copyright 1999
First off, let me thank you for your interest in the Wood shop Telescope design. Originally presented in Sky & Telescope, March 1999 issue, this design has been favorably received by the amateur telescope building community, demonstrating that a home-made telescope need not appear homemade. The prototype was built in Nov. & Dec. of 1996, a 4.25 inch f/4. Another quickly followed, as well as an f/5. Those little scopes were so popular during comet Hale-Bopp's apparition, that the 6-inch version was drawn up. Two of those were built, and with some prodding by fellow observers, I wrote up the article for S & T. Since then, I have received requests for more information from all over the world.
While the telescope can be built with standard home-wood shop tools and a lot of sweat, use the best tools and equipment available. If you are a novice to woodworking tools, or have no experience fabricating components from plastic and aluminum, then this project could try your patience, and I advise you to build something simpler as a first effort. I have built seven telescopes using this design, up to ten inches aperture, and while each successive instrument goes a little faster than the last, they are still very labor-intensive. The main ingredient in building these telescopes is elbow grease. Don't expect to complete it in a couple of weekends. This project could take months, but don't let it discourage you. The results are worth the effort, and I guarantee that the final product will be the best looking scope at the star party, if you put the required effort into it. If you want a scope quickly, don't care how it looks, and don't want to actually have to work, go buy some plywood and a sonotube, and throw this package away. I'm serious. This design is a LOT OF WORK, but that's the price to be paid if you want your scope to stand out from the crowd.
I expect most of you will elect to customize your telescope a bit, so I am not giving you a cut-and-dried materials list. Some of the materials will have to be scrounged or sought-out, and may take some time to locate if you don't live in a manufacturing area. Also, these directions assume a basic knowledge of tool use. I will not be micro-managing your project, and take no responsibility for personal injury or errors made on the part of the builder. To build this design, you will be using woodworking tools on new materials, and they don't behave like wood. Make practice cuts if you are not sure of the outcome. Advanced amateurs and craftspeople will undoubtedly have better ideas on fabricating some of the parts. I would be very interested in hearing how you made them, for possible inclusion in future versions of this design. Contributors will be credited in future versions, if any.
This free design includes 20 detailed sketches and 13 photos. As these are large files they are grouped in Zip files that are available as automatically unzip versions (extension .exe).
The easiest form for reading them is Adobe .PDF, which can be read by your browser if you have the free add-on.
To get the sketches or the photos, please use our little freebie form. The three drawing formats (.DWG, .SKF, and .DXF) can be read with various CAD systems. All files are sent as executable (.EXE) files that automatically unzip when you run them.
If you want the photos, request photos in the comment box on the order form. These photos are in .jpg format and total 0.47 megs in size so they take a while to send and receive.
If you have questions or comments about this telescope design, please, send them to the designer Chuck Hards, chuckh@companionsystems.com. Woodware Designs is just the distributor for this design.
Before beginning any shop project, make sure you have the proper safety equipment: Safety glasses, particle mask, rubber gloves and protective clothing, ear plugs, and an organic vapor filter cartridge respirator if you are spray-painting your creation.
This scope is a 6 inch, f/8 Newtonian reflector on a Dobsonian mount. It is a great first scope, or smaller, quick set-up scope if you already have a large light-bucket. I recommend using Pyrex optics, although if you build the mirror cell with a fan, plate glass may be substituted. Use what you are most familiar with. Other focal lengths can be accommodated, by adjusting the tube length and secondary mirror size accordingly.
The drawings were done by a professional industrial designer, who, unfortunately, no longer works with me. They are not perfect, and to make them so would mean that you would not be reading this now. I have made the decision to go ahead and publish them as they are, reasoning that most builders will be able to deal with the misprints and typos. You get what you pay for. (These plans are free.) Those who preferred e-files to paper can correct problems themselves, if they are proficient at CAD-type programs. (I am not, and have no idea what program was used to create the drawing files. Good luck.) I suggest having the drawings plotted out full-size; 2 copies. You can then use one set as layout aids when making the jigs.
Refer to the photos in Sky & Telescope. I was hoping to get a photo package together for these instructions, but ran out of time. I will keep working on it in the coming months, and send it out, if and when it gets done.
With the disclaimers and apologies out of the way, lets begin.
