Telescope Mounts

Telescope Mounts

The type of mount you choose for your telescope is just as important as what type of telescope you choose. Some mounts are better for tracking objects, others are easy to maneuver when you are hopping around the sky.

Equatorial

An equatorial mount has one axis (the polar or right ascension axis) lined up parallel to the Earth's axis, pointing at the celestial pole, and a second axis (declination) at right angles to it. The benefit of this set up has to do with keeping an image in the eyepiece. Since the earth is constantly turning on its axis, when we magnify a planet, star, or other object, it tends to drift out of the eyepiece. The more magnification we use the faster it will drift out of the field of view. With an equatorial mount, once an object is located, you only need to turn the polar axis to keep it in the eyepiece, and a single motor on that axis can do this for you.

For this to work, the mount must be polar aligned: its polar axis aimed at the celestial pole. In the Northern Hemisphere that point is less than a degree from Polaris; many mounts have a polar scope to help, and astrophotographers often refine the alignment with a camera and software.

The most common type is the German equatorial mount (GEM), where the telescope sits on one side of the declination axis and a counterweight on the other. GEMs carry everything from small refractors to large reflectors and are the standard choice for astrophotography.

Alt azimuth Mount

This is the simplest type of telescope mount. It has only up/down (altitude) and left/right (azimuth) motions. A manual alt-azimuth mount is easy to set up and intuitive to use, which is why it is used for Dobsonian telescopes, many small refractors and terrestrial spotting scopes.

With a manual alt-az mount, we need to adjust both axes to keep an object in the eyepiece, which takes some practice at high power. Computerized alt-az mounts solve this by driving both axes at once and can track well enough for high-power viewing. For long-exposure photography, though, an alt-az mount lets the field slowly rotate around the target, which is one reason astrophotographers prefer equatorial mounts.

Dobsonian

The most popular alt azimuth mount, popularized by John Dobson, is called a Dobsonian mount or "Dob." It is a low, simple box (often called a rocker box) that the telescope tube pivots in, with smooth bearing surfaces such as Teflon pads. While basic Dobsonian mounts don't track objects on their own, they are a very easy arrangement for finding and pointing to an object, and many observers enjoy hunting down targets by hand. Because a Dob-mounted telescope is easy to set up and maneuver, it is a popular telescope among amateur astronomers. Some Dobsonians now come with digital setting circles, smartphone pointing aids, or full GoTo motors.

Fork Mount

A type of mount most often used on short-tube compound telescopes such as Schmidt-Cassegrains, in which the telescope tube is held between two arms (or, on some smaller models, from a single arm). Used directly on a tripod, a fork mount works as an alt-azimuth mount. Tilted on an equatorial wedge so that its base points at the celestial pole, it becomes an equatorial mount, which avoids field rotation for long-exposure photography. Older fork mounts had a motor drive on the right ascension axis and setting circles for locating objects by their right ascension and declination coordinates; most modern fork mounts are computerized GoTo mounts.

Go-To

A "go-to" mount is any computerized mount with motors on both axes and a database of objects. After you align it on a few bright stars (or let a camera or GPS do part of the work), it can slew to thousands of objects and keep them in view. Go-to mounts come in many forms: alt-azimuth (single-arm and fork mounts, and GoTo Dobsonians) and equatorial (German equatorial mounts and fork mounts on a wedge). Because the computer controls both axes, an alt-az go-to mount can follow objects accurately for visual observing, while equatorial go-to mounts are preferred for long-exposure astrophotography.