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Equatorial mount
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An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation. This type of mount is used with telescopes, satellite dishes, and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion by driving one axis at a constant speed.
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Encyclopedia
An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation. This type of mount is used with telescopes, satellite dishes, and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any object in the sky that has a diurnal motion by driving one axis at a constant speed. When used with satellite dishes, an equatorial mount allows the dish to be pointed at several geosynchronous satellites by slewing along one axis.
Telescope mounts
In equatorial mounts for telescopes, the equatorial axis (the right ascension) is paired with a second perpendicular axis of motion (known as the declination). Equatorial mounts are often equipped with a motor drive for automatic tracking of objects across the sky. They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates. Equatorial mounts differ from mechanically simpler altazimuth mounts, which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography, the image does not rotate in the focal plane, as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator is installed.
Equatorial telescope mounts come in many designs. In the last twenty years motorized tracking has increasingly been supplemented with computerized object location. There are two main types. Digital setting circles take a small computer with an object database that is attached to encoders. The computer monitors the telescope's position in the sky. The operator must push the telescope. Go-to systems use (in most cases) servo motors and the operator need not touch the instrument at all to change its position in the sky. The computers in these systems are typically either hand-held in a control "paddle" or supplied through an adjacent lap-top computer which is also used to capture images from an electronic camera. The electronics of modern telescope systems often include a port for autoguiding. A special instrument tracks a star and makes adjustment in the telescope's position while photographing the sky. To do so the autoguider must be able to issue commands through the telescope's control system. These commands can compensate for very slight errors in the tracking performance, such as periodic error caused by the worm drive that makes the telescope move.
In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications. Massive new instruments are most stable when mounted in an alt-azimuth (up down, side-to-side) configuration. Computerized tracking and field-derotation are not difficult to implement at the professional level. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography.
German mount
In the German mount the primary structure is a T-shape, where the lower bar is the right ascension axis, and the upper bar is the declination axis. The telescope is placed on one end of the declination axis, and a suitable counterweight on other end of it. The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis.
Open fork mount
The Open Fork mount has a Fork attached to a right ascension axis at its base. The telescope is attached to two pivot points at the other end of the fork so it can swing in declination. Most modern mass-produced catadioptric reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles an Altazimuth mount, but with the azimuth axis is tilted and lined up to match earth rotation axis with a piece of hardware usually called a "wedge."
Many mid-size professional telescopes also have equatorial forks, these are usually in range of 0.5-2.0 meter diameter.
English or Yoke mount
The English mount or Yoke mount has a frame or "yoke" with right ascension axis bearings at the top and the bottom ends, and a telescope attached inside the midpoint of the yoke allowing it to swing on the declination axis. The telescope is usually fitted entirely inside the fork, although there are exceptions such as the Mt. Wilson 2.5 m reflector, and there are no counterweights like German mount has.
The original English fork design has the disadvantage of not allowing the telescope to point too near the north or south celestial pole. Later modifications have overcome this problems, for example the Hale telescope which has had its mount changed into a huge horse shoe so that it can point to the north celestial pole.
Cross-axis mount
The Cross-axis or English cross axis mount is like a big "plus" sign (+). The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately mid point with the telescope on one end of the declination axis and a counter weight on the other.
See also
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