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A Danjon astrolabe is a type of instrument for measuring very precise horizontal coordinates of stars at precise times, sufficient information to determine coordinates within other celestial coordinate systems. The Danjon astrolabe's precision is accomplished through the use of a prism to split the light, in conjunction with a mercury bath to provide a precise horizontal reflective surface. It was developed by astronomer André-Louis Danjon in the 1950s as an improvement on transit telescopes for such work. (The term astrolabe is more commonly used for hand-held instruments used centuries ago to make equivalent measurements.)