurethane resin is spin-cast over the basic container. After this plastic parabola has hardened, liquid mercury can be poured into the basin. Such simple designs make liquid mirrors relatively affordable and easy to construct. The rudimentary observatory (bottom right), built at Laval University in 1987, supported the first astronomical research performed using a liquid mirror. A mirror 2.7 meters in diameter would cost approximately U.S.$15,000 to assemble.

Homemade Telescope Liquid Mirror

LIQUID MIRROR, tended by graduate student Luc Girard, acts as the receiver for an atmospheric monitor. The 2.7-meter mirror is at the University of Western Ontario.

ror at an angle 7.5 degrees away from the central axis. Indeed, this pioneering work shows that it is possible to correct the severe aberrations introduced when light is reflected at a sharp angle from a fluid parabolic surface.

Optimized correctors face few limitations in theory. They should give liquid-mirror telescopes access to much of the visible sky, albeit over narrow fields of view, valuable for spectrosco-py or very high resolution imagery. On the practical side, Min Wang, Gilberto Moretto and I, in collaboration with Gerard Lemaitre of the Marseilles Observatory, are exploring novel adaptations of conventional corrective mirrors. This optical technology, pioneered by Lemaitre, is based on warping mirrors into complex shapes capable of removing reflective errors. Recently Wang, Moretto and I have designed on a computer such a high-performance corrector that uses two auxiliary mirrors. It can give excellent images in regions of the sky located as much as 22.5 degrees away from the central axis.

A holographic device could at least in theory serve as the perfect mediator, reconciling the differences between reflected light and its original source. A prerecorded hologram could be cast in the path of reflected light. As light passed through the hologram, it would filter out predictable errors. Mosaics of computergenerated holograms could compensate for aberrations that occur as light travels great distances from the zenith, over large fields of view. At Laval, Guy-lain Lemelin, Roger A. Lessard and I are exploring the promise of holographic correctors. Unfortunately, we find that practical versions of these instruments must await technological advances that we are only beginning to glimpse.

I am often asked if a liquid-mirror telescope could be placed in space. The possibility is intriguing because liquid mirrors offer excellent optical qualities, low masses and simple packaging. Despite inhospitable temperatures, the moon could certainly host a liquid-mirror telescope. Mirrors made from light gallium alloys, or perhaps even lighter alkali alloys, would remain liquid in lunar telescopes because such alloys have low melting temperatures. Until recently, though, I thought placing a liquid-mirror telescope in orbit would be impossible. Gravity supplies the necessary acceleration to form a parabola on the earth or the moon; an orbiting telescope free-falls and thus is not affected by gravity. Using an engine for acceleration would be impractical because the engine would eventually run out of fuel.

The potential of solar sail-powered crafts has changed my mind. In 1992, I published an article in the Astrophysical Journal that examines the plausibility of using solar sails to propel a liquid-mirror telescope in orbit. Our sun supplies an inexhaustible source of energy that a solar sail could harness to accelerate a liquid surface into a parabola. Although this concept may appear to belong more to science fiction than to science, it rests on reasonable assumptions. No solar sail-powered craft has ever been launched successfully, but a study carried out by nasa in the late 1970s showed that solar sail-powered crafts are feasible.

A mirror accelerated by solar sails would not gain momentum and leave the solar system, provided the vessel traveled more slowly than its proper orbital speed. The solar sail would then counteract just enough gravitational pull to keep the telescope in orbit. It could possibly replace all gravitation, yielding a stationary instrument capable of long integration times. Colin McInnes of the University of Glasgow has shown that crafts rigged to solar sails could navigate through a rich variety of paths and switch orbits mid-course. In this case, an orbiting liquid-mirror telescope could be pointed like a conventional, glass-mirror telescope. I have fearlessly considered placing mirrors having diameters as vast as one kilometer in space. It is staggering to imagine what scientists could discover with such gigantic mirrors.

In the June 1987 issue of Physics Today, associate editor Per H. Andersen wrote a news note entitled: "Will Future Astronomers Observe with Liquid Mirrors ?" Now, nearly seven years later, a handful of liquid-mirror telescopes have been built for research purposes. The next question is how many astronomers will make observations using liquid-mirror telescopes. Only the future knows, but at the very least, I expect that liquid-mirror telescopes will perform specialized astronomical tasks such as surveys. At the other extreme, I dream that one day liquid-mirror telescopes will support most astronomical research, relegating tiltable telescopes to specialized niches. That may seem preposterous, but I believe the simplicity and cost of liquid-mirror telescopes will be persuasive. In the meantime, amateur astronomers, engineers and all scientists alike should be aware of the capabilities of liquid mirrors. They may yet reveal greater wonders than did Lewis Carroll's looking glass.

FURTHER READING Liquid Mirror Telescopes: History. B. K. Gibson in Journal of the Royal Astronomical Society of Canada, Vol. 85, No. 4, pages 158-171; August 1991. The Case for Liquid Mirrors in Orbiting Telescopes. E. F. Borra in Astro-physical Journal, Vol. 392, No. 1, pages 375-383; June 10, 1992. Liquid Mirrors: Optical Shop Tests and Contributions to the Technology. E. F. Borra, R. Content, L. Girard, S. Szapiel, L. M. Tremblay and E. Boily in Astrophysical Journal, Vol. 393, No. 2, pages 829-847; July 10, 1992.

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Responses

  • Cirilla
    Can telescopes be made of gallium?
    7 years ago

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