Traditional equatorial and alt-azimuth mounts lock the scope into position and drive it using gears. For small changes in direction, you move the scope by driving the gears with motors or manual slow motion controls. For large changes in direction, some mounts allow you to disengage the gears, pivot the scope, and then re-engage the gears. But while you are observing, the scope is always clamped into position.
Conversely, a Dobsonian mount uses simple friction bearings that allow the optical tube assembly (OTA) to move freely. When you want to point the OTA in a Dobsonian mount to a different part of the sky, you simply push the OTA up or down, right or left. The bearing friction must be small enough to allow smooth motions, but at the same time the friction of the altitude bearings—the bearings that support vertical movement—must be large enough to keep the OTA from shifting position under its own weight.
The altitude bearings must also have sufficient friction to accommodate small changes in weight. For example, removing one eyepiece before inserting another unbalances the scope because the front of the scope is suddenly lighter. The altitude bearings must have sufficient friction to keep the scope from swinging upward. Conversely, if you replace a light 1.25" eyepiece with a heavy 2" eyepiece, the front of the scope may have an extra pound or more pressing it down. If the altitude bearings have insufficient friction, the front of the scope nosedives.
The best way to accommodate these opposing requirements—low friction for good motions versus higher friction for tube stability when the weight changes—is to balance the OTA precisely by moving the altitude bearings forward or backward on the OTA as necessary to achieve balance. For example, if your heaviest eyepiece weighs 18 ounces, you'd place the alti-tude bearings so as to exactly balance the scope with a 9 ounce eyepiece installed. That way, the front of the OTAwould never be more than plus or minus 9 ounces from exact balance, with or without an eyepiece in place. The friction of your altitude bearings could be low enough to provide only 10 ounces or so of resistance—enough to keep the tube from drifting on its own, but light enough to provide very easy altitude motion.
The problem is that the Dob manufacturer can't balance the tube for you because they don't know what you're going to hang on the front or back end. For example, you may decide to add cooling fans to the primary mirror cell, which makes the OTA tail-heavy. Or you may decide to replace the light optical finder supplied with the scope with a heavy 50mm optical finder and a Telrad, which makes the OTA nose-heavy.
The best solution is to make the altitude hubs movable, and that is, in fact, the method used by high-quality tube Dobs. With movable altitude hubs, you can simply slide the OTA backward or forward to reach the balance point. But adjustable altitude hubs are a relatively costly feature to implement. The ubiquitous, inexpensive Chinese and Taiwanese Dobs—which are sold under numerous brand names including Orion, Celestron, Sky-watcher, and others—use a cheaper and less desirable method. Instead of providing a mechanism to balance the tube, they use springs or clamps to increase the friction on the altitude bearings.
shows the spring used to adjust altitude bearing tension on our 2000-vintage Orion XT-10 Dob. (This scope was made by the Taiwanese company Guan Sheng; current Orion Dobs are made by the Chinese company Synta, and they use clamps rather than springs.) The two small white pads visible at left and right center of the image are the Teflon pads upon which the circular altitude hub rotates.
Figure 1. A spring pulling down on the altitude hub increases friction on the altitude bearing
But increasing altitude bearing friction doesn't eliminate balance problems; it simply conceals them. With the springs or clamps in use, the OTA doesn't drift under its own weight or when you change eyepieces, but this is achieved at the expense of smooth altitude motion. It's difficult or impossible to get decent altitude motion if you use these springs or clamps.
What are the alternatives, then? You could drill a series of holes in the OTA and use trial and error to locate the proper position for the hubs to achieve perfect balance. That may be a viable solution for you if your Dob is tail-heavy. Unfortunately, most econo-Dobs are nose-heavy. Their makers balance them with a light optical finder and the light eyepieces bundled with the scope. By the time you add a heavy 50mm RACI finder, a Telrad, and a heavy premium eyepiece, the OTA is nose-heavy, and often by a lot.
Figure 327 shows a typical Dob (ours) after upgrades. Left to right are a 50mm RACI finder, the Telrad, and a 14mm Pentax XL eyepiece that weighs about 13 ounces. All told, this scope has about 20 ounces more on the front end than the manufacturer balanced it for. That doesn't sound like much, but it's enough extra to make the scope nosedive unless it is counter-weighted.
