Hack 43. Upgrade Your Dobsonian Bearings  
Use Ebony Star and Teflon for butter-smooth motions. Dobsonian telescope mounts move in altitude and azimuth on friction bearings. There are two double altitude bearings, one on each side of the telescope tube or mirror box. The scope pivots vertically from horizon to zenith on these altitude bearings, one of which is shown in Figure 3-30. The scope rotates horizontally on one triple azimuth bearing, shown in Figure 3-31. On the left is the bottom of the circular baseboard, which is covered in Ebony Star laminate. The triangular groundboard, the top surface of which is shown on the right, has Teflon bearing pads applied directly over its feet. The two boards are loosely joined by the azimuth bolt, visible in the center of the groundboard. When the two components are assembled, the groundboard sits flat on the ground, with the baseboard above it. When the Dobsonian scope is rotated in azimuth, the laminate surface rides on the Teflon pads, providing smooth movements. Figure 3-30. An altitude bearing 
Figure 3-31. The azimuth bearing 
This arrangement seems simple enough in theory, but in practice there are three requirements for ideal Dob bearings: Ideal bearings have perfect friction, not too much and not too little. The friction must be low enough to allow you to start the scope moving with the gentlest of fingertip pressure, but high enough to stop the scope's motion instantly when you stop applying pressure. Also, altitude bearing friction must be high enough to allow you to swap eyepieces in and out without the scope moving as the weight on the front of the tube changes. Ideal bearings exhibit zero stiction (STatic frICTION). Stiction is, in effect, variable friction. In a scope that suffers from stiction, you must apply significant pressure to start the scope moving. But as the scope begins to move, it "breaks loose" and suddenly begins moving much more freely. The motions of such a scope feel jerky. It is very difficult to track objects, particularly at high magnification, in a scope that suffers from stiction. Ideal bearings require exactly the same effort to move the scope in altitude as in azimuth. There should be no feeling of having to move the scope discretely on two axes, but rather a sense that the scope pivots freely in any direction.
Numerous combinations of materials have been used for Dob bearingsin fact, some astronomers experiment constantly with different bearing materials and surface treatmentsbut the most commonly used and recommended combination is virgin Teflon pads bearing on Ebony Star laminate. The combination of virgin Teflon and Ebony Star has an unusual property: its dynamic coefficient of friction and static coefficient of friction are nearly the same and can be made almost identical by applying paste wax to the Ebony Star surface. Unfortunately, virgin Teflon and Ebony Star laminate are more costly than some alternatives, so the makers of inexpensive Dobs make do with cheaper materials, such as nylon or mechanical Teflon pads riding on some sort of smooth laminate surface. Alas, those combinations have inferior friction/ stiction characteristics, so inexpensive Dobs nearly always have inferior motions. Their manufacturers use several tricks to cover up bearing (and balance) problems, including springs, clamps, and other workarounds. Ultimately, though, none of those workarounds actually works very well because cheap bearings simply don't have the proper friction/stiction characteristics. The real solution is to balance your Dob properly [Hack #41] and to replace the bearings with virgin Teflon riding on Ebony Star laminate. Fortunately, it's easy to do that, and it costs only $20 to $40 for a typical small to mid-size Dob. Here are the materials and tools you'll need: Asheet of Ebony Star laminate large enough to provide a circle the size of your baseboard and two strips wide enough to cover the bearing surfaces on your altitude hubs and long enough to cover most or all of their circumference. For a typical small or mid-size Dob, a 2-foot square sheet suffices. That will provide a 20" or 22" circle for the baseboard and the two 3/4" or 1" strips needed for the altitude hubs. Seven or eight small virgin Teflon pads, 3/16" thick. You'll need three pads for the azimuth hubs, four for the altitude hubs, and (optionally) a center pad [Hack #42]. For small to mid-size Dobs, the azimuth pads should be about 1" square and the altitude pads 3/4" x 1". That means you can cut all seven pads from one 1" x 6" strip of virgin Teflon. If you use a center pad, make it 1.5" square.
 | For larger Dobs, increase the size of the pads and use a center pad. For Dobs from 75 to 125 pounds, use 1.5" square azimuth pads, 1" x 1.5" altitude pads, and a 2.5" square center pad. For Dobs from 125 to 250 pounds, use 2" to 2.5" square azimuth pads, 1.5" x 2" altitude pads, and a 3.5" to 4" square center pad. The goal is to load the azimuth and altitude pads at roughly 15 pounds per square inch and to have a center pad with roughly the same area as the combined area of the three azimuth pads. |
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Laminate adhesive to secure the Ebony Star laminate to the baseboard and altitude hubs. Large hose clamps or cable ties to hold the Ebony Star laminate in place on the altitude hubs until the adhesive sets. (Ebony Star laminate is reasonably flexible, but when you bend it at the tight radius of the altitude hubs it wants to spring loose.) Small finishing brads to secure the Teflon pads to the Dob. Atack hammer and nail set to drive the brads and countersink their heads well below the surface of the Teflon. Sandpaper to smooth and bevel the edges of the Teflon pads.
