How to build a Dobsonian mount for a 5-inch telescopeNovember 2, 2010
By reader request, I am posting instructions on how to build the Dobsonian mount shown in photo above. The design was made up mostly on the fly to suit my particular needs and the materials I had on hand. Your requirements might be different, so I’ll also discuss the design decisions I made along the way, so that you can a build a mount that fits your telescope and your needs.
If you have the freedom to design a telescope from the ground up, you’ll be much better served by the free instructions provided by the Sidewalk Astronomers, or in any of the several telescope-making books out there. I’m thinking here of Build Your Own Telescope by Richard Berry, The Dobsonian Telescope by David Kriege (yes, that David Kriege, founder of Obsession) and Richard Berry, Sam Brown’s aging but accessible compendium, All About Telescopes, and several others. There are also some awesome resources online, including the Cloudy Nights ATM (amateur telescope making) forum, and plenty of solo ATMers who have posted pictures and descriptions of their designs on their own blogs or webpages (I like these, these, and this one in particular).
But as Rich Hoover said in a comment,
Most everything on the net is catered to 6″ or 8″ OTAs, so finding some GOOD plans for a 130mm OTA is rather difficult. Those plans that I’ve seen also make some rather large assumptions on the user’s part. It’d be great if there were plans with a complete parts/price list.
So it seems there is a need here, and I will do my best to address it–although the emphasis on GOOD plans might mean I’m doomed from the start!. I have zero experience with woodworking and anyone who knows what they’re doing will be appalled at the crimes against wood–and probably good taste–that I have committed. It is the height of ridiculosity for me to be instructing others in how to do this. Still, if you have some scrap wood, a saw, a drill/driver, no relevant experience and a desire to experiment, then you have everything I did when I started and you will probably do as well or, most likely, better.
Many thanks to David DeLano for much help along the way, and to Rich Hoover and Dan Walker for their continued interest in this project. Hope you find this useful!
PARTS OF A DOBSONIAN MOUNTED TELESCOPE
The first thing to do is to get familiar with the basic parts of a Dob mount. In addition to the OTA (optical tube assembly), there are three basic assemblies: the altitude bearings, the rocker box, the ground board.
The altitude bearings are attached to the OTA and allow the telescope to rock up and down. The rocker box is really the main part of the mount, and the one that requires the most design decisions and the most work to build. At the top end it provides bearing surfaces for the alt bearings, and at the bottom it rotates on the ground board via the azimuth bearing (which is not listed as a separate part because it is created by the rocker box and ground board). The ground board is the simplest piece, the easiest to build, and the hardest to screw up.
When we get down to the actual building, there will be separate sections on how to make each of the three pieces, plus separate sections on bearing surfaces and finishing the mount.
MEASURE TWICE, CUT ONCE
If you’re building a Dob mount for an existing OTA, the tube dictates everything else. So the first steps are to measure the tube, find its balance point, and decide how tall you want your mount to be. Only with these measurements in hand will you be ready to start cutting wood.
In my case, the OTA came from a SkyWatcher 130N-EQ2 setup. I bought this last summer on the cheap, and they’re still pretty darned good deals. However, I didn’t want the EQ mount, so I sold it and started building a Dob mount.
Finding the width of the rocker box. The OTA on this particular scope is 33″ long and 6.5″ in diameter (blue arrows). I decided to attach the alt bearings to the tube rings, which have 1/4-20 sockets, for simplicity (if you don’t have tube rings, you’ll have to build a box around the tube, like this). The tube rings add an extra half an inch on either side, bringing the width of the thing which must be mounted to 7.5″ (red arrows). But that’s not all–I also knew that I would be using thin plates of wood as bearing retainers to keep the OTA and alt bearings from slipping off their supports on one side or the other. The bearing retainers are each 0.25″ thick, meaning that the rocker box had to enclose a space at least 8″ wide (yellow arrows). I wanted a little breathing room on top of that; I was using wide bearings so if the rocker was a little too wide, no problem, but if it came out too narrow, I’d have to start all over. So I made mine with an internal width of 8.5″ (white arrows), although in retrospect 8.25″ would have been enough.
