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!

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.

Later modification: 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.

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. You may need omnidirectonal idler bearings in order to prevent the ball from skidding. You could also get declination control by moving the drive axle in or out (being careful to maintain its polar alignment while it moves). Be careful to limit the range so you don't push the ball off the idlers!

Bearing angle

You might think that the two idler bearings should be angled so they roll 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.

NEW INFORMATION: Another way to look at it is to figure where the axis of rotation sticks out the bottom of the ball (it's the center of that target in the photo), and aim the axles of the idler bearings at that point.

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 . Here's a direct link to that discussion.

Click here to go to the discussion of the optics.

Click here to go back to the trackball front page.

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