The optical tube is designed and built first. Aluminum irrigation pipe with a .064 inch thick wall is used. Cut to length with a jig-saw or hacksaw, the ends are squared with a long sanding block and coarse (60 grit) wet-or-dry abrasive paper. One tube diameter is typically left ahead of the focuser, to cut down on scattered light and improve contrast. The 1.25 inch low-profile helical focuser is an off-the-shelf part, with the base contoured to fit the tube O.D. This is accomplished by cutting the mounting rails free with a hacksaw or band saw, then re-shaping the seat on a scrap piece of main tube, with 60-grit paper contact-cemented to it. Squareness is periodically checked by inserting a long 1.25 inch dia. cylinder into it. A piece of drain pipe works well for this. You can also use the old method of drilling a small hole in the tube exactly opposite the focuser, and sighting for squareness, but then you have an otherwise unused hole in the tube, and have to plug it later. Alternatively, this could be done on a scrap piece of tube. If you do a good job of contouring, no further shimming or adjusting of the focuser will be required at final assembly.
A contoured base helps keep dust out of the tube, looks better than black tape or caulking, and reduces focuser height. Including 1/8 inch removed from the top of the focuser base, the entire unit is now 1/4 inch lower than stock. Keeping the focuser as low as possible allows a smaller secondary mirror, which improves definition at the eyepiece.
After laying out the focuser position on the main tube, I suggest leaving the back (mirror) end of the tube overly long for the time being. The exact separation of the primary and secondary mirrors depends on your primary mirror's precise focal length, and what eyepieces and barlow you intend to use. The dimensions on the drawings should be close enough if your focal length is close to 48-1/4 inch.
The end rings, mirror cell back, and hinged tube rings are all shaped from commercial PVC sheet, using the router. You must first make a jig for each part, and this is where attention to detail pays off. The actual parts will be made from these jigs, and will be precise copies, so if the jig isn't right, the part won't be, either.
A high-quality router is essential for this project. I use a 13-year-old Bosch router, with a 1/2 inch collet. I prefer to use bits with a 1/2 inch shank whenever possible. They last longer, and vibrate less than bits with a 1/4 inch shank, but are more expensive. Some operations are made easier if the router is mounted inverted, under a table.
The jig is a disk of 3/4 inch MDF, (Medium Density Fiberboard) cut to the OD of the main tube. Since sanding and finishing the ring will lessen it's diameter slightly, you can make it about 1/32 inch oversize. Cut a blank of 3/4 inch PVC about 1/16 inch larger than the jig, and tape the two together using double-sided tape. Make sure to use high-tack tape, since if the jig and blank slip or change position with respect to one another, you won't get a good part. Clamping the two together for a while can improve the bond. Route the PVC with the jig attached, using a flush-trim bit with ball-bearing. You now have an MDF disk taped to a PVC disk of the same size. Gently wedge the two apart with a putty knife, taking care to not break the jig. They won't come apart quickly, so don't be concerned. You may have to worry them apart, bit by bit. When separated, make another PVC disk for the rear end ring.
The end ring needs to have a shoulder routed into it, about halfway up, so that part of it can slide into the telescope tube. Alternatively, (and this is what I did on some of them), you can route another disk to fit the inside tube diameter. After the interior holes are routed, they are glued together with Loctite 454 to form the complete ring.
The inside hole is routed using what I call a capture jig. Because the ring is small in cross-section, there is insufficient area available for attaching a jig with tape. The capture jig holds the PVC disk by it's circumference, and the router guide for the bit bearing is a hole in the bottom of the jig. See the photo in S & T, 3/99, page 123. The small bolt in the front of the jig is what closes the upper half, clamping the PVC
disk in place. This arrangement is much safer, and you don't have to pull off that gummy double-sided tape after routing.
I made my rear end ring with a slightly smaller central hole than the front, but this is a matter of personal preference. The rear end ring needs no shoulder. It attaches directly to the back of the mirror cell, with Loctite 454.
Outside corners were rounded over with a 3/16 inch radius round over bit in the router. You can modify this radius to suit your self, or available tools.
The back part of the mirror cell is made from 1 inch thick PVC sheet. It needs to be this thick for structural reasons, and to be able to accommodate the battery pack for the fan. Route the OD using a disk jig, this time made to fit the tube ID. Making it about 1/64 inch undersize will help when installing or removing the cell during construction, or for routine maintenance. Routing a 22-1/2-degree bevel on the front side will facilitate this, as well.
Once the disk is routed, it is time to route the pockets for the switch and fan. You'll have to make jigs for this operation, using the drawing as a guide. Note that the fan pocket is square, with rounded corners, but the air hole is round. The square pocket does not go all the way through the disk, it stops just deep enough to accommodate the fan. A 22-1/2-degree bevel on the front side of the round air hole helps distribute the airflow as it exits the cell back, and hits the mirror itself.