Finding the height of the rocker box. Here you have a lot more freedom. The length and balance point of the OTA set the minimum height of the rocker box, in that you want the bottom end of the OTA to be able to swing freely all the way to vertical without bumping into the bottom of the rocker. On the other hand, you can make the rocker box as tall as you want, within reason. I wanted to make the entire mount as small and light as possible, so I went with the minimum height, and I’ll explain how I got there in a moment.
One persuasive reason not to make the mount as small and light as possible is that such a short mount will require you to either get down on the ground to observe, or to set the mounted telescope on some kind of stool or table. Neither solution is optimal for most people; getting up and down off the ground can be tiring and dirty (although some people aren’t bothered by either thing), and few stools or tables are stable enough to serve as observing platforms. Tiny flexings and rockings that are completely unnoticeable during normal daily use can be annoying or intolerable when magnified 200 times, which is what they’ll be when you’re observing at 200x magnification. If you want a mount that will be comfortable to use when seated, then park yourself in a chair, sit up comfortably, measure the height of your eyeball off the ground, and go from there. Remember that your seated eyeball height should correspond to the maximum height of the eyepiece; it’s easy to bend over a few inches if the eyepiece is lower, and hard to stretch up those same few inches if it’s too high. From your eyeball-to-ground measurement, subtract the length of the OTA from the focuser to the balance point, and knock off a couple more inches for the ground board and azimuth bearing surfaces, and that’s how tall your rocker needs to be. For an excellent example of a DIY Dob mount meant for seated observing, see Rob Nabholz’s Kid Peek II.
To find the minimum height of the rocker box, you have to know the balance point of the OTA. Please don’t do it like I did in the above photo! Three things are wrong in that picture: first, it was ridiculously dangerous to balance the OTA on a <1″ surface suspended over concrete. Do it on thick carpet, and balance the tube on a paperback book or something. You don’t have to be exact, and inch or so of slop in either direction is unavoidable. Which brings us to the second thing that is wrong with that picture: the balance point is way too far back because the scope is lacking the finder and an eyepiece. But in practice you will have a finder, maybe more than one, and an eyepiece. So load up the front of the scope with whatever you will actually use on a night-to-night basis, and don’t forget to remove the dust cover from the front of the scope (the third thing I did wrong).
Finding the exact balance point down to the last micron is a lost cause, because eyepieces vary so much in weight. My lighest eyepieces only weigh 3 or 4 ounces, and my heaviest weigh over a pound. The entire OTA of this scope only weighs 10 lbs, so changing the weight at the front end by up to a pound is going to shift the balance point, bigtime. If you do as I did and attach the alt bearings to tube rings, no problem, you can just scoot that ring fore or aft on the tube to rebalance (that’s a capability you might want to retain if you have to build a box around your OTA). Other ATMers use springs or adjustable friction thingamabobs, but remember, here at 10MA you’re getting caveman-level engineering. Speaking of, you’ll notice that I put the extra tube ring down at the bottom of the OTA, to shift the balance point backward and allow a shorter rocker box.
In fact, with the finder in place and a middleweight eyepiece in the focuser, the balance point on the tube was just aft of the halfway point, so about 16.5″ of tube would hanging down into the rocker box. I made my rocker with a height of 18″ so that the OTA would clear it even if I had to shift the tube down in the rings to accommodate a heavier eyepiece. If you’re going for a minimum-height mount, it’s worth erring on the side of caution. If the mount is an inch too tall you’ll never notice, whereas if it’s an inch too short you won’t be able to balance your scope without hanging extra weights off the front end. IMHO, it’s kinda silly to design a minimal mount that requires you add extra, nonfunctional mass to the OTA, although some people deliberately do this when the mount absolutely has to be small (to fit in to luggage, for example).
