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 observers.
Above: A typical fork 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. 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 declination axis. By moving the telescope up and down between the forks the scope points north or south.
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 theGerman 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 more quickly.
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 from 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 permanent observatory.