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What Is an Equatorial Mount Telescope?

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  • Written By: Jo Dunaway
  • Edited By: C. Wilborn
  • Last Modified Date: 13 November 2016
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    Conjecture Corporation
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An equatorial mount telescope is one equipped with a clock drive that keeps the equatorial axis of the Earth, known as its right ascension, in synchronized motion with the perpendicular axis of the sky, known as its declination. The clock drive will keep the mount in synchronization with any fixed objects in the sky. For taking photographs using a mounted camera, this motion resolves the rotational blur on a photograph, so that pictures are clear and sharp. Viewing objects can be quickly arranged by use of a setting circle that allows the viewer to enter celestial coordinates to find and lock into objects. A computer can also be used to locate and track objects; adjustments can be made manually, or the computer can be set to automatically adjust to keep the object in view.

One of the most common mounts for reflecting telescopes with large diameter mirrors is the open fork. This equatorial mount telescope has a fork at the base of the right ascension axis, with two pivot points at the opposite end of the fork to change the declination. Open fork telescopes are generally less expensive than other types of equatorial mount telescopes, and are relatively easy to set up.

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The German equatorial mount is shaped like a T, with the telescope at one end of the upper bar, which is the declination axis, and a counterweight at the other end; the ascension axis is the lower bar. Running parallel to the Earth's spin, these mounts are more complicated to set up initially, and require the know-how to mount a camera correctly to produce good space photography. Once configured, however, the setting circles can quickly bring desired objects into view, and once an object is found, focusing adjustments can be made easily. After the image is focused, the motor on the clock drive will keep the image stable for as long as desired. These motors can usually be powered off any vehicle's cigarette lighter using a power converter or by using installed battery packs.

Yoke or English mount telescopes are mounted in a tuning-fork shaped frame. The bearings for the right ascension axis are at the top and bottom, and the telescope is mounted between them. This equatorial mount telescope allows swing on a declination axis, but does not have the counterweight of a German equatorial mount. There is a disadvantage to this yoke design; the telescope cannot be pointed to near south or north celestial poles. The open "horseshoe" shape of the similar horseshoe mount telescope avoids this limitation.

A telescope can also be mounted on an equatorial platform mount, which are specially designed platforms that allow the device to track the equatorial axis. The platform pivots about a virtual polar axis, and can rotate a small camera on a tripod, a telescope, or an entire observatory building. Most of these platforms can only track for one hour before their range of motion are exceeded. Pivoting back to the east, however, allows resetting the mechanism of the clock drive to once again view another hour.

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