Loads of other telescope-building resources, including many of those linked above, will show you how to make elegant alt bearings. I’m going to show you how to make easy, cheap alt bearings. My alt bearings are 4″ grated PVC endcaps, which you can get at any hardware store for two or three bucks apiece. They’re held on to the OTA with 1″ 1/4-20 thumbscrews threaded through washers. I also put on alt bearing retainers to keep the scope from slipping sideways off the alt bearing surfaces. The alt bearing retainers are 7″ squares of 1/4″ thick wood, that I cut from disposable fish cutting boards that I found at Big Lots, half a dozen for a dollar. Any reasonably stiff, reasonably thin material would work; if all else fails you could cut two circles of corrugated cardboard and poke holes in the middle. I drilled the center holes in the bearing retainers with a 3/8″ bit.
In case it’s not clear from the pictures, the thumbscrew on either side goes first through the washer, then through the middle slot in the PVC endcap, then through the center hole in the alt bearing retainer, and finally screws into the 1/4-20 socket in the tube ring.
Total parts and price list:
2x 1″ long 1/4-20 thumbscrews (~$1.50 for a pack of four)
2x washers (~$1.00 for a pack of eight)
2x 4″ grated PVC endcaps (~$2.00 apiece)
2x 7″ square alt bearing retainers (made from disposable cutting boards, $1.00 for pack of six)
Total cost: about $7.50.
A rocker box minimally consists of four pieces: a front, two sides, and a bottom. You need the sides to support the alt bearing surfaces and thus hold up the OTA, the bottom to form part of the azimuth bearing, and the front to brace the whole thing and keep the sides from bowing or leaning under the weight of the OTA. So, at a minimum, all you have to do is build a four-sided box that matches the minimum internal height and width established above for your tube. Mine needed to have an internal width of 8.5″ and an internal height of 18″.
Rather than make a single front piece, I made three 8.5″-long braces. One connects the sides a little more than halfway up and sports a handle, and the other two sit at the bottom of the box in front and in back. Whether you go with a single front piece or a series of braces, make sure that you don’t put them right at the top of the rocker, because you want the OTA to be able to swing down to the horizon as well as up to vertical. I cut the braces from some 2.5″ wide x 7/8″ thick wooden bars left over from a broken futon, but just about anything would work.
The sides of my rocker box are 18″ lengths of an old wood shelf, 9 3/4″ wide x 5/8″ thick. Once you have cut the basic rectangle to length, the only necessary cuts are the V-cuts in the tops that will hold the alt bearings. I made these inboard of the corners a bit, to leave four flat surfaces on top. Partly because I didn’t want to stab myself if I fell on the mount, and partly because those flat surfaces made very handy supports when I turned the mount upside-down for finishing and for assembling the azimuth bearing (see below).
One thing I haven’t discussed yet is the effect of the V-cuts on the effective height of the rocker box. I made my V-cuts 4″ deep in the center, but obviously the alt bearing isn’t going to ride right down at the point of the V. It will ride higher up, at a point fixed by its diameter and the width of the V. By doing a little testing in advance I worked out that the 4″ PVC bearings sitting on some kind of bearing material (furniture sliders, as it turned out) would put the center of rotation back up at or near the top of the rocker box sides, and if you look closely at the top photo, you’ll see that that’s about where it ended up.
I had a 1.25″ spade bit that I was planning to make an eyepiece rack with, but in the end I used it to drill a bunch of holes in the side pieces of the rocker box. Mostly because I thought it would look cool. They probably help lighten the mount a little, although they also exposed more surface area for sealer and paint to bond to, so who knows. I’ll tell you this: if I had to do it over again, I’d leave out all the holes, because they made painting this thing a right sod.
