On The face of it, a telescope with a liquid mirror sounds about as useful as a chocolate teapot. The enormous rigid mirrors that capture and focus light inside large telescopes weigh several tonnes, and take many years of painstaking effort to cast, grind and polish—a process that shapes their surfaces to within a few billionths of a metre. So how could a pool of liquid mercury do the same job? The answer is surprising. Pour any liquid into a cylindrical container, rotate the container at a constant speed, and the surface of the liquid will become a paraboloid, which just happens to be the perfect shape for a telescope's mirror.
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