Liquid mirror telescopes are telescopes with mirrors made with a reflective liquid. The most common liquid used is mercury. The container for the liquid is rotating so that the liquid assumes a paraboloidal shape. A paraboloidal shape is precisely the shape needed for the primary mirror of a telescope. The rotating liquid assumes the paraboloidal shape regardless of the container's shape. Liquid mirrors can be a low cost alternative to conventional large telescopes. Compared to a solid glass mirror that must be cast, ground, and polished, a rotating liquid metal mirror is much less expensive to manufacture.
"Isaac Newton noted that the free surface of 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 concept was further developed by Ernesto Capocci of the Naples Observatory (1850), but it was not until 1872 that Henry Skey of Dunedin, New Zealand constructed the first working laboratory liquid mirror telescope."
"Another difficulty is that a telescope with a liquid metal mirror can only be used in zenith telescopes that look straight up at the zenith, so it is not suitable for investigations where the telescope must remain pointing at the same location of space ... Currently, the mercury mirror of the Large Zenith Telescope in Canada is the largest liquid metal mirror in operation. It has a diameter of six meters, and rotates at a rate of about 8.5 revolutions per minute."