Ebony Star (http://www.wilsonart.com) can be purchased at most hardware stores and home supply centers, although it is often a special-order item. Don't be surprised at the price. When he was building his 17.5" Dob, our observing buddy Steve Childers ordered a 4 x 8 foot sheet of Ebony Star. It cost $80. Fortunately, many home supply centers will sell you a 2 x 2 foot piece for $20 or so. | A local cabinetry installer may be willing to give you scrap pieces large enough for your needs. Sink cutouts are often large enough, for example. Acabinetry installer also has laminate cutting tools and may be willing to trim your Ebony Star to size for a small charge. |
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Actually, it's not critical that you use Ebony Star laminate specifically. What's important is not the brand name but the surface of the laminate. Standard laminates such as Formica have a smooth surface that produces stiction. Ebony Star and similar laminates have a "nubby" surface that nearly eliminates stiction. Although we've never used anything but Ebony Star laminate, by all reports any laminate with a similar surface texture works about as well. In fact, many astronomers substitute the inexpensive "glass board" laminate or fiber-reinforced plastic (FRP) laminate that is commonly used in public bathrooms. The type of Teflon you use for the pads is critical. There are two types available. Mechanical Teflon (also called recycled Teflon or remanufactured Teflon) is relatively inexpensive but has unsuitable friction/stiction characteristics. You want the second type, called virgin Teflon, which is considerably more expensive than mechanical Teflon. There are two ways to get it: If you're willing to pay for convenience, buy a "Teflon kit" with the pads already cut to size and pre-drilled for mounting screws. Astro-Systems (http://www.astrosystems.biz), for example, sells a small Dob kit for $25 shipped that includes three 1" square azimuth pads, four 3/4" x 1" altitude pads, and stainless steel mounting screws. They also offer a large Dob kit and a custom kit with larger pads. All pads are 3/16" thick. If you're cheap, like us, buy a raw chunk of virgin Teflon and cut your own pads. The best source we've found for bulk Teflon is McMasterCarr (http://www.mcmaster.com). We ordered a 6" square piece of 3/16" virgin Teflonthe equivalent of six AstroSystems small Dob kitsfor $16.08 shipped.
| Teflon is hard to cut. The best method we found was to use a fine point marker to mark the measurements, score the lines with a razor knife, and then use a fine hacksaw blade to make the actual cuts. After you cut them, use a sharp knife or sandpaper to smooth and bevel the edges. You may also want to use sandpaper to roughen the surface of the Teflon pads slightly. |
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Installing the upgraded bearings is straightforward. The hardest part is getting the Ebony Star trimmed to size and the center hole drilled in the azimuth portion. You can do the trimming with a razor knife, but a router and laminate bits make the job much easier and neater. If you've never cut laminate before and don't have a friend with the necessary tools and skills, we recommend you pay a kitchen installer a few bucks to make the necessary cuts and drill the center hole. Once the Ebony Star is prepared, proceed as follows: Disassemble your Dob, removing the baseboard and rocker box assembly from the ground board. Clean the bottom surface of the baseboard thoroughly, using Formula 409, Fantastic, or a similar household cleaner. Make sure you've removed all oil, lubricant, or other contaminants that might prevent the laminate adhesive from adhering. Remove the old nylon or plastic azimuth pads from the ground board and the altitude pads from the rocker box. These pads are usually secured with small countersunk brads or staples. Use a screwdriver or pry bar to pop them loose. Apply the laminate adhesive to the baseboard and/or the rear surface of the Ebony Star laminate, following the directions on the can. Insert the center bolt temporarily in the baseboard, and use it as a guide to press the Ebony Star laminate into position, centered on the baseboard and with the center hole in the laminate matching the center hole in the baseboard. With everything aligned, press the Ebony Star firmly into contact with the baseboard. Be careful when doing this. Once the two surfaces make contact, it may be difficult or impossible to correct any misalignment. Remove the center bolt and place the rocker box assembly on a flat surface. The weight of the rocker box is sufficient to clamp the Ebony Star laminate into close contact with the baseboard. Leave the assembly undisturbed for at least the time specified for the adhesive to set. Following the directions on the can, apply laminate adhesive to the back of each Ebony Star laminate strip and/or the surface of the altitude hubs. Press the Ebony Star laminate strips into position around the full circumference of the altitude hubs, making sure that the seam is located where it will never contact the altitude bearing pads. | The strips needn't be exactly the circumference of the altitude hubs. It's OK to trim them a bit shorter than the full circumference because the altitude pads don't bear on all parts of the laminate strip as the Dob is elevated from the horizon to zenith. To calculate the length of the strips, multiply the diameter of your altitude hubs by p (about 3.14). For example, the 5" altitude hubs on our Guan Sheng 10" Dob have a circumference of about 15.7". |
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If your Dob has typical 4" or 5" diameter altitude hubs, you'll find that the Ebony Star doesn't want to stay stuck when bent to such a tight radius. You'll need to clamp it into place until the adhesive sets. The best way to do that is to use hose clamps large enough to fit the diameter of your altitude hubs. If you don't have such clamps and don't want to buy them, you can instead use nylon tie-wraps. Pull them as tight as possible, putting the junction opposite the ends of the Ebony Star laminate strips. Attach the three azimuth pads to the upper surface of the ground board and the four altitude pads to the rocker box. Use two finishing brads to secure each pad, and countersink the heads of the brads well below the surface of the Teflon. | The Ebony Star laminate increases the diameter of the altitude hubs slightly. Depending on the thickness of the original pads and how they were installed, you may need to file down the mounting locations a bit to allow the altitude hubs to sit fully in the cutouts in the Dob base. |
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Apply a coating of silicone paste wax to all of the Ebony Star laminate surfaces.
