Liquid mirrors

It has long been known that spinning a shallow dish of a reflective liquid, such as mercury, can create a parabolic mirror, like those used in reflecting telescopes. A number of attempts to construct liquid mirror telescopes were made in the 18th and 19th centuries. Liquid mirrors cost less to construct than conventional solid glass mirrors which are expensive to cast, grind and polish into a precisely parabolic shape. However, as Robert Wood of John Hopkins University observed in 1908, slight changes in angular velocity and inaccuracies in levelling the container supporting the fluid caused surface ripples that distorted the image.

Recently, a Belgian and Canadian project has developed a liquid mirror telescope that is being installed in Devasthal, India. The primary mirror has a diameter of 4 m and consists of a base fluid covered in a 1 mm layer of mercury.