Altazimuth mount

An altazimuth mount points a telescope by rotating it around vertical azimuth and horizontal altitude axes. Its simple geometry makes it common in visual observing and large modern telescopes.

Azimuth: The horizontal angle of an object measured around a vertical axis from a chosen direction. The mount's base rotates in this direction to change where the telescope points horizontally.
Horizontal coordinate system: An astronomical coordinate system that locates objects by altitude and azimuth relative to an observer's horizon. The mount's two axes correspond directly to this coordinate system.
Very Large Telescope: The European Southern Observatory's array of four 8.2-meter optical telescopes at Paranal Observatory in Chile. Its large telescopes use altazimuth mounts, which are practical for massive structures.
Fork mount: A telescope mount whose tube is held between the arms of a fork attached to a rotating base. A fork mount can use altazimuth or equatorial geometry, so its form is not the same as its axis system.
Altitude (astronomy): The angle of an astronomical object above the horizon. The telescope tilts in altitude to aim higher or lower in the sky.
Celestial sphere: An imaginary sphere centered on an observer, used to represent the apparent positions of celestial objects. Pointing the telescope selects a direction on this apparent sky.
Keck Observatory: An astronomical observatory on Mauna Kea, Hawaii, whose twin telescopes each have a 10-meter segmented primary mirror. The Keck telescopes demonstrate how altazimuth mounts support very large instruments.
German equatorial mount: An equatorial telescope mount with a counterweighted shaft and a rotating telescope saddle. Its polar-aligned axis tracks celestial motion differently from an altazimuth mount.
Equatorial mount: A telescope mount with one axis aligned parallel to Earth's rotation axis. Unlike an altazimuth mount, it can track stars by rotating around one axis.
Earth's rotation: The daily rotation of Earth about its axis, which makes celestial objects appear to move across the sky. The mount must continually change its aim to compensate for this apparent motion.

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