A horizontal telescope for measuring the instant at which a star reaches a specific altitude on the celestial sphere. A prism is mounted with one face vertical immediately in front of the telescope's objective lens. Below the prism is a horizontal mirror formed by a dish of mercury. The star's image, and its reflection in the mercury mirror, are reflected internally in the prism. The two images coincide in the focal plane of the telescope when the star's zenith distance is half the angle at the leading edge of the prism (30° for a 60° prism).
prismatic astrolabe: Light from a star is directed into a telescope by a prism. Some starlight falls directly on to the prism, and some is reflected into the prism via a mercury mirror. The two images converge when the star reaches a precise altitude that is fixed by the angle of the prism, usually 30 ° from the zenith. In practice, the light path is folded inside the astrolabe for compactness
Subjects: Astronomy and Astrophysics.