The Trackball Telescope pt1

[Jamie]

The Trackball Telescope

Designed and built by Jerry Oltion

Featured in the August, 2006 issue of Sky & Telescope Magazine

Here is the original link

http://tinyurl.co.uk/t0hg

Kathy and I had been enjoying amateur astronomy for about a year when I decided it was time to try my hand at building a telescope. I wasn't happy with equatorial or Dobsonian mounts--both types have difficulty reaching certain parts of the sky, and equatorial mounts make you do some back-breaking contortions to reach the eyepiece once you do find your target--so I decided to see if I could design something that would be easy to point and easy to look into no matter where in the sky it was aimed.

I didn't want to be influenced by what others had done, so I purposefully didn't look for other designs until I had come up with one on my own. I followed a few false leads, but I eventually figured out that a spherical base resting in a socket would let me point the scope anywhere in the sky with equal ease, and it would let me rotate the eyepiece to a comfortable position no matter where I was looking.

That was half the battle, but one of the things I didn't like about Dobsonian mounts is that you have to keep shoving them by hand to track the stars. How could I make my mount track automatically? The answer to that came in a flash of inspiration: The stars move because the Earth--a sphere--rotates. But I was building a spherical telescope, so if I made it rotate in the opposite direction, it would track. Once I realized that, the solution was obvious: rest the sphere against an axle that points at the celestial pole, and rotate the axle.

Confident that I had just reinvented the wheel--almost literally--I went online to see how other people had done it. Surprise! Nobody had. Spherical telescopes were old news, but nobody was talking about my kind of mount. I showed it to other astronomers, figuring there must be some fundamental flaw in the design that would explain why nobody was building them, but nobody I talked to could find anything wrong with the concept. And none of them had ever heard of this design, either.

So I searched the U.S. Patent and Trademark Office database, and came up blank there, too. As near as I can tell, I have come up with a new design for a mount that not only eliminates the pointing problems of equatorial mounts and Dobsonian mounts, but also tracks. I call it the "trackball" because there's no better name to describe it. Happily, another search of the USPTO database shows that the term "trackball" has fallen into the public domain, so apparently anyone is free to use it as a generic term for this type of telescope.

Many people have urged me to patent the design, but I would much rather put it into the public domain so anybody can build (and sell) them as they wish. So I have done just that. In July of 2005 I displayed the scope and mount at a public star party, effectively placing it in the public domain. The article in Sky and Telescope reinforced that action, as does this web site. My actions only affect the parts of the design that are new with me, of course, but that much at least is free for everyone to use, and nobody may patent it.

So how do you build a trackball? The most important thing is not to be afraid to experiment. My design philosophy has been to make it work first, then worry about making it pretty. I'll describe how I built mine, but if you have a better idea for any of aspect of the design, go for it! Look around your shop for whatever will work rather than following my recipe to the letter. So with that in mind, here's a general idea of how to build a trackball. I've divided the instructions into two pages to make them easier to load and view. Click on the part of the above photo you're interested in, or click on the following links. Below these links I've written some instructions for using the completed telescope and mount, and some other ideas and random thoughts about the design.

How to use the telescope

The trackball is amazingly easy to set up. Just put the mount on a level surface, aim the drive axle at the celestial pole, set the telescope on the mount, and turn on the motor. This simple alignment is all you'll need for visual observing even at relatively high power.

To find your target, just push the scope where you want it to go. If the eyepiece isn't in a comfortable position, rotate the scope until it is. Most times you can sit down while you're observing, which is by far the most comfortable way to view.

Once you've found your target, just let go and the scope will track. You can center it east-west (right ascension) with the fast or slow tracking buttons, but north-south (declination) adjustments are made by nudging the scope by hand. Also, if there's any play in the gears (and there probably will be), then you'll want to overshoot to the west and then pull the scope back to the east to load the gears so the scope will begin tracking the moment you let go. You'll quickly learn to combine these adjustments into one smooth, quick motion, so you don't even think about it anymore. You'll just point, let go, and observe.

If your target drifts out of the field to the east or west, adjust the tracking speed. If it drifts to the north or south, nudge the mount sideways or raise or lower the axle a bit to bring it back. That's called drift aligning, and it's a piece of cake. If you've never done it, click here for instructions.

