Fork mounts have become very common for their versatility and ease of use.
German equatorial mounts (GEMs) used to be more common, but the advent of
computerized telescopes has made the fork mount usable for a variety of
applications and it is quickly becoming the most popular mount for most
Above: A typical fork mount
There are two basic categories of telescope mounts:
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. An advantage of the fork
mount is that it can be quickly converted from alt-azimuth to equatorial.
How a Fork Mount Works
Above: Alt-azimuth and equatorial configurations. See
text below for descriptions.
Almost all fork-mounted telescopes are computerized now. This allows
the alt-azimuth (up-down, left-right) configuration to automatically track the
sky. This configuration is by far the easiest to set up and use.
However, long-exposure photography is typically not possible in this mode. Tracking
must be done in two axes because the mount is not aligned with Earth's rotation
axis. As the telescope tracks the sky, the view will appear to rotate,
causing stars in an image to trail. To keep the stars pinpoints, the
telescope must track in only one axis, eliminating field rotation. By
adding a component called an equatorial wedge, the fork mount can be converted
to equatorial mode. In this configuration, the fork arms are aimed toward
Polaris, the north star, aligning this axis with Earth's rotation axis. By
rotating the telescope at the same rate as Earth (once every 24 hours), the
telescope tracks the sky in just a single axis, called the
polar axis or right ascension axis. Moving the telescope about this axis aims to
the east or west. Perpendicular to this axis is the
By moving the telescope up and down between the forks the scope points north or
Advantages of a Fork Mount
Compared to a German equatorial mount, the advantages of a fork mount are
cost, ease of use, ease of setup, and mounting versatility. (See the
German Equatorial Mount page for more
details on GEMs.)
Normally, a fork-mounted telescope will be less expensive than an equivalent
German-equatorially-mounted telescope. The trade-off is stability, since
an equivalent GEM will have less vibration and flexure.
A fork mount, especially in alt-azimuth configuration, is extremely easy to
use. Pointing is very intuitive (and usually a computer will be doing it
for you anyway) and the eyepiece is always in a convenient position for easy
access. The range of motion of the eyepiece of most fork-mounted scopes is
very short as well, making it easy to set the telescope at a convenient height
for sitting to observe or for sharing the view with others.
Fork mounts are also very easy to set up. In alt-azimuth mode there are
only two components, and assembly involves putting the scope/mount onto the
tripod. The orientation of the tripod and telescope is unimportant.
No polar alignment needs to be done. Even in equatorial mode only one
other piece, the equatorial wedge, is added. Fewer components are
required than a German equatorial mount. Setup time is easily half or less
than that of a German equatorial mount.
Another advantage is versatility. Unlike a German equatorial mount, a
fork mount can be quickly converted from alt-azimuth mode for visual observing
to equatorial mode for photography. This means no tricky setup for nights
when you just want to view. With a German equatorial mount, balancing the
telescope and polar aligning the mount is required every time you set it up,
whether for photography or not. With a fork mount, this more
time-consuming setup need be done only for photography, and still usually goes
Another potential advantage for photography is that fork equatorial mounts
can track through the meridian uninterrupted, unlike a GEM which must be flipped
from east to west when the target crosses from one side of the sky to the other.
This is especially advantageous for automated search programs (looking for
asteroids or supernovae) or other scientific research.
Disadvantages of a Fork Mount
The primary disadvantages of a fork mount compared to a German equatorial
mount are stability, weight, and telescope versatility.
Because the distance from the main mount axis to the telescope is shorter on
a GEM than a fork-mount (especially for larger-sized instruments), a German
equatorial mount tends to be more stable than a fork.
While the total weight of an assembled German equatorial
mount system is usually more than an equivalent fork-mounted scope (thereby
giving the scope more stability), the individual components of the GEM are smaller and lighter
than the fork mount. With a fork mount, the forks and optical tube are
almost always a single component. Compare a fork-mounted 14" SCT in which
the heaviest component (the fork/scope) weigh in at 110 pounds to a similar
scope on a GEM. The German-equatorially-mounted version has more components
(tripod, counterweights, mount, telescope) but the heaviest single piece is only
45 pounds. In larger instruments, a GEM is often the only way one person
can set up the telescope. Any fork mounted scope larger than about 10-11"
becomes a two-person setup.
Another advantage of the German equatorial mount over the fork is that a
variety of instruments can be interchanged. A fork-mounted scope cannot
normally be separated from its fork arms. Using a second telescope, such
as a small refractor for wide-field observing or imaging, requires a separate
mount. Using a GEM allows multiple telescopes to be used with a single
mount, offering a potentially substantial cost savings.
Fork Mount Prices
Unlike German equatorial mounts, fork mounts are an integral
component of the telescope system, so they are not sold separately. When
comparing costs between the two systems, be sure to look at the cost of the
telescope/mount combination, not just the mount itself. The most common
fork-mounted scopes are Schmidt-Cassegrains
(and sometimes Maksutov-Cassegrains),
which are very compact and versatile. Other more sophisticated optical
designs are available as well for advanced observers. They range in size
3.5" to 16" and all current models are computerized. Cost of smaller
models in the 3.5" to 6" range is about $600-1200. The most popular sizes
are in the 8"-11" range and costs run from $1400-5600. The largest sizes,
from 12"-16" cost $3800-17,000.
Is a Fork Mount Best for Me?
Fork mounts are the most popular design, and for good reason. They are
very easy to use and relatively inexpensive. If you want a telescope that
is quick and easy to set up for visual observing but also has the versatility of
being used for deep-sky astrophotography, a fork mount is a great choice.
The main reason to choose a German equatorial mount instead would be if you were
primarily interested in imaging and wanted the most stable mount possible.
Also, in large sizes, fork mounts get difficult for one person to handle, so a
GEM might be a better choice in a larger size if you don't have the option of a