Paul Hickson in American Scientist:
For almost a century, astronomers have experimented with the intriguing idea of building telescope mirrors out of mercury. The reason is that it is relatively easy to get a liquid to take on the required parabolic profile—all one needs to do is to place it on a spinning platter—whereas solid mirrors require meticulously grinding the shape from glass. The obvious shortcoming of liquid-mirror telescopes is that they can only point straight up. Fortunately, for many studies this is the best direction to look, because the line of sight passes through the least amount of atmosphere. What’s more, the practice of “drift scanning” with modern CCD detectors circumvents the inability of liquid-mirror telescopes to track a target as it moves across the sky. The author reports the progress his group has made constructing a 6-meter liquid-mirror telescope, and he discusses plans for other instruments of this kind to be built at more suitable sites—perhaps even on the Moon.