Although the amount of friction provided by the pads is determined solely by their sizes and the force bearing on them, the placement of both the altitude pads and azimuth pads determines how freely the bearing moves. Increasing the separation between the pairs of altitude bearing pads stiffens altitude motions. Increasing the radial distance between the center bolt and the azimuth pads stiffens azimuth motions. For altitude bearings, the amount of normal force on the four altitude pads determines the amount of friction provided by those pads. As you increase or decrease the distance between the altitude pads, you also change their angle with respect to the vertical, which in turn changes the force on the bearing pads. Consider the normal force the pad exerts on the hub. That normal force can be resolved into two components, a vertical component, which acts to support the scope, and a horizontal component, which is equaled and opposed by the other pad on the same hub. The vertical component is independent of angle because each of the four altitude pads must support one quarter of the (constant) weight of the tube. The horizontal component depends on the angle. Because we are talking about a force normal to the surface, the horizontal force must change if the vertical force remains constant but the angle of the normal with respect to the vertical changes. If the pads are at the bottom of the hub, there is no horizontal component: the normal force on each pad is one-quarter the weight of the tube. As you increase the separation between the pads, the vertical component remains constant, but the horizontal component increases, increasing the normal force on the pads and therefore the amount of friction.
For azimuth pads, the radial distance of the pads from the center bolt determines the amount of mechanical advantage (leverage) that is applied when you move the tube. The amount of force needed is determined by the ratio between the separation between the bearings and the length of the lever. Unfortunately, although the position of the azimuth pads remains fixed, the length of the lever (and therefore the mechanical advantage it provides) varies with the elevation of the tube. That is, when the scope is pointed at the horizon, the effective lever length is quite long, and the scope is very easy to move in azimuth. Conversely, when the scope is pointed at zenith (straight up), the length of the lever is zero, and you have no mechanical advantage. (The zenith is called Dobson's Hole for just this reason. When the tube is pointed straight up or nearly so, it becomes almost impossible to move in azimuth.) There are a couple of steps you can take to minimize the problem of variable azimuth motions: Alter the distance of the azimuth pads from the center bolt. Traditionally, the azimuth pads are placed near the outer edge of the ground board, directly over its feet, with the goal of transferring the scope's weight directly to the feet, avoiding flexure of the ground board. But the farther the azimuth pads are from the center bolt, the stiffer the scope's azimuth motions will be. If your azimuth motions are too stiff, you can improve them by moving the azimuth pads closer to the center bolt. Use a center pad to shift some of the weight of the scope off the azimuth pads. If you use a center pad with an area about equal to the combined area of the three standard azimuth pads, you effectively cut the friction of those pads in half, making the scope much easier to move. The total friction remains the same, of course, but half of it is produced by the center pad. Because the center pad is located right at the center bolt, even a small lever arm gives a huge mechanical advantage, for all intents and purposes, eliminating the friction of the center pad.
Most Dob users do nearly all of their observing between 20° and 70° elevation. Below 20° the object is down in the muck and the views are generally poor. (Of course, you may have no choice, because a particular object may never rise very high at your latitude.) Above 70° Dobson's Hole becomes a problem, but it's easy enough just to wait until the object is a bit lower in the sky. Accordingly, it's usually best to optimize azimuth motions for 45° elevation, the midpoint between 20° and 70°. Ideally, you want the effort required to move the scope in azimuth when it is at 45° elevation to exactly match the effort required to move it in altitude. If you strike that balance, there will be no sense of having to move the scope up and down versus right and left. The scope simply moves smoothly in any direction, and controlling it becomes entirely automatic on your part. Even when the scope is pointed as low as 20° or as high as 70°, the effort required to move it in azimuth is close enough to ideal that you'll likely be unaware of any differences in motions. Getting your scope to this point may require some trial and error, but the results are worth it. | For another take on this issue, see Gene Baraff's description of his experiments with Ebony Star, virgin Teflon, and PFA (an alterative to Teflon) at http://groups.yahoo.com/group/telescopes/message/103010. |
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