Equatorial mount
A mount that tracks celestial objects by aligning with Earth's rotation.
An equatorial mount is a type of support for instruments like telescopes and cameras that counteracts the spin of the Earth. It does this by having one of its rotation axes—called the polar axis—set to run parallel to the planet's own axis of rotation. The key benefit is that by turning just that one axis at a steady speed, the instrument can stay locked onto any celestial object as it moves across the sky. This mechanism is known as a sidereal or clock drive. To make it work, the mount's rotational axis must be aligned with the Earth's axis, a step called polar alignment.
In telescope mounts specifically, this equatorial axis (which corresponds to right ascension) is paired with a second, perpendicular axis for declination. The equatorial axis often has a motorized clock drive that rotates it once every 23 hours and 56 minutes, matching the sky's apparent motion. Many mounts also include setting circles to help find objects using their celestial coordinates. Unlike simpler altazimuth mounts, which need variable-speed motion on both axes to track a target, an equatorial mount only requires constant speed on one. This also means that for astrophotography, the image doesn't rotate in the focal plane—a problem that altazimuth mounts must fix with a field derotator.
Equatorial mounts come in several designs. Over the last two decades, motorized tracking has been enhanced with computerized object location. Two main types exist: digital setting circles, which use a small computer with an object database and encoders to monitor the telescope's position (the user still pushes the telescope), and go-to systems, which typically use worm and ring gears driven by servo or stepper motors, letting the operator change position without touching the instrument. The computer is often in a handheld paddle or a laptop that may also capture images. Modern systems often include an autoguiding port, where a special instrument tracks a star and sends commands to correct small tracking errors, like those from the worm drive.
In large professional observatories, equatorial mounts have become less common in recent decades. Massive new instruments are more stable on alt-azimuth mounts, and computerized tracking and field derotation are straightforward at that level. Among amateurs, however, equatorial mounts remain popular, especially for astrophotography.
- Rotation period
- 23 hours and 56 minutes
- First developed by
- Joseph von Fraunhofer
- First completed in
- 1824
- First telescope
- Great Dorpat Refractor
Lore & Background
In astronomical telescope mounts, the equatorial axis (right ascension) is paired with a second perpendicular axis of motion (declination). The equatorial axis is often equipped with a motorized clock drive that rotates one revolution every 23 hours and 56 minutes in sync with the apparent diurnal motion of the sky. Equatorial mounts may also have setting circles to locate objects by celestial coordinates. They differ from altazimuth mounts, which require variable speed motion around both axes to track a fixed object and cause image rotation in the focal plane unless a 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. Two main types are digital setting circles, which use a small computer with an object database attached to encoders (the operator must push the telescope), and go-to systems, which use worm and ring gear systems driven by servo or stepper motors, allowing the operator not to touch the instrument. Modern telescope systems often include a port for autoguiding, which tracks a star and makes adjustments to compensate for tracking errors such as periodic error from the worm drive. In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications, as massive instruments are most stable in alt-azimuth configuration. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography.
Reader's Guide
Equatorial mounts are notable for their ability to track celestial objects with a single constant-speed drive, making them essential for astrophotography where long exposures are needed without image rotation. The article describes several designs: the German equatorial mount (GEM), a T-shaped structure developed by Joseph von Fraunhofer for the Great Dorpat Refractor in 1824, with the telescope on one end of the declination axis and a counterweight on the other; the open fork mount, common in modern mass-produced catadioptric reflecting telescopes, which resembles an altazimuth mount but with the azimuth axis tilted and lined up to match Earth's rotation using a wedge; the English or yoke mount, which has a frame with bearings at top and bottom and no counterweights, but cannot point too near the celestial poles; the horseshoe mount, which overcomes that limitation with an open horseshoe structure, as used in the Hale Telescope; the cross-axis mount, shaped like a plus sign with the right ascension axis supported at both ends; and the equatorial platform, a specially designed platform that provides equatorial tracking to any device sitting on it, from small cameras to entire observatory buildings, often used with altazimuth mounted amateur telescopes like Dobsonians. The article notes that computerized tracking and field-derotation are not difficult at the professional level, but equatorial mounts remain popular among amateurs for astrophotography.
Did You Know?
- The equatorial mount compensates for Earth's rotation by having one rotational axis parallel to Earth's axis.
- The Hale Telescope uses a horseshoe mount to allow access to stars near the celestial pole.
- Equatorial platforms can provide equatorial tracking to any device, from small cameras to entire observatory buildings.
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