prismatic astrolabe

An instrument for the accurate determination of the positions of stars. The telescope can be rotated in azimuth, and the time when a star reaches a particular predetermined zenith distance automatically recorded. An equilateral prism is placed in front of a telescope with one of its faces vertical. Two images of a star are formed from light passing through the prism, one from rays reflected directly off the lower internal face of the prism and one from rays first reflected from a horizontal mercury surface and then from the upper internal face of the prism. The two images coincide when the star's zenith distance is the predetermined value. The design was refined by the French astronomer André Danjon (1890-1967). A prototype was constructed in 1951 at the Observatoire de Paris, where such an instrument remains in operation.