An old idea for astronomical imaging is undergoing a technology-driven renaissance
Nestled in the mountains of southwestern British Columbia is a nondescript building that might easily be mistaken for a church or a ski chalet. Little about its appearance would suggest that hidden within is one of the largest astronomical telescopes in North America. Yet on clear nights, the steeply sloping roof rolls back, allowing the telescope to record images of the stars and galaxies passing overhead. More remarkable, however, is the fact that the heart of this telescope—the primary mirror, which collects and focuses light—is a rotating dish of liquid mercury.
Conventional astronomical telescopes, of course, employ glass mirrors for this purpose. The largest, the Keck twin 10-meter telescopes atop Mauna Kea in Hawaii, actually use segmented primary mirrors, each composed of 36 hexagonal elements. These mirrors must be carefully ground and polished, to an accuracy of a few tens of nanometers, before being coated with a thin layer of aluminum or silver to make them reflective. Such mirrors also require a complex support system to prevent temperature changes or the force of gravity from distorting the surface. And for most large, modern telescopes, a system of sensors and actuators actively controls the shape of the mirror on a fine scale so as to counteract the distortions created by the atmosphere, a strategy dubbed "adaptive optics." These instruments are technological marvels, but they cost an enormous amount to build, roughly $10 million for one with a 6-meter-diameter mirror. Amazingly, comparable precision can be achieved simply by rotating a dish covered with mercury.
The principle is elementary and known to all first-year physics students. The surface of a liquid in equilibrium is a surface of constant potential energy. (Any variation of the potential along the surface would constitute a force, which would cause the liquid to flow). Normally, the potential energy of an object is just proportional to its height. So the surfaces of most liquids are flat or essentially so. But suppose that one rotates a liquid at a constant angular speed about a vertical axis. The potential energy of any tiny parcel of fluid now has two components, one that increases with height and another that decreases with distance from the rotation axis—or rather, with the square of that distance. That particular combination of dependencies makes the surface take on the shape of a paraboloid.
By happy coincidence, a paraboloid is exactly what is needed to focus light. Incident rays that are parallel to the axis are reflected so that they come together at one spot, the focal point of the mirror. Parallel rays arriving from other directions are not focused so perfectly. But with the addition of three or four lenses placed close to the focal point, good image quality can be obtained over an extended field of view. Most large astronomical telescopes thus employ parabolic (or nearly parabolic) primary mirrors and secondary mirrors or lenses to correct off-axis aberrations.
The idea of using a rotating liquid to focus light is an old one. The Italian astronomer Ernesto Capocci of the Osservatorio di Capodimonte in Naples was the first to describe this possibility in print, in 1850, although he never put the idea into practice. The concept was initially demonstrated in 1872, when Henry Skey of the Dunedin Observatory in New Zealand constructed a 35-centimeter-diameter liquid mirror in his laboratory. In 1909, the American physicist Robert W. Wood of Johns Hopkins University built the first complete liquid-mirror telescopes. Wood's most successful model used a mirror that was 51 centimeters in diameter. It rotated on a mechanical bearing and was turned by a motor using a drive belt consisting of fine threads of India rubber. With this telescope, Wood was able to resolve the e Lyrae quadruple star system, which has component stars separated by as little as 2.3 arc-seconds. Such performance is quite impressive—within a factor of 10 of the theoretical diffraction limit for a mirror of that size.
Still, Wood's telescope was not very practical. It was plagued by vibrations and a small but noticeable wobbling of the mirror. What's more, imprecise speed control gave rise to fluctuations of its focal length. Because the rotation axis had to be vertical, the telescope could observe only a small area of sky directly overhead, and the rotation of the Earth resulted in constant motion of the images. Such problems took much of the luster off the idea of using liquid-mirror telescopes for astronomy, which explains why this strategy was abandoned for the next 73 years.
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