What is "zenith telescope"

Zenith \Ze"nith\ (?; 277), n. [OE. senyth, OF. cenith, F. z['e]nith, Sp. zenit, cenit, abbrev. fr. Ar. samt-urras way of the head, vertical place; samt way, path + al the + ras head. Cf. Azimuth.]

1. That point in the visible celestial hemisphere which is vertical to the spectator; the point of the heavens directly overhead; -- opposed to nadir.

   From morn To noon he fell, from noon to dewy eve, A summer's day; and with the setting sun Dropped from the zenith, like a falling star.
   --Milton.

2. hence, figuratively, the point of culmination; the greatest height; the height of success or prosperity.

   I find my zenith doth depend upon A most auspicious star.
   --Shak.

   This dead of midnight is the noon of thought, And wisdom mounts her zenith with the stars.
   --Mrs. Barbauld.

   It was during those civil troubles . . . this aspiring family reached the zenith.
   --Macaulay.

   Zenith distance. (Astron.) See under Distance.

   Zenith sector. (Astron.) See Sector,

3. Zenith telescope (Geodesy), a telescope specially designed for determining the latitude by means of any two stars which pass the meridian about the same time, and at nearly equal distances from the zenith, but on opposite sides of it. It turns both on a vertical and a horizontal axis, is provided with a graduated vertical semicircle, and a level for setting it to a given zenith distance, and with a micrometer for measuring the difference of the zenith distances of the two stars.

A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both.

A classic zenith telescope, also known as a zenith sector employs a strong altazimuth mount, fitted with levelling screws. Extremely sensitive levels are attached to the telescope mount to make angle measurements and the telescope has an eyepiece fitted with a micrometer. They are used for the measurement of small differences of zenith distance, and used in the determination of astronomic latitude.

Other types of zenith telescopes include the Monument to the Great Fire of London that included a central shaft meant for use as a zenith telescope. High-precision (and fixed building) zenith telescopes were also used until the early 1980s to track Earth's north pole position e.g. Earth's rotation axis position. Since then radio astronomical quasar measurements have also measured Earth's rotation axis several orders of magnitude more accurately than optical tracking. NASA Orbital Debris Observatory, which used a 3 m diameter aperture liquid mirror, and the Large Zenith Telescope uses a 6 m diameter aperture liquid mirror are both zenith telescopes since the use of liquid mirror meant these telescopes could only point straight up.