The final piece of the rocker is the bottom, which on my mount is a 9″ by 9 3/4″ piece of 1/2″ plywood. It’s a little narrow, so the rest of the rocker partly overhangs it on either side. I could have gotten a bigger piece of plywood and cut a bottom that exactly matched the dimensions of my rocker (9 3/4″ long, fixed by the width of the shelf from which I made the sides, and 9 3/4″ wide, fixed by the 8.5″ internal width plus 5/8″ on either side for the thickness of the rocker sides), but I already had an old plywood shelf that was 9″ wide. The bottom is screwed to the two lower braces anyway, and not to the sides of the rocker box; it’s really the braces that hold the whole thing together.
The rocker box is glued-and-screwed together. Get some long wood screws, drill pilot holes, do a test fit, put some wood glue on the surfaces to be bonded, and screw the thing together. If I can do it, anyone can.
Total parts and price list:
Braces: 2x 8 3/8″ long, 2 1/2″ tall, 7/8″ thick ($0)
Sides: 2x 18″ tall, 9 3/4″ long, 5/8″ thick ($0)
Bottom: 1x 9 3/4″ long, 9″ wide, 1/2″ thick ($0)
Wood screws (~$1.00 for a whole bunch)
Wood glue (~$4.00 for a good-sized bottle)
Total cost: for me, about $5.00, because I already had all the wood. If you don’t have scrap wood laying around, you could build a normal 4-sided rocker box (2 sides, 1 front, 1 bottom) entirely from 1/2″ or 3/4″ plywood. If you had worked out how big everything needed to be in advance, you could probably even get the hardware store to make the cuts for you. Or you could scavenge scraps from castoff bins and cut them yourself. Either way, it should still be possible to get out for under $10.
This one is dead easy. You need a piece of wood big enough to support the bottom of the rocker box. You can make it a circle if you want to be fancy, or a square if you want to be simple. I cut a 10″ length of my 9″ wide plywood shelf and drilled a hole in the middle. For feet I used four 1″ diameter screw-in type furniture feet, which I found for a buck at Big Lots. In theory, three would be more stable because a 4-footed structure can rock on uneven ground. In practice, however, I found that the 3-footed version tended to tip toward the unsupported corners. I could have avoided that problem by making the ground board larger than the minimum necessary to support the rocker, but that would have made the whole mount wider and heavier. Do whatever suits your needs. Don’t screw in the feet until you’re done finishing the mount (see next section)!
Total parts and price list:
1x 10″ long, 9″ wide piece of 1/2″ plywood ($0)
4x 1″ diameter screw-in furniture feet ($1)
Total cost: about a buck, since I had the wood. Again, this is something you could cut out of any spare board or sheet of plywood.
FINISHING THE MOUNT
By finishing here I mean sealing, sanding, and painting, not completing. You can’t complete the mount without the bearing surfaces and so on, but you can’t put those on until after it’s painted. Painting is optional, but sealing is a good idea, because a working mount is going to dew up from time to time, maybe a lot depending on where you live.
Before sealing and painting I beveled off the top outside edge of the uppermost brace and put on a cheap drawer handle. The handle is a hollow U-shaped piece of plastic, like a very thick drinking straw folded back on itself. It’s threaded inside and the normal procedure would be to put bolts or screws through the thin front of a wooden drawer and tighten them into the handle. I didn’t have that option, since I was going through a couple of inches of very hard wood. What I did instead was hammer in a couple of nails, liberally goop up the nails and the sockets in the handle with 5-minute epoxy, and then slide the handle down over the nails. Solid as a rock.
Also, I like nice smooth edges, so I sanded off all the sharp corners on the entire mount and ran little fillets of spackle along all of the joints. Totally optional.
At the suggestion of frequent commenter David DeLano, I sealed the mount with a mix of wood glue and water in equal measure, lightly sanding between coats. Then I gave it a couple of coats of water-based white primer, again with very light sanding between coats. I used Kilz2 latex primer, but anything would probably do. Since I was priming anyway, the sealing might have been overkill, but whatever. It was cheap, didn’t take long, and if helps this thing last a little longer, great. Then two coats of black latex exterior house paint. I hate messing around with paint thinner so I did everything with water-based products.
