Liquid mirror telescope
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A liquid mirror telescope is a reflecting telescope whose primary mirror is a rotating pool of a reflective liquid, usually mercury.
Isaac Newton first realized that a rotating liquid forms a circular paraboloid and can therefore be used as a telescope, but he could not actually build one because he had no way to stabilize the speed of rotation (the electric motor did not exist yet).
The main advantage of using a liquid mirror is the cost. The University of British Columbia's 6-meter liquid mirror telescope cost about a fiftieth as much as a conventional telescope with a glass mirror.[link]
Of course, because the shape of the liquid depends on gravity, the telescope can only point straight up. This is not actually as big a drawback as it sounds, because the part of the sky at the zenith changes with the season and time of day.
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