A 12.5" Trackball with Super-light Secondary Cage and Focuser

Designed and built by Jerry Oltion

Featured in the May, 2014 issue of Sky & Telescope Magazine

After building the Scaled-up Astroscan, my next scope project was a 12.5" trackball. I had a great big full-thickness mirror that I'd ground to f/4.6, and I had another 20" ball to put it in, but at 58" focal length I knew that balance would be a significant problem. I could always add a ton of lead shot to the bottom of the ball, but the mirror already weighed 18 pounds and I didn't really want to add any more weight than I had to. I decided to see if I could make the mirror be the only counterweight necessary. I did the math: with 18 pounds only 6 inches from the ball center (which was as low as the mirror could go), I could only balance 2.5 pounds up at the middle of the secondary cage 43 inches away. Holy crapola! 2.5 pounds for a complete secondary cage? With mirror, focuser, finder, eyepieces, and at least half the weight of the trusses?  Even the weight of the paint would be a significant portion of that. I considered a minimalist secondary with just a ring and a light baffle, but any ring I could find that was rigid enough was over two pounds just by itself. I considered doorskin, but even that ultra-thin (0.11") plywood would have made a cage weighing 2.4 pounds just in the wood. My secondary mirror weighed 6 ounces -- I would be overbudget without even considering a spider or focuser or finder. I considered using styrofoam insulation, and even built a prototype secondary cage with Pink Panther insulating foam, but while it only weighed in at 11 ounces, it dented really easily and it looked huge perched on top of the trusses. I needed to think of something else.

Then I discovered Duraplast. It's a plastic foam board with plastic facing, it's only half as thick as Pink Panther foam, and it's light, light, light. A 4 x 8 foot sheet of the stuff weighs only 8.5 pounds. Enough material to make a secondary cage would only weigh 1.4 pounds (20 ounces). That's almost twice what the Pink Panther foam weighed, but it still left me with some spare weight budget for the rest of the components. Plus it comes in flat black, eliminating the need for paint. That saved a couple of ounces right there! The Duraplast is made by Gilman Brothers and it cost me $63 for a 4 x 8 sheet, but I only needed 1/4 of the sheet for the scope so my cost for this particular secondary cage was only $16.  Not bad. I found out later that there's another kind of foam core board that's easier to find and probably even stiffer: it's called "Gator Board" and it's available at nearly any art supply store. It's a bit more expensive, but not outrageously so. I cut eight panels, using 22.5 degree cuts on the edges so I could glue them edge-to-edge with solid glue joints. I made them just under a foot high so I could get an extra couple of panels from the middle of the circles, which are 13.5 inches inside diameter and 17 inches outside diameter. Duraplast glues well with epoxy, especially on the edges, where the bond is stronger than the foam. The flat black face has a dusty texture to it that doesn't grip the glue as well. You can peel the glue off the face if you work at it. The only places where that might be a problem are on the top and bottom rings, and there's so much surface area there that I didn't think it would be a problem, and so far it hasn't been.

I used 24-gauge wire to make the spider. I even found some that was anodized black. You don't want your wires to go all the way to the center or you get torsional vibration, so I used some sheet aluminum to make the upper strap and lower X that the wires attach to. The secondary mirror holder is made from a vitamin bottle that I cut at a 45-degree angle. The all-thread rod extends through the cap to a nut with a big washer that spreads out the force across the whole cap, which lets the adjustment bolts hold everything snug. I was surprised at how stiff this arrangement is, and it only weighs 2 ounces (counting the wires). I use four adustment bolts rather than three. I much prefer this setup, even though I have to be careful to snug all four to equal tightness. The mirror's motion is intuitive and precise with four bolts. (Make sure one set is in line with the focuser.) I don't actually use the adjustments much, though; it's easier to just tighten and loosen wires to tilt the secondary when you need to collimate. The wires don't need to be very tight. Just enough to take out any sag, but not nearly tight enough to play a tune on. They barely pull on the foam at all. I made little pads on the outside for the adjusting wingnuts to rest against, thinking I would need to spread out the pressure so it didn't dimple the foam, but I doubt if that was necessary. It's nice to have a flat surface for the wingnuts to rest against, though. You can see them in the photo below.

Here are the flat surfaces for the wingnuts to rest against. They're made from a big wooden dowel. Notice the angle of the bolts, so the wires can criss-cross on their way to the secondary mount. (Crossing the wires prevents vertical sag of the secondary mirror.) The cut angle for the dowels was a complex angle, since the flat foam panel they rest on isn't perpendicular to the bolt angles in either the horizontal or vertical direction. You can calculate the angle, or just wing it and sand until you get it right. I drilled out the center of the bolts so the wires would pull directly down their length rather than off to the side. The holes are angled enough to break out along the side of the bolt head, where I wrap the wire once around and come back out the head to tie against itself. When I tested this arrangement for strength, the wire stretched and broke in the middle rather than snapping at the bolt head.

Stringing the wires and centering the secondary mirror is easier than you might think. I made the all-thread shaft long enough to reach the top plane of the cage when the mirror was in the right position, so it was a simple matter of hanging it from a stick spanning the top of the cage while I strung the wires. Each wire is a separate entity; I didn't loop around and go back out with the same wire. I did a prelimiary set of wires with long extra tails on both ends so I had plenty of adjustment room; then once I got the lengths right I removed one wire at a time and replaced it with a final wire cut to the right length and with the ends finished off with tight little twists that don't get in the light path.  I use the adjustment bolts for fine tuning, and if I ever reach the end of a bolt's adjustment I simply re-string that wire with a new one that puts the adjustment back in the middle of the range.

