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
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
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.