A few other random thoughts

Cooling

There's not a lot of air flow inside the ball. My scope takes about half an hour to cool on an average night, but then again, my mirror is pretty thin. If yours is thicker, it might take longer, and you might need to install a fan. I've experimented with a 1.5" muffin fan mounted at an angle inside the ball where it's out of the light path and can swirl air down under the mirror and back out the other side of the ball, and that seems to work okay. You might be able to drill holes in the ball and mount a fan to blow air straight out (or in), but if you do, use a small drill so the holes don't mess up your tracking. They have to be small enough that the grid of them will ride across the bearings without bumping.

You might try putting insulation over the counterweight to keep it from radiating heat into the ball, but that might just slow down the cooling process. It depends on how fast its heat will escape through the outer surface of the ball. Experiment with removable foam before you spray permanent stuff in there!

Nesting

You could cut a big enough hole in the ball for the secondary cage to nest in there during transport, but that would probably cut into your ability to look close to the horizon. I thought collapsibility would be a neat thing when I was designing my scope, but I quickly gave it up when I saw how difficult it would be, and in truth I've never needed to take the telescope apart except to tinker with it anyway. Maybe if it was longer, but at 38 inches it fits into the back seat (or upright in the passenger seat) of my Volkswagen just fine.

Carrying

I haven't put a handle on mine yet, but the part of the ball directly below the eyepiece never rides on the bearings, so a handle there shouldn't get in the way of anything. On the other hand, it's pretty easy to carry the scope by the lip of the ball, so I may just keep doing that. One other thought on the subject: I'm not sure how strong an acrylic sphere is. My fiberglass sphere is plenty strong enough to support the entire scope's weight from a couple of handle attachment points, but you might want to err on the side of caution with an acrylic sphere.

Ideas to experiment with

(Any of these that are not already patented I also place in the public domain)

The ball could be made of an open grid of wires or hoops or whatever. You would need larger bearing surfaces--maybe slides--so the gaps wouldn't affect the tracking, but it would really help with cooling (and maybe with weight, too).

The axle could be a conveyor belt or a sling or anything else that pushes the ball. It could be spring-loaded to press into the ball while tracking and moved away while slewing. (This would allow the use of a very grippy substance for the axle, which could help eliminate fussiness over balance.)

The mount could be any kind of a cradle, even a toilet-seat-style hole, with the axle pressed against the scope (maybe spring loaded as mentioned above).

Rotation could be constrained to the correct direction with multiple bearing surfaces oriented in the proper direction, or with a suction cup on a bearing that stuck to the bottom of the ball. (That would probably have to be on a lever so it could be moved away while slewing.)

The axle could be cone-shaped, so variable speed could be accomplished by moving the axle longitudinally rather than changing the motor speed.

You could paint a star map on the ball and use a pointer on the mount to help you aim the scope. Of course when you rotate the scope for a comfortable eyepiece position, your map will move...

You could use the telescope itself for precise latitude alignment. Drift align the scope once, and after you've got it tracking perfectly, put Polaris (or Sigma Octantis if you're in the southern hemisphere) in the center of the view of a medium-power eyepiece. Mark the point where the ball and the axle come together, then next time you set up just sight on Polaris (or Sigma Octantis) and move the axle up and down until the marks line up. You could probably use a similar method for left-right alignment using the idler bearings.

You could use magnets for fine-tuning the counterweight system. Overdo the weight inside the ball by a pound or so, and put a metal strip up near the focuser. Stick magnets on the metal strip, and remove them when you put a heavy eyepiece in. Or you could embed a metal strip in the ball (easiest if you're making a fiberglass ball) on the side opposite the focuser and stick a few magnets to that when you use a heavy eyepiece.

Total cost

I put about $500 into my trackball. Some of that was blind alleys (making a fiberglass sphere instead of buying an acrylic one, experimenting with Teflon instead of bearings, etc), so I'm guessing a person could shave $50-100 off that figure if they were careful. If you buy a finished primary mirror rather than make your own, it will probably run you more. Or you could rob the mirrors and other hardware out of your Newtonian and build the whole thing for practically nothing. Once you use a trackball, you'll never go back to that old scope anyway. :-)

Drift alignment

Drift alignment can seem pretty involved if you get a complicated set of instructions, but it's actually very simple.