The secondary cage, with mirror, weighed 32 ounces. That only left 8 ounces for the focuser, eyepiece, and finder. That seemed nearly impossible, especially since I wanted a Crayford focuser. Even the minimalist ones I'd seen were way over 8 ounces. I realized I would have to come up with something different, so I asked myself what's actually essential and what isn't. The essential parts of a Crayford focuser are the drawtube, the bearings, something to hold the bearings in the right place, a drive axle, something to hold the axle, and knobs. I already had the bearings, which I got from Surplus Shed. They already came on inch-long threaded shafts, which led to the idea of bolting them to an angle-iron spine. Iron is heavy, though, so I got some angle-aluminum instead (1 ounce). I splayed the sides out in back so I could bolt the assembly to the cage. I considered a 1.25" drawtube to save weight, but that would have led to vignetting problems. Fortunately 2" ABS  pipe is really light. A five-inch length of it only weighs 1.75 ounces. An aluminum file hanger strap from a filing cabinet proved stiff enough to support the crank shaft (a size 2 knitting needle donated by my wife, Kathy). I put some plastic oxygen hose over the knitting needle for grip.  A piece of high-density plastic made a good tensioner bearing. Even the knobs are as ultra-light as I could make them. They're cut off the core of a paper spool, and are mostly hollow. With that, the focuser was done.  It weighed in at 5 ounces. Turns out I didn't really need the tensioner. Once I set it to the right pressure I've never touched it again, so I could have done that by carefully drilling the axle holes in the first place (saving maybe 1/10 of an ounce!)

This is the finished focuser. I had a machinist friend make a hollow aluminum 2" to 1.25" adapter that only weighs an ounce. I mounted the focuser with a hard plastic plate on both sides of the Duraplast to distribute the force of the bolts. Since there are only two bolts there's a little bit of rocking motion if you really tug on the focuser, but with the weight of any 1.25" eyepiece I've got, it's rock solid. I could in theory use 2" eyepieces in this scope, but few 2" eyepieces are light enough for it to remain balanced. I do have some leeway, though, especially when I'm observing near the zenith, so I occasionally put a wide-field Nagler in there. Even then, the focuser holds its position pretty well.

This is the finished secondary cage with the finder (my glow in the dark design, more info on that here) and focuser. Note how the trusses just bolt straight to the Duraplast. I kept the bolts from crushing the foam by drilling 1/4" holes in the foam and putting nylon inserts into the holes. The bolts tighten against the inserts, not against the foam. The inserts are epoxied into the foam so they're nice and rigid. I added an extra layer of foam right at the pressure point to give the trusses a little more contact area, but that probably wasn't necessary. Note also the bracket holding the finder on. That's homemade out of aluminum and wood, and weighs about half what a commercial dovetail bracket weighs. (And costs about fifty cents if you don't count your time.) The whole works (with finder) weighs 39 ounces, leaving me...one ounce for my eyepiece. Fortunately I had been overly conservative in my estimate of how light the upper end needed to be. When I assembled the scope and set it on the kitchen floor, it rolled upright like a Weeble. Turns out I can use an 8-10 ounce eyepiece and the scope will still balance just fine. That includes almost any plossl design and quite a few Explore Scientifics and Naglers. With my lightest eyepieces I actually have to clamp extra weight on one of the trusses to balance the scope.

The rest of the scope is pretty much a basic Trackball. The ball is a 20" polycarbonate sphere from 1000Bulbs.com that I fortified with fiberglass inside. Since that's the same size ball that I used on my Big Astroscan, I didn't have to build a new base for it. I just use the Astroscan drive. The extra weight of the scope (35 pounds total) doesn't affect the drive at all. It could make a Volkswagen track if the Volkswagen was balanced well enough. I use ski wax on the outside of the ball so it'll glide easily on the mount's drive axle and rollers. The mirror cell is made from an aluminum frying pan. (More on that here.) The collimation bolts stick upward so I can just reach inside the ball to adjust the mirror. The trusses are just wooden dowels. So how does the scope work? Really well. I love using this telescope! The 12.5" mirror gathers a lot of light, and the trackball design makes it a joy to use. I can sit down to observe, and the target stays in the eyepiece for as long as I want. Slewing from target to target is like breathing; that long lever arm makes it feel almost frictionless. There's no lurching even at the slowest nudge speed. Zero! The lightweight upper end doesn't seem to have added any vibration at all. If anything, this scope is steadier than my other trackballs, maybe because the foam damps out what little vibration the rest of the scope might generate. So there it is: the ultra lightweight secondary and focuser on a 12.5" trackball. I'm very happy with it, and will probably build foam secondaries and lightweight Crayford focusers on any future scopes that require very light front ends. It's a good solution to the problem.

How to contact me

I'd love to hear from people who are interested in this scope design. Please feel free to email me at the address on the right. (Sorry you can't click on it or copy and paste it; it's a graphic file to thwart spambots that search the internet for addresses to send junk mail to.) I have no idea how much mail this idea will generate, so I can't guarantee a response, but I'll do my best to answer everyone who writes with a genuine question or comment about the design.