Total parts and price list:
1x plastic drawer handle ($1.00)
Wood glue (same as above, for rocker box)
1 quart Kilz2 latex primer (~$6.00)
1 quart black latex exterior house paint (~$8.00)
Total cost: about $17.00. Although I still have a lot of everything left over for future projects. If you count the fraction actually expended on this mount, it probably comes in under ten bucks.
Aye, there’s the rub: how much friction do you need or want on your bearing surfaces? Too much and the mount sticks or, worse, tips; too little and you can’t keep the scope pointed at anything long enough to see it.
The alt bearing surfaces were easy: I got four nail-in furniture sliders from Big Lots ($1.00 for the package) and, well, nailed ‘em in. They’re the little white nubs in the V-cuts. David had kindly sent some Teflon tape, but it proved too slippery for this mount. I could have scuffed up either the tape or the PVC bearings but went for the simpler solution instead.
The azimuth bearing surfaces are also pretty simple. From Rob Nabholz I got the idea to use furniture sliders against an unwanted phonograph record. Here you almost certainly do want just three, because even minimal rocking at the azimuth bearing is going to wreck the motions of your scope. Another one-buck package of sliders from Big Lots covered this and I have one left over for something else. Pound these into the bottom of the rocker box, as far apart as you can get them. The LP I got from the local library book sale for a dollar, but you could probably get one even cheaper at a garage sale.
To hold the stack together you minimally need a 2″ long 1/4-20 bolt, a 1/4-20 wingnut, and a couple of washers. I also like to put in some milk-jug washers to keep things turning smoothly. Milk-jug washers are a cinch: cut out flat circles from the walls of plastic milk jugs, and cut or punch holes in the middles so they’ll fit over the azimuth bolt. If your Dob, of any size, has sticky azimuth friction, you can stack up some milk-jug washers and/or old CDs around the azimuth bolt, between the ground board and the rocker, to take some of the weight off the lateral pads. You want the stack just thick enough to free up the motion a little, but not so thick that it makes the rocker box rock back and forth on the ground board (the only rocking normally associated with the rocker box is the rocking up and down of the OTA). For this scope and mount, I found that one old CD and two milk jug washers were perfect.
This photo shows mount upside down, read to thread the bolt through. From the bottom up (top down in the photo), the bolt goes through:
1 – flat metal washer
2 – 2″ milk jug washer
3 – ground board
4 – record (glued to ground board)
5 – 5″ milk jug washer
6 – old CD
7 – 5″ milk jug washer
8 – base of rocker
9 – 2″ milk jug washer
10 – flat metal washer
11 – wingnut
The reason the wingnut goes on top is so that you can adjust the azimuth friction by simply reaching into the bottom of the rocker box and tightening or loosening it.
Total parts and price list:
7x Teflon furniture sliders, 4 in the V-cuts for the alt-bearing surfaces, and 3 on the bottom of the rocker box for the azimuth bearing surfaces ($2.00; $1.00 each for two packs of four)
1x old phonograph record ($1.00)
2x flat metal washers (from the same package bought for the alt bearings, see above)
1x 2″ 1/4-20 bolt (~$1.00 for a pack of three or four)
1x 1/4-20 wingnut (~$1.00 for a pack of four or five)
milk jug washers ($0)
Total cost: about $5.00, less if you have some of the hardware already lying around.
That’s it, the whole shebang. If you can use a saw and a drill, you can build this mount, and probably just about anything else you put your mind to. If I forgot anything or if any of the steps are unclear, let me know in the comments and I’ll do my best to fix it.
Total cost of the entire project: for me, about $25, since I already had the wood. If you have to buy wood, add another $5-10. Remember that you can find scrap wood all over the place; heck, your neighbor probably some in the back of his garage that he’d be happy to give you.
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