photographic zenith tube
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photographic zenith tube(PZT) A specially designed telescope that is used for the accurate determination of time of transit of fundamental stars, so ascertaining clock error arising from the Earth's irregular rotation and Chandler wobble. It is also used to determine the zenith distance of those stars at transit. The telescope is fixed in a vertical position so that the observed stars are at or near the zenith of the observatory. The positions of the stars are recorded on a photographic plate.
photographic zenith tube[¦fōd·ə¦graf·ik ′zēn·əth ‚tüb]
A type of zenith telescope in which light is reflected from a pool of mercury, and the photographic plate is held in a carriage just below the objective that is alternately rotated through 180° and moved slowly across the field of view to follow a star image; used for the accurate determination of time.