Figure 2. A front-end heavy Dob after typical upgrades
You might think the easy solution would be to relocate the altitude hubs nearer the front end of the OTA, which would indeed work. Unfortunately, doing that requires additional clearance at the rear of the OTA, and the bases supplied with econo-Dobs have almost zero extra clearance. When we measured our 10" Guan Sheng Dob, for example, we found that we could relocate the altitude hubs at most about 1/2" farther up the tube—not enough to do us much good. Achange of more than 1/2" prevents the OTA from reaching vertical because the rear end of the OTA hits the baseboard of the Dob.
Time for Plan B. If the front of the OTA is too heavy and we can't shift the balance point, the only solution is to make the rear of the OTA heavier to bring things back into balance. The way to do that is to add counter-weights, ideally ones that are adjustable in weight and position to allow rebalancing as necessary.
Dob owners use an incredible array of objects to counter-balance their scopes, from scuba diving weights to bean bags filled with lead shot to heavy magnets. We prefer magnets, which are cheap, compact, dense, and easy to move around as needed to rebalance the scope when you make changes to it. Magnets are a particularly good choice if you have one of the Chinese or Taiwanese Dobs that uses a steel tube. You just stick them to the OTA and they stay stuck. If you have a Sonotube (cardboard) OTA, you'll need to use tape, bungee cords, cable ties, glue, or other means to affix the weights.
Many Dob owners who add counterweights treat their OTAs like a see-saw, concerned only with balancing them on the longitudinal axis (over the length of the OTA). That's a mistake. You also need to consider the radial axis, which is to say where the various weights are located around the circumference of the tube.
ADob can move from 0° altitude, when it is pointed at the horizon, to 90° altitude, when it is pointed at zenith. As you move the OTA through its range of vertical motion around the altitude bearing axis, the relative positions of the various weights and their moment arms change. For example, if you counterweight the OTA at the rear center of the tube so that the OTA is properly balanced when it is elevated 45°, you have too little counterweight when the OTA is below 45° and too much when it is above 45°.
Some cunning astronomers use a short, heavy chain attached to the rear of the OTA. When the scope is horizontal, the full weight of the chain counterweights the tube. As the OTA is elevated toward zenith, more and more of the chain comes to rest on the base of the Dob, reducing the amount of counterweight.
The problem with counterweighting at the rear center or rear top of the OTA is that the axis between the counterweight and the offsetting weight at the front of the OTA doesn't pass through the center of mass. Ideally, you want the center of mass to be located at the exact midpoint between the altitude bearings. The best way to approximate this ideal is to eyeball the weights on the front of the scope to determine the approximate position of their center of mass. In our case, looking from the front of the scope, that center is at about 1 o'clock, roughly 6" from the front of the tube.
Once you have determined the center of mass of the front-end components, imagine an axis passing from that center of mass through the balance point between the altitude hubs. Where that axis intersects the rear of the OTA is the point where the counterweight should be. shows our counterweight at about the 7 o'clock position (as viewed from the front of the scope).
Figure 3. A counterweight placed opposite the center of mass of the front-end components
We used a 19-ounce bucking magnet, wrapped in duct tape to prevent scratching. These magnets are the shape of a flat donut, and they have a very strong grip. We bought six of these magnets from Parts Express (http://www.partsexpress.com) for about $1.50 each, plus shipping, but you may be able to find them at a local electronics store. With the steel backing plate in the mirror cell, one 19-ounce magnet suffices. With the backing plate removed, which is how we normally use the scope, a second magnet is needed.
A magnet works well if just one is heavy enough. If you need two or more for extra weight, magnets can become problematic because they attract or repel each other strongly, depending on their orientation. Stacking two magnets with the north pole of one against the south pole of the other is an easy solution, if the stack fits the available space. (We couldn't use a double stack at the 7 o'clock position, because the stack wouldn't clear the Dob base when we swung the OTA vertical.) In that situation, simply move the magnets around until you find a configuration where the scope balances properly and the magnets do not interfere with each other or with moving the OTA to vertical.