Schmidt-Cassegrain telescopes and refractors are the most popular systems for CCD imaging. Most SCTs come as a complete system with optical tube and fork-mount as one assembly. Less expensive refractors may include a small German equatorial mount, but many high-end refractors are sold simply as optical tubes only and require a separate mount to be purchased. For advanced imagers looking for the best performance, a high-quality German equatorial mount is the most popular choice. In addition to being a very stable mount for imaging, German EQ mounts allow multiple telescopes to be interchanged, allowing wide-field imaging with a refractor and narrow-field imaging with an SCT or Ritchey-Chrétien type scope using the same mount.
Advantages of a German Equatorial Mount
German equatorial mounts (GEMs) tend to be more stable than fork equatorial mounts, all things being equal. This is due to the fact that there is a shorter fulcrum between the mount and the telescope. On a fork mount there may be 18 inches or more of lever arm from the base of the mount to the telescope. On a GEM there is rarely more than 8 inches between the center of the polar axis and the telescope mounting plate.
Above: The lever arm is much shorter on a German EQ mount, making a GEM more stable than a fork mount.
The balancing system is typically more versatile on a GEM as well. Balancing is often easier to achieve and more accessories may be added to the optical tube without taxing the capabilities of the mount.
Disadvantages to a German Equatorial Mount
Fork-mounted scopes tend to be much easier and quicker to assemble than GEM scopes. The exception to this is very large (12″+) telescopes where the weight becomes difficult for one or two people to handle. Also, fork-mounted telescopes can cross the meridian without any problems. A German-equatorially mounted scope must be moved from the western to the eastern side of the mount when the target crosses the meridian, meaning imaging must be temporarily suspended while the telescope is repositioned. These drawbacks are minor to most CCD imagers who would rather sacrifice a little bit of convenience for the stability of a good GEM.
For advanced CCD imaging, tracking and pointing accuracy are key. Advanced imagers are typically after a mount which provides more stability, more accurate pointing, and more precise tracking than what is usually offered as a stock system by the manufacturer. Advanced telescope mounts usually have impressive technology in terms of goto capability and computer interface. Permanent observatories can benefit from the precision offered by software which can model pointing and tracking errors, telescope flexure, and other quantities in order to minimize their detrimental effects.
Tracking accuracy is much higher on advanced mounts. While a 30-60 second unguided exposure is typically the limit on many less-expensive mounts, exposures as long as 5 minutes or more can often be captured with sophisticated GEMs.
What Size Mount?
As with a telescope and a CCD camera, bigger is better. However, other factors such as portability and price come into play, as usual. The best advice is to get as big a mount as you can. For imaging, it is always preferable to have a mount that is too big rather than too small. Manufacturers list maximum recommended capacities for their mounts. Ideally the mount will be more than sufficient for the payload you intend to put upon it. Overkill is a good thing when it comes to mounts. If a mount has a weight rating of 70 pounds, and your scope with all accessories attached weights 50 pounds, you will have no problems. However, plenty of good images have been taken with 60-pound scopes on 50-pound-capacity mounts. But only under the most ideal conditions!