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Liquid mirrors are mirrors made with reflective liquids. The most common liquid used is mercury, but other liquids will work as well (for example, gallium alloys). Liquid mirrors can be a low cost alternative to conventional large telescopes. The liquid is stored in a cylindrical container made of a composite material, such as Kevlar. The cylinder is spun until it reaches a few revolutions per minute. The liquid gradually forms a paraboloid, the shape of a conventional telescopic mirror. The mirror's surface is very precise and small imperfections in the cylinder's shape do not affect it. The amount of mercury used is small, less than a millimeter in thickness.
The greatest advantage of a liquid mirror telescopic is its small cost, about 1% of a conventional mirror. This cuts down the cost of the entire telescope by 95%. Unfortunately, the mirror can only be pointed straight up. If it tilts from the zenith, it loses its shape. The mirror's view changes as the Earth rotates and objects cannot be tracked. However, some types of astronomical research are unaffected by these limitations, such as long-term sky surveys and supernova searches. Since the Universe is isotropic and , the investigation of its structure by cosmologists can also use liquid mirror telescopes. However, research is underway to develop telescopes that can be tilted.
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