What's "German" about a GEM? It was invented by a German priest, Christoph Grienberger, in the seventeenth century, not long after Galileo did his refractor thing. Perhaps it should have been called the Grienberger equatorial mount, but "German" does have a nicer ring.
Whatever you call it, the GEM is a simple if complicated-looking device. In Plate 10, you can see that it is composed of two axes connected to form a "T" shape. The
"vertical" bar of this T forms the RA axis, while the "cross" bar of the T is the declination axis. The telescope is attached to the crossbar, and the vertical bar is tilted to point at the North Celestial Pole. Arranged in this fashion, the GEM can track the stars by turning on its RA axis. Add a single motor, and it can track automatically. Hook up a computer and add a declination motor to the German mount, and it can go-to as well as any fork mount can.
The details are a little more complicated, but not much. A shaft runs through the vertical bar of the T, rotates on bearings, and forms the actual RA axis. The RA axis is physically attached to a strut or housing that allows it to tilt to point at the pole, and there are usually fine-adjustment knobs that allow this tilt to be precisely set for exact polar alignment. The RA axis can also be adjusted finely in azimuth. There is a lock to secure the scope in RA during tracking and go-to.
The crossbar of the T houses the declination axis shaft and its bearings. There is a lock on the declination axis just like the one on the RA axis that must also be tightened down for go-to. The declination shaft extends through and out of its housing, and "counterweights" are mounted on this extension to counterbalance the scope that is attached to the opposite end of the axis. The counterweight end of the declination shaft is usually equipped with a safety bolt that prevents the heavy weights from accidentally sliding off the end of the declination shaft. That is a very good thing. As a very young man, I had a bad experience with a counterweight on a GEM-mounted 4-inch reflector that did not have this feature. In the course of adjusting the scope's declination balance, I let go of the 10 pound counterweight, which I had loosened, and it slid right off the shaft and onto my big toe. Ouch!
Drives on GEMs are not much different from those on fork mounts. There is a separate servo or stepper motor for each axis, and these motors are controlled by a computer in a hand control. One advantage GEMs, even low-cost imported ones, have over less-expensive fork mounts is that almost invariably the smaller and larger gears on both axes are both worms. One drawback to GEMs is that the RA gear is usually smaller than those gears used in the fork mounts. A fork usually has an RA drive gear as large as the drive base—6-inches in diameter or larger. RA gears on all but the largest GEMs are typically half that diameter or smaller. That does not seem to affect GEM tracking, however. A GEM's declination gear is typically identical to its RA gear.
Unlike forks, German mounts are still available without go-to, mostly from third-party manufacturers. Non-go-to GEMs typically feature manual slow-motion controls on both axes, while go-to models do not. Often, manufacturers, especially of cheaper mounts, use the same basic mount for go-to and non-go-to models, with the slow-motion controls removed to allow go-to motors to operate without the addition of the clutches that would be needed if the slow-motion controls were retained. Even without slow-motion controls, most GEMs are easy to point and hand track without power. They are considerably easier to balance than fork-mount scopes, and that helps a lot when pushing the scope along after the battery dies.
Is a German mount better than a fork? In some ways, it is. Despite the aforementioned smaller RA gears on some GEMs, most of the German mounts seem to track better than similar-quality forks. In large part, this is because they are easier to balance. Unlike a fork, a telescope on a German mount is easily balanced in RA by moving the counterweight up and down its shaft. Declination balancing is also easy and is accomplished by the simple expedient of moving the scope back and forth in the dovetail/cradle that is used to attach tube to mount. A fork may need the addition of weights to the fork and to the OTA to achieve balance in both RA and declination—assuming the telescope can be properly balanced in both axes, something that is not always possible with fork-mount Schmidt Cassegrains.
One tremendous advantage for the GEM is that, unlike most fork-mount scopes, the tube can be removed from the mount easily. That makes the scope/mount combo much easier to transport since it can be broken down into several easy-to-handle pieces. Being able to remove the scope from the GEM also allows the same mount to be used for a number of different telescopes. Most GEMs use one of two standardized and easily available dovetail/cradle-mounting systems to attach scopes to the mounts.
Who is the GEM for? The GEM is for two groups. The first is those folks, beginners or just the budget conscious, who want a mount that does everything a fork can do, including go-to and computer control, but for a less-expensive price. Meade and Celestron offer medium-size imported (Chinese) GEMs for their SCTs that work well and are as full featured as more expensive forks.
Since, as we all know, "there ain't no such thing as a free lunch" (TANSTAAFL), there must be a penalty of some kind. There is: GEMs are harder for beginners to learn to use than forks. Since the GEM cannot be set up in alt-az mode, the novice must learn to at least roughly polar align if the mount is to track the stars accurately. A halfway decent polar alignment is also needed for good go-to performance on some models. Go-to alignment is also a bit more complicated with GEMs, with many models requiring the user to sight as many as six or seven stars for optimum accuracy rather than the fork's two or three.
The biggest fans of German mounts, however, are serious imagers. They often eschew the GEMs offered by Meade or Celestron and place their SCT tubes on large (and expensive) third-party mounts sold by companies such as Losmandy, Asro-Physics, Software Bisque, and Mountain Instruments. These big guns are often in the $10,000 price range.
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