What is a liquid mirror telescope? It's a reflecting telescope whose primary mirror is made of liquid mercury! A reflecting telescope is a telescope which uses the focusing property of parabolas and paraboloids of revolution (namely, that all light rays traveling parallel to the symmetry axis will be reflected by the parabola or paraboloid to the focus) to concentrate and magnify light. A schematic of one type of reflecting telescope appears below.

A now-famous example of a reflecting telescope is the Hubble Telescope whose home is in outer space. The all-important reflective property of parabolas can be proved very simply and elegantly using vector calculus: See for yourself!

But how can a liquid be a parabolic mirror, and why would you want it to? The answer is that if you have a spinning container of liquid like the one pictured below, the surface of the liquid will form a perfect paraboloid, even if the inner surface of the container is imperfectly shaped. Thus there is no need for tedious lens-grinding, and the mirrors can theoretically be made larger than solid mirrors.

To see why this lucky phenomenon takes place, click here!

So how good a mirror does this process make? Very good! The photograph below shows a liquid mirror with a diameter of 2.5 meters and a focal length of 3 meters at Université Laval in Canada. Ph.D. student Luc Girard, Prof. Ermanno F. Borra and Dr. Robert Content have their images reflected in the mirror.

The spinning mirror in the picture is a very thin film of liquid mercury supported by a solid parabolic bowl. The solid bowl is made by a spin-casting process which makes use of the same paraboloid-producing phenomenon. The reason for using thin films is that mercury is very heavy: A liter of mercury (a pop bottle-full) weighs about 30 pounds! Liquid mirror telescopes are much cheaper than ordinary reflecting telescopes, and, as I mentioned above, they can theoretically be made much larger. For more information about liquid mirror telescopes, visit the Liquid Mirror Home Page at the Université Laval in Québec, and the Liquid Mirror Telescopes page at the University of British Columbia.

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