The front of the cell back has a pocket for the switch. I made the switch pocket on both sides; you need to recess the switch on the back side for mirror clearance, and I did it on the outside so that the toggle would not extend out too far, where it could be damaged or switched inadvertently. The spring seats are formed with a #4 Uni-Bit, after drilling a pilot hole first. I like to accomplish as much as I can while the OD jig is still attached, so I use it as a drilling guide for the collimation bolt holes. The fan hole is routed from this jig, as well as the battery compartment and switch pocket on one side. Examine the same S & T photo referred to earlier. Note that this jig uses an insert in the center, so that I can use the same jig for routing both the air hole, and fan seat. Tube mounting holes are drilled at 120-degree intervals around the perimeter, but it may be easier for some builders to do this later, after drilling the holes in the main tube. It is then an easy matter to insert the cell back, and just trace where the holes go on the PVC.
The mirror carrier itself is routed from a piece of 1/4 inch thick aluminum plate, drilled and tapped for collimation bolts, and edge brackets. The brackets are cut from a piece of aluminum channel on the table saw, to the dimensions shown on the drawing. The mirror carrier has a rounded-over bottom edge, to allow the brackets to sit close to the mirror. Mirror face clips are made from at least 18-gauge sheet steel, bent in either a bending break, or bench vise. You only need about 1/8 inch extending onto the surface of the mirror. Note that edge bracket and clip supports are dependent on your mirror thickness. Adjust to suit.
The battery contact clips were cut from thin sheet metal with tin snips, and formed with a vise, and needle-nose pliers. Small springs were soldered to the negative contacts. The clips are screwed in place with tiny sheet metal screws, available from hobby shops, and normally used to mount servos in radio-controlled airplanes. Wires from the switch and fan are wrapped around the appropriate contact screws, and the screws are tightened down. Making the contacts will try the patience of anyone who has never tied a dry fly!
The battery compartment cover is jig-routed from 1/8 inch thick aluminum plate.
Mount the focuser on the main tube, and install the diagonal. Now, mount the primary mirror in it's cell, and temporarily insert it into the back of the tube. Tighten all collimation nuts all the way down. (Use strong springs) Back them off 1-1/2 turns. This should be all the adjustment leeway you need, if you mount the cell squarely, and were careful in fabrication. Make sure the cell is reasonably snug and won't fall out. (Duct tape works great.) Collimate roughly. You can now interchange eyepieces, and determine exactly where the final position for the mirror cell should be, and thus, main tube length. Drill the mounting holes on the main tube, and on the cell back, if not done already. Remove the cell, and trim the tube to its final length, making sure both ends are square, but especially the mirror end.
The hinged clamping rings are a real luxury on a Dobsonian, and part of the charm of the design presented here.
The tube rings are routed from another MDF jig. You need to make 4 identical pieces. The jig is secured with double-sided tape, and clamped together for a few minutes in a vise, insuring good contact. The hinge hole is drilled while the PVC is still attached to the jig. It is very important to have the hinge hole perfectly centered. Remove from the jig before trimming the hinge knuckle. The hinge knuckle is relieved on the table saw. Do all four in one session, taking only about 1/16 inch off with each pass (1/16 inch of depth. The full height is removed with each pass. Stop when exactly half of each knuckle has been trimmed away. Outside corners are relieved to suit with the router. I prefer a 45-degree bevel, but the prototype used a round over bit. Your choice. The hinge pivot holes are enlarged to accommodate the bolts used. Use a Nylock nut on each hinge. The clamping knob holes are drilled on the drill press, if available. The upper ring is drilled to completely clear the bolt diameter, while the hole in the lower ring is threaded to match the clamping knob. The holes used to mount the rings to the bridge plates are drilled last. Thread these also, to match the bridge plate mounting hardware.
All of my plastic parts are sanded and buffed to a high gloss, and left natural. Do this by dry sanding with 120 grit garnet paper to remove machining marks, then wet-sand using a grit progression, starting with 220, then 320, 400, and finally 600 grit, eliminating progressively smaller scratches. Just like mirror grinding, change water and rags between grits. Buff and polish using automotive rubbing compound. If done right, the parts look like they have been injection molded. Alternatively, you could paint them, using any high-quality enamel.
The hinged tube rings are lined with felt strips, adhered with double-sided tape. You can adjust the fit of the tubes on the ring by using the appropriate thickness of tape. Clamp on the tube overnight for best adhesion.
Cut the square bottom for the rocker first, and set aside. Now cut out one side board. You can glue the full-size plot to a piece of MDF, and cut it out following the lines, to make it easier. Temporarily screw it down to another piece of MDF, and route out a copy using the jig you just made as a master. Separate the two, and glue each to another piece of MDF. You now have two assemblies consisting of a routed side glued to a rough-cut side. When the glue dries, route the rough piece using the top, finished piece as a guide. When both are routed, glue them down to the bottom board, and clamp overnight. A few strategically placed wood screws will help keep it from cracking along joint lines later. Add the cross-bracing at the front and rear bottom of the rocker, which is also cut from MDF. round over all outside corners to suit, and fillet all inside corners with automotive body-filler. Route the lateral stop pockets.
