In the past, most telescopes used a German equatorial mount (GEM). With the advent of computerized telescopes, there are more convenient ways to mount telescopes, but the German equatorial still has many advantages and is still quite common.
Above: A typical German equatorial mount
There are two basic categories of telescope mounts: alt-azimuth and equatorial. Alt-azimuth–short for altitude-azimuth–mounts allow a telescope to move up-down and left-right. This movement is parallel and perpendicular to the horizon, making this mount intuitive to point, leaving the eyepiece in a convenient position, and making it well-suited to terrestrial observing. However, it makes tracking objects in the sky more difficult; automatic tracking is only possible with a computerized telescope. Equatorial mounts, on the other hand, have their axes aligned with Earth’s rotation axis, allowing easy automatic tracking of the night sky. Disadvantages include less intuitive pointing, more weight, and sometimes an awkward eyepiece position. But for accurate tracking, especially for photography, an equatorial mount is essential.
How a German Equatorial Mount Works
Above: The right ascension and declination axes. See text below for description.
The most distinctive feature of a German equatorial mount is the counterweight. The weight is used to balance the mass of the telescope. The telescope and weight mount on opposite sides of the main axis of the mount, called the polar axis or right ascension axis. This axis is aimed toward Polaris, the north star, thereby aligning the mount with Earth’s axis of rotation. By turning the mount at the same rate as Earth (once every 24 hours), the mount keeps the telescope pointed at any celestial target all night. Moving this axis aims the telescope to the east or west. The other axis, running between the scope and counterweight, is the declination axis. This allows pointing perpendicular to the rotation axis, letting the scope point to the north and south. This motion is not very intuitive to begin, but with some practice it becomes easy to point the telescope where desired.
Advantages of a German Equatorial Mount
An equatorial mount of some kind is necessary for long-exposure photography. While a computerized alt-azimuth mount can track, the sky appears to rotate relative to the scope, meaning the stars in the image will appear to trail. To keep the stars perfect pinpoints, an equatorial mount retains alignment of the telescope with the sky while tracking. The two basic types of equatorial mounts are the fork equatorial and the German equatorial. (See the Fork Mount page for more details on the fork equatorial mount.) The advantages of the German equatorial mount are stability, versatility, and weight.
Because the distance from the main axis of the mount to the telescope is shorter in a German equatorial mount than a fork mount (especially for larger instruments) there is less possibility of flexure, making the telescope more solid and less prone to vibration. The counterweights keep the entire assembly balanced around a central point, increasing stability.
Another advantage of the German equatorial mount is versatility. Unlike a fork mount, various instruments and be interchanged on a German equatorial mount. This allows multiple telescopes to be used with a single mount. Often an astronomer will use a small refractor for wide-field viewing or photographing, while employing a larger-aperture SCT or reflector for deep-sky viewing or photography of smaller targets. Using a single mount can be a considerable cost savings, especially when the required mount is large.
German equatorial mounts can be relatively heavy. This is intentional because they are designed to be stable, and also because of the extra mass of the counterweights. However, because the German equatorial mount breaks down into more and smaller pieces than a fork-mounted system, the individual weight of components will be lighter. This becomes more important in larger instruments. Take for example, a 14″ Schmidt-Cassegrain telescope. A typical fork-mounted instrument has two components, the tripod and the telescope/forks. The heaviest component weighs 110 pounds. With a German equatorial mount, there are more components (tripod, mount, counterweights, and optical tube), but the heaviest single component is only 45 pounds. This can mean the difference between a one-person setup and having to bring a friend every time you want to assemble your telescope.
Disadvantages of a German Equatorial Mount
The primary disadvantages of the German equatorial mount versus a fork mount are cost, ease of setup, and ease of use.
All things being equal, a German equatorial mount will be more expensive than a fork mount. On the other hand–again, all things being equal–a German equatorial mount will be more stable. If long-exposure photography is a primary goal, spending more on a German equatorial mount may well be justified.
Setting up a German equatorial mount is more time consuming. There are more components to assemble, for one. Also, the mount must be balanced and polar aligned every time it is used. With a fork mount, this is true if photography is going to be done, but for visual observing it is unnecessary. With a German equatorial there is no getting around these setup procedures.
While German equatorial and fork equatorial mounts are similar in their ease of use, compared to an alt-azimuth fork mount the equatorials are more difficult and awkward to use. An advantage of a fork mount is that it can be converted from an alt-azimuth mount for viewing to an equatorial mount for photography. This is not true of the German equatorial mount. The eyepiece position can be awkward when pointing to certain parts of the sky, and simply pointing the telescope is less intuitive. Of course, having a computerized telescope which points for you eliminates most of this hassle.
One other issue that is minor for most applications is that a German equatorial mount cannot track through the meridian–it must be flipped from east to west as the target being tracked crosses from one side of the sky to the other. For viewing this is normally not a big deal. Even for photography, if timed right, running into the meridian is usually not problematic. However, for certain applications such as scientific observations or automated search programs (looking for asteroids or supernovae) the telescope often must be able to track through the meridian uninterrupted. For these applications and heavy-duty fork mount is better suited.
German Equatorial Mount Prices
German equatorial mounts have a huge range in prices. Very small and simple non-motorized mounts, used for small telescopes (3-4″ aperture) might only cost $150-200. At the other end of the spectrum are the very high-end models designed for extremely precise tracking, stability, and for holding very large instruments (up to 20″). These mounts might cost $10,000-15,000. Normally these mounts are permanently installed as they can weigh several hundred pounds. In the middle are the most common German equatorial mounts, designed to hold mid-sized refractors and SCTs. These typically run $700-3000, depending on the size and quality. More expensive mounts not only hold more weight but track more precisely. Also, whether a mount is computerized or not will contribute to the price, although the difference is becoming small thanks to less expensive computer technology.
Is a German Equatorial Mount Best for Me?
The German equatorial mount is the best choice for someone who wants a relatively large instrument that is portable, or wants a very stable mount for photography. It is also a great choice for someone using multiple instruments, whether for visual observing or imaging. For someone only interested in viewing with a single telescope, a fork mount is probably a better choice as long as the instrument is small enough (usually under 12″ aperture) to handle easily.