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prismatic astrolabe

A Dictionary of Astronomy | 1997 | 578 words | Copyright

prismatic astrolabe 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).

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