When you have sanded the rocker smooth enough, turn it over, and glue down the Ebony Star Formica to the bottom, ground-facing side of the rocker box. Trim it flush using the router. Temporarily mask it off, and the mount is ready for finishing. I prefer a gelcoat finish. Gelcoat is a two-part polyester paint, typically used as a surface coat on fiberglass products. It has the additional property of being much tougher than traditional paints. It must be applied with a spray gun. Local boat repair shops can probably spray it on your rocker box, for a fee. An alternative is to spray the rocker with grey lacquer primer, sanding the dried primer and re-spraying as necessary to block out and smooth the entire mount. A color coat of regular spray-paint, from a can, can then be used.
When fully dry, the gelcoat finish is wet-sanded and buffed to a gloss, just like the plastic parts. A spray-can finish does not require handwork after curing, if applied correctly.
All handles, Teflon bearings, and bumpers can now be added. Carefully bore out the pivot hole to accept a nylon bushing. I found these at the hardware store. The bushing is cut to length, and glued in with Loctite 454 or equivalent super-type glue. Clean up any smearing quickly with acetone. Acetone will not harm a cured gelcoat finish, but it will instantly dissolve paint from a spray can. The ground board is 3/4 inch plywood, covered with Formica. Nothing special here. Install the azimuth pivot bolt, and the Teflon azimuth bearings. Slide the rocker box over the pivot bolt, and install a washer and Nylock nut to keep them together.
Cut the bridge plates from 1/4 inch thick aluminum plate, on the table saw. Use an 80-tooth blade, with carbide teeth, and zero hook angle. Make sure it is square. Drill the clamping ring mounting holes where indicated. Drill a centered hole for mounting the altitude bearings, and tap it with 1/4-20 threads or larger. I like to finish my aluminum by wet-sanding, just like the plastic parts. You can even buff it to a high gloss, if desired. Mount the clamping rings on the bridge plates.
Make the cores from MDF. I first route a 3/4 inch thick disk to fit the inside of a piece of 6 inch diameter irrigation pipe. Route the inside hole, then bevel it with a 45-degree chamfering bit. Now glue it down to another piece of 3/4 inch MDF, and route the second piece flush with the first. This assembly is then glued into an aluminum ring made from 6 inch diameter aluminum irrigation pipe, with about 1/16 inch to 1/32 inch of the pipe extending beyond the surface of the MDF on both sides. Mask the aluminum, and gelcoat the wood faces. You need to build the gelcoat up to a thickness of at least 1/16 inch. You'll have to do one side at a time, allowing the first side to cure before handling it to turn it over.
Sand both faces until you just expose the aluminum ring. Now go through the grit progression, polishing the gelcoat to a gloss. Repeat the grit-progression for the outer aluminum ring. This surface is what contacts the Teflon bearing, so make sure it is smooth. Locate the center of the bearings, and drill a hole to allow bolting them to the bridge plates. They should be as round as possible, but if they are off by 1/32 inch or so, their function will not be adversely affected. Shim as necessary to make the overall width of the cradle assembly match the rocker. Use stainless steel fender washers under the altitude bearings.
I used stainless steel hardware on my scopes, which drove up costs, but will not rust any time soon. I like the looks of Allen-head hardware, so I used them whenever possible. The clamping knobs were made by inserting a piece of threaded rod into a female plastic hand knob, and jamming it in place with a coupling nut. I then turned the coupling nut to a round section on my mini-lathe, but you can leave it hexagonal, without too much charisma penalty.
I lined the tubes with flocked paper, and made it a bit easier by dividing the tube into three sections with PVC baffles. They only shade a small part of the tube interior wall, but it was much easier to install three small pieces of flocked paper, than one huge one. The baffles are not needed for top performance, if you use the flocked paper liner.
The springs used on the mirror cell need to be strong. It should take two hands, squeezing very hard, to compress one.
The fan supplier is Marlin P. Jones.
The optics for the prototypes came from University Optics, and Earth and Sky Adventure Products.
All nuts, bolts, screws, knobs, washers, etc., came from my hardware drawer, and Eagle, a hardware store chain. Lowes, in the east, should have similar products. They also carry MDF.
The aluminum plate that I used was salvaged manufacturing scrap.
Loctite 454 is a super-type glue in a gel formulation. Regular wood glue can be used on the MDF parts.
The irrigation pipe is available from irrigation & sprinkler system contractors.
Sheet PVC is available from most plastics dealers.
Remember that all dimensions are approximate. Tube layout dimensions depend on the exact focal length of your mirror. Commercial 6 inch f/8 mirrors can vary in focal length by up to two