In the northern hemisphere, do this:

Aim the telescope at a star to the south, and somewhere near the celestial equator. (That's 90 degrees away from the pole, along a line that runs straight overhead.) Look for north-south motion of the star. We're only concerned with north-south motion. If it drifts east or west, your drive is set too fast or slow. And I'm talking about actual motion, i.e. the direction you have to push the telescope to re-center the star. If the star drifts south, then rotate the entire mount so the axle points farther west. If it drifts north, rotate the mount so the axle points farther east.

Now look at a star near the horizon to the east, and see if it drifts north or south. This time if your target drifts south, then angle your axle farther upward (toward a higher latitude on your scale). If it drifts north, lower the axle.

If you were a long way off in either direction, do the whole procedure again to fine-tune it.

In the southern hemisphere, do this:

Aim the telescope at a star to the north, and somewhere near the celestial equator. (That's 90 degrees away from the pole, along a line that runs straight overhead.) Look for north-south motion of the star. We're only concerned with north-south motion. If it drifts east or west, your drive is set too fast or slow. And I'm talking about actual motion, i.e. the direction you have to push the telescope to re-center the star. If the star drifts south, then rotate the entire mount so the axle points farther east. If it drifts north, rotate the mount so the axle points farther west.

Now look at a star near the horizon to the east, and see if it drifts north or south. This time if your target drifts south, then angle your axle farther downward (toward a lower latitude on your scale). If it drifts north, raise the axle.

If you were a long way off in either direction, do the whole procedure again to fine-tune it.

Acknowledgments

I've had a lot of help in developing the trackball idea. My wife, Kathy, has been incredibly supportive and has helped brainstorm many a gadget at unlikely hours of the day. David Davis and Mel Bartels, thin mirror gurus and telescope builders extraordinaire, have also been most generous with their time and enthusiasm. Thanks also to Tom Conlin, Ted Touw, Craig Daniels, Chuck Lott (of the Lott 35mm finder fame), Dave Cole, Bill Murray, and the entire Eugene Astronomical Society for help and support. Thanks also to Gary Seronik, my editor at Sky & Telescope, who knows how to make a writer feel appreciated. Thanks to you all, I've had a ton of fun on this project.

I hope anyone else who builds a trackball will get as much enjoyment from it as I have had with mine.

[Jamie]

The Trackball Telescope Optics

The trackball telescope itself is basically a Newtonian reflector with the mirror inside a ball instead of a tube or a box. Trusses hold the secondary the correct distance away from the primary mirror, and a counterweight below the mirror balances the whole works so it will stay put wherever you aim it. The beauty of this design is that you can aim it anywhere simply by pushing it there (you're not locked into altitude and azimuth or right ascension and declination motions), and you can rotate it so the eyepiece is in a comfortable position no matter where in the sky you're pointing. Even if you don't build the tracking mount, the spherical base in a simple cradle is much more fun to use than an alt-az or an equatorial mount.

First, decide how big you want it to be

A spherical scope has to have its balance point in the middle of the sphere. That means the longer the focal length, the more counterweight you have to put in the bottom to balance the weight of the secondary and focuser and finder. F/6 or so is probably the practical limit. My scope is f/3.8, which is probably a bit shorter than it needs to be, but that's where my mirror wound up when I was done grinding and polishing it. (No, I didn't start with a salad bowl, but I followed what can only be described as a steep learning curve.)

There's probably a practical limit to the mirror diameter, too. Mine is 10" and the finished scope weighs 35 pounds. It's easy enough to carry without taking the secondary cage off, but I'd bet a 12" or larger would only be movable in pieces. That's not necessarily a big deal, since the trusses are simple to remove, but I do enjoy being able to carry the thing outside already assembled and start observing within a couple of minutes. Decide what's most important to you: aperture or portability, and plan accordingly.

Start with a sphere

I did this the hard way, coating a big rubber ball with fiberglass. You don't have to do that. Go to lightingdiffusers.com, rotonics.com, or cicball.com and buy a ready-made sphere. It will be much cheaper, more perfectly spherical, and lightingdiffusers.com will even cut the opening to your specifications. (The others may do so as well; I haven't checked.)

How big a sphere? As mentioned above, the scope's center of gravity has to be in the center of the sphere, which means the mirror has to rest below the center to help counter the weight of the secondary and the focuser and the finder. 1.5 times the mirror diameter will let you place the mirror pretty low in the sphere and still leave room for more counterweight below if you need it.

How big should the opening be? Make it about 1" bigger than your mirror diameter. That will give you half an inch of clearance all the way around for the light path. You could cut it bigger, but if you cut away too much of the sphere you won't have enough bearing surface to let you look at objects close to the horizon.

My fiberglass sphere was strong enough to not need any stiffening material around the opening, but a factory-made sphere might be thinner and/or more flexible. If it is, you'll want to attach a ring around the top to reinforce it.

The layout inside the sphere

Everything inside the sphere is glued, rather than bolted into place. That leaves the outer surface smooth to ride on the bearings and the axle without bumping over screw heads. If you don't trust glue, go ahead and screw, but recess the heads and smooth them over as well as you can. Any deviation that rides over the axle or the roller bearings will mess up your tracking.

The counterweight goes in the very bottom, and the mirror sits above it. Because you can't collimate the mirror from behind the way you do in a regular Newtonian scope, you hang the mirror from three supports and collimate it from above. It's easier to see in a cutaway drawing than in a photo. I show the mirror mounts and the truss mounts on opposite sides of the ball for clarity, but they're really 120 degrees apart.

My original counterweights were two disk-shaped weights off a set of barbells, mounted on a short length of pipe that I glued to the bottom of the ball. That worked fine until one day when I tilted the scope toward the horizon and the pipe broke loose. Now I use lead shot that I mixed with epoxy. It's not going anywhere!

The shot doesn't look centered in the drawing. That's because it's not. The focuser and the finder will offset the telescope's center of gravity to the side, so you should put the counterweight a little to the other side to compensate for that. How much? You could calculate out all the moments of inertia of the various parts of the system, or you could just assemble the scope, put an average-weight eyepiece in it, and balance the scope upright on a hard surface. You will have to tilt the focuser and finder inward a little to get it to balance. The point on the ball that's touching the floor when it balances is where the center of your counterweight should go.

Don't paint the inside of the sphere until everything is glued in! The paint bond won't be strong enough to hold. As for what kind of glue to use, I used epoxy and Shoe Goo on my scope. I don't know what the best glue for acrylic or polycarbonate or other materials would be; you'll have to experiment. If people let me know what they learn about various materials, I'll post it here. I do know that Shoe Goo only cures on contact with air, so it's not good for bonding two wide surfaces together. It will only cure around the edges.

Making and mounting the mirror cell

I used a triangular piece of plywood for a mirror cell, with felt pads to support the mirror and three clips to hold it in place. The mirror hangs out over the side of the triangle and could be hit by the counterweights if they ever broke loose, so I'm thinking about making a new cell that's as wide as the mirror all the way around. Even if you do that, you'll still need three "ears" on it for the collimation screws to attach to.

Aficionados of "PLOP," the fabulous program for calculating mirror support points, will shudder when they see this support pattern. This was supposed to be temporary, and I was going to run PLOP to find out where the support points should really go, but when I took the scope outside for the first time I got such good images with it that I didn't want to mess with it.

I think that was blind luck. You should probably run PLOP to see where to put your supports. Here's the link: http://www.davidlewistoronto.com/plop/

Here's a closeup of the mirror clamp. The top part reaches over the mirror by 1/8" or so, and the bottom part rests against the mirror's edge to keep it from sliding sideways when you tilt the scope toward the horizon. To make it I bent a piece of galvanized metal strap into the right shape, drilled it so the deck screw would fit through without binding, and padded the face that touches the mirror with a piece of rubber (glued on). The screw goes straight into the wood, which provides enough friction to let me snug the clamp down without fear of it coming loose. If the screw hole does loosen up over time, a little wood glue should snug it right back up again.

You could embed a nut in the wood and use a bolt instead of a screw, or figure out a completely different mirror clamping system. If your mirror is much thicker than mine, you probably should, because a screw in 3/4" of wood isn't going to hold a heavy mirror if you jounce it hard. The important bit is to make sure the mirror doesn't go anywhere, but doesn't get pinched by the clamps. When you tighten them, just snug the rubber down to the mirror. Don't compress it.

Note the washer at the bottom of the photo. That's where the spring for the collimation screw rests. (See below.) On the underside of the wood I've embedded a nut for the collimation screw to thread into.

Here's the mirror mount inside the ball. I shaped the back edge of the wood to match the curve of the ball and glued it in place. There's room for a triangular wedge below it, also glued in, for extra strength. This (and the other two like it) are all that holds your mirror in place, so make sure it's solid.

The hole through which the collimation screw goes should be snug, so the screw has no sideways slop. This will keep your mirror from shifting from side to side when you tilt the scope from horizon to horizon. On the other hand, the hole shouldn't be so snug that the threads bind in it. The actual collimation adjustment takes place when the screw pulls upward on a nut embedded in the bottom of the mirror cell, so you don't want it riding up and down on the upper mount as well

You could make it work that way if you would rather, and it might even hold the mirror more firmly in position if you did. But one end or the other has to be free to slip, or you won't actually have a collimation adjustment.

Put washers on both ends of the spring and under the knob to prevent any metal parts from rubbing on wood. Glue the washers in place. It's almost impossible to assemble everything if all the parts are loose!

Paint all the wood and hardware flat black to cut down on internal reflections. Glue first before painting! Note the nifty anti-reflective texture on the ball's inner surface. It's tempting to recount how many hours of painstaking work it took me to etch that in there, but in fact it's just the texture of the rubber ball that I started with, faithfully reproduced when I coated it with fiberglass .

The collimation screws are just all-thread (I used 10 x 24) with three nuts tightened together on one end to make a convenient knob. Put a piece of rubber hose over the nuts to improve your grip.

The springs should be relatively stiff. 5 pounds only compresses these guys about 1/4".

Make the screws long enough to engage the nuts in the mirror cell with the springs all the way extended. That will allow you to put the mirror cell in place without compressing the springs. But make sure the screws are short enough to miss the inner surface of the ball.

The trusses

The bottom of the trusses clamp to the outside of the ball. It may seem counterintuitive, but the stationary part of the clamp goes to the outside; otherwise you won't have enough wood for the mounting screws to bite into. (If you wind up putting a reinforcing ring around the opening, you might be able to do it the other way.) It doesn't look like there's much form-fitting in the clamps, but I did actually dig grooves in the blocks for the trusses to fit into. I also put tiny nails in the stationary side to pin the trusses in place so if I lift the scope by the secondary, the ball won't fall off.

Alas, the trusses aren't interchangeable. I tried, but when you're hand-building things, there are too many variables. Do your best, but if you have to number them, don't feel bad. So did I.

The trusses should probably be painted black to cut down on glare from reflected light. I plan to do that real soon now. :-)

The secondary cage

Your secondary cage can be just about anything, down to something as minimal as an embroidery hoop with a black card sticking out of it for a backdrop. The only real requirement is that it be light, because every ounce up there is going to require 5-10 times as much weight inside the ball to balance it. I used the top of a 5-gallon bucket. The fins around the top make it surprisingly stiff, and the lip where the handle went makes a great place to mount the trusses. The bucket tapers, so you'll have to shim the focuser a bit to make it aim directly across, but that's easy.

I used 1/2" oak dowels for trusses. The top of the trusses can be filed flat to fit inside the bucket rim and attached with bolts and wingnuts for easy assembly. Note that they don't meet on top. When I tried that, they forced the bottom of the bucket into a cloverleaf shape. A longer focal length would probably avoid that problem, but there's no reason why the trusses have to come together at the top (or at the bottom, either, for that matter). As long as they've got some angle to them, the scope will be rigid.

While you're deciding where they'll go, look down from the top and make sure they don't cut through your light path. If they're angled too steeply, they will.

The plastic bucket will probably take paint without scuffing it first, but I hit mine with some fine grit sandpaper just to make sure. Use several very light coats of spray paint rather than one heavy one. When the inevitable scrapes and bangs expose white plastic, I touch it up with a black Sharpie.

The spider and focuser

I bought my spider from Murnaghan Instruments. This is their 11.5-12.5" model, and it fit in the bucket without any modification whatsoever. It's lightweight and easy to collimate. People who are used to 3 adjustment screws may wonder if four will work as well, but I've used both kinds for a while now and I actually prefer 4. You can adjust one axis without budging the other, and it's much more intuitive which axis you're adjusting!

The heavier the spider, focuser and finder, the more counterweight you're going to need down inside the ball. I decided it was worth the trouble to use a good rack-and-pinion focuser (yeah, yeah, I know some people think "good rack-and-pinion" is an oxymoron, so get yourself a Crayford or a Feathertouch), but that did lead to some balance trouble. With my original counterweights centered in the ball, the weight of the focuser and finders would pull the scope to the side, so I had to add more weight to the secondary cage on the side opposite the focuser, which meant I had to add more weight inside the sphere, and so on. I was actually glad when my first (inadequately glued) counterweights inside the sphere broke loose, because that meant I could put the replacement weight off to the side a bit so the scope would balance better. Now I don't need extra counterweights on top at all.

The finders

I use two finders. The main one is a really lightweight finder made by a friend of mine named Chuck Lott. Chuck is a genius with finders. This one is made from a 35mm film can, a lens, and some clear plastic sheeting, and it weighs all of 3 ounces. If you're interested in the design, Chuck would be happy to hear from you. He can be reached at the email address below. (This is a graphic file to thwart spambots, so you'll have to copy it by hand.)

3 ounces is such an insignificant weight that I went ahead and mounted my green laser pointer beside it for a "lazy man's finder." Turns out the laser is great for rough pointing, but the 35mm finder is much more precise, so I may remove the laser pointer.

The stars are stickers that Kathy and I put on whenever we both see something cool through the scope.

The dust cover

I use a plastic bag over the secondary, secured with a rubber band taped to the top of the bag. I'm sure there are more elegant ways to do this, but as the old saying goes, "Temporary arrangements tend to become permanent."

I used a piece of masonite for a primary cover. I cut notches in it so the truss mounts would help hold it in place, and I made it stick out over the rim of the ball a ways for two reasons: it gave me a place to put some half-round closed-cell foam insulation to seal it tight against the ball, and it made the disk wide enough that there's no way it will fit through the hole and bang into the mirror if I accidentally drop it. It fits tight enough that the bolt heads on the truss clamps hold it in place.

One problem with masonite: it has to be sealed or it's dusty, but it sucks up varnish like a sponge. I must have used half a pint on that tiny disk alone, and then it outgassed so bad I was afraid to get it near the mirror until I'd let it cure for a couple of weeks in the sun. I think I would use plastic if I did it again. (Maybe the bucket lid? I didn't get one with my bucket, or I'd probably have tried that.)

Paint and wax

I used automotive touch-up paint for the outer surface of the ball, figuring it would be more durable than standard spray paint. I don't know if it is; but it does seem to hold up pretty well. I have also waxed it with Turtle wax to let it glide over the PVC bearings better than the paint alone did. I was afraid that might make it too slick, but it's just about right.

That's pretty much the scope. You can come up with all sorts of variations that would still work on a trackball mount. The key is to make something that has its center of mass (and thus its center of balance) in the middle of the sphere, so there's no tendency for it to rise or fall or twist no matter what position it's in. A little bit of imbalance is okay, but if it's too much the scope will slip against the bearings and won't track properly.

[Jamie]

The Trackball Mount

This is the new part of the trackball system. The basic principle of the whole works is to rest the spherical bottom of the telescope against an axle that points at the celestial pole, and to rotate the axle so the sphere turns once a day--exactly counteracting Earth's rotation. The axle is mounted on a shuttle that can be moved along an arc, allowing adjustment for the viewer's latitude.

A sphere needs at least three points of support to hold it in place. The axle is one of those points. I tried Teflon glides for the other two, but at the incredibly slow speed of the ball's rotation, even Teflon proved too sticky. Rather than track smoothly, the scope would lurch forward, then pause, then lurch, then pause. Wax helped, but then the ball's balance became supercritical. There's undoubtedly a happy medium somewhere, but I gave up and went to roller bearings. That turned out to be a good move. Roller bearings allow frictionless motion in the right direction and resist motion in other directions, which actually helps force the scope to track more precisely as well as more smoothly. (No, they don't swivel, and yes, the ones in the photo are really angled in the correct direction for my latitude. See below for details.)

A little experimentation showed that PVC pipe provided the right amount of friction to keep the ball in place, yet let it slew smoothly with just a light push.

The center disk is just a simple plywood round purchased pre-cut from a lumber store. The legs were all cut from one 2' x 2' piece of plywood. I cut notches in the legs so they would fit nice and tight over the disk, then used chair-stiffening brackets underneath to stiffen them even further (just visible under the left side of the disk). I still had a bit of vibration, so I later added the turnbuckle tensioners.

I keep meaning to drill some holes in the disk for eyepiece storage while I'm observing, but so far I haven't gotten around to it. There's plenty of room for it, though, and no reason not to.

The axle

The axle is pretty simple: I used a carriage bolt, some washers, a Bic pen barrel, a hose, and a piece of PVC pipe. I had to ream out the pipe to get the hose to fit.

The gear is from a hobby shop. It's the drive gear for a radio-controlled car, and costs about $2.50. Drill out the hub so it fits snugly over the pen barrel, and drill a couple of tiny holes in the spokes for equally tiny brads to pin it to the pipe so it will actually turn the axle instead of just spin in place. In the S&T article I said to pin the gear to the hose, but I later realized there's too much flex that way. Pin it to the pipe. (You could also glue it if you want.)

Wax the bolt before you slip it inside the pen barrel. I just ran a birthday candle up and down it a few times.

The bracket that holds the axle (see below) is a piece of 3/4" x 3/32" bar stock bent into a shallow "U" and screwed to a block of wood. Leave room for the worm gear (a 3/8" bolt) to clear the bracket. (If you use a different gear than what I used, you may need a different size bolt to mesh with the teeth.)

The washers keep things from slipping apart and provide spacers so the axle fits snugly in the bracket. Any play there will become slop in declination when you're using the scope.

You can remove the lettering on the pipe with a simple rubber eraser.

The motor

The ball has to turn once per day. If you have a 16" ball and a 7/8" axle, that means your axle has to turn 18.28 times per day. A 50-tooth drive gear means your worm gear needs to turn 914 times per day, or 0.635rpm. It's hard to find a motor that turns that slowly, so I made a second worm gear that brought the motor speed up to 30 rpm. It's a lot easier to find 12-volt DC motors in that range. I bought one that was rated for 20-45 rpm so I could tweak the tracking speed quite a ways if my calculations were off. (I got mine from Techmax, which sells salvaged motors. Jameco also has a good selection of new motors.)

You could undoubtedly use a step motor if you want. I went with a simple DC gear motor because the circuitry is so much simpler. A variable resister between the motor and the battery is all you really need for a speed control. I got a little fancier, but not a whole lot more so. (See circuit diagram below.)

I had to drill a hole in the second worm gear (i.e. another 3/16" bolt) so it could fit over the motor shaft. I had a heck of a time getting the hole centered until I realized I could put the bolt in a drill press and lower it onto a stationary drill bit. Perfectly centered hole! Epoxy it onto the motor shaft, and occasionally run the motor while the glue is setting so you can detect (and correct) any wobble. This worm should be as perfectly centered on the motor shaft as you can get it. A little wobble won't matter, but too much will give you a periodic tracking error.

I mounted the motor on a pivot so the weight of the motor would hold the worm against the drive gear. That turned out not to be quite enough weight, so I added a bolt and a few nuts for more weight on the back of the motor. The pivot is on the same shaft that holds the other worm gear's mounting brackets, which explains that thicket of nuts and washers between the motor and the shuttle.You can probably figure out a better system than this!

Late-breaking news: I added a spring-loaded idler bearing to push the motor's worm-gear shaft against the drive gear, and that works much better than simply adding weight to the back.

The worm that drives the PVC axle is held into place by two more pieces of 3/4" x 3/32" bar stock. The angle of the bar stock allows the worm to be adjusted up and down so it meshes smoothly with the drive gear's teeth. I leave a little play in those gears, because the weight of the telescope compresses the axle and tightens them up.

Latitude adjustment

The latitude arc should be concentric with the sphere, so when you move the axle shuttle up or down, the sphere stays put. You don't have to be overly finicky about this; just get it close. (Don't use the same radius as the sphere for this arc! The radius of the arc has to be bigger than the radius of the sphere by the thickness of the axle-shuttle combination.)

My shuttle uses two pads on the back side (visible three pictures above) and one on this side with a screw-down clamp to hold it in place. Use wood for the pads, rather than rubber, or you'll get a lot of vibration in the scope. The clamp is just a bolt with a wooden disk on the end of it. I got fancy and drilled a hole in the end of the bolt so I could screw the disk onto it. It's harder than I thought to tap threads into a bolt!

The knob is just a nut snugged up to the bolt head and both of them covered with a piece of rubber hose.

I made a power distribution box out of an old telephone adapter. Power goes into the box from a 12-volt battery below, then up the phone cord to the hand controller and back to the motor.

The other two bearings

The other two bearings are just idlers. They let the ball move without friction in the direction of tracking motion, and they provide just enough grip to constrain that motion to the angle you set them at. They're made of PVC pipe couplings with rollerblade bearings inside. Rollerblade bearings are cheap--about $1.25 each--and you can use a 5/16" bolt for an axle. There are a couple of nuts down inside to keep the edge of the PVC from rubbing on the bracket. The angle brackets are made from the same 3/4" x 3/32" bar stock as the drive axle mount. I made mine adjustable in two axes, but after using the scope a while I realize that I only needed the top adjustment. I may redesign mine to see if removing one angle bracket will reduce vibration any.

The pipe couplings have to be reamed out a little for the bearings to fit. You don't need the whole coupling--half or 3/4 of it will do. The main reason to have it at all is to give the ball a nice slick PVC surface to ride on, and to make that surface wide enough that the edge of the bearing can't dig into the paint. (Rollerblade bearings alone are way too narrow.)

The bearings don't have to be positioned at any particular height, but bear in mind that the higher they are, the more they'll get in the way when you point the scope toward the horizon. I made mine so I could tilt the scope horizontal without dropping off the bearings, but high enough so I wouldn't push the scope off the mount when I was slewing it.

The angle of the bearing is determined by your latitude. More on that below.

LATE BREAKING NEWS: It turns out there's an easy way to add declination control to a trackball. Thanks go to Pierre Lemay for this idea. If you mount the idler bearings so they can be moved toward or away from the drive axle (actually I think moving them toward or away from the celestial pole would be more direct) their motion will force the ball to roll up or down the axle, which translates into motion in the declination axis.

Bearing angle

You might think that the two idler bearings should be angled more horizontally. I thought so, too, until I watched the way the ball skidded across them. At my latitude (44 degrees north), the bearings need to be oriented almost vertically.

Why is this? Look at the photo to the left. Think of the ball as a globe that spins just like the Earth does (but in the opposite direction). I've pointed the telescope at the celestial pole to make this easier to visualize, and I've drawn latitude lines on the ball to show where something touching it would track when the ball spins. The bearings need to roll along those latitude lines.

Now imagine taking the telescope to Alaska, where the celestial pole is much higher in the sky. The scope will be angled much higher, so the bearings will need to "toe in" a ways to continue tracking lines of latitude. Go to Hawaii and you get the opposite effect: the celestial pole is practically on the horizon, so the bearings need to toe out in order to follow the latitude lines.

Of course it doesn't really matter where the scope is pointed, since the surface is a sphere. As far as the drive is concerned, it's just spinning a globe along the celestial axis no matter how that globe is oriented.

The easiest way to figure the correct bearing angle for your latitude is to set up the scope with the drive axle aligned to the pole and just watch the surface of the globe move past the bearings. Adjust them so they're rolling rather than skidding, and you've got it.

The hand controller

The hand controller is pretty simple. It's got an on/off switch on top, a button to kill the power and let the sky's motion center an object in the field, a fast-forward button to center objects that are already too far ahead, and a variable resistor to adjust the tracking speed. The wire is just a modular telephone cord.

The control circuit is also fairly straightforward: the kill button is just a normally-closed momentary switch in series with the power switch, and the fast-forward button shorts out the dropping resistors. I wired the variable resistor in parallel with a fixed resistor so the variable wouldn't have to take all the current going to the motor, but at 50 milliamps I doubt if that was necessary. The electrolytic capacitor provides a little boost to start the motor when it's cold.

That's pretty much the mount. I've included a few paragraphs on the introductory page about how to use a trackball

[Kaptain Klevtsov]

Brilliant concept, I love it.

Captain Chaos

[geppetto]

Wonderful 8)

Great write up, you've put some work into that.

Thanks for sharing

[Jamie]

Wonderful 8)

Great write up, you've put some work into that.

Thanks for sharing

Sorry to miss lead you Philip but thats not my work.

I came across it whilst surfing and thought i would share it with you.

[geppetto]

Fair enough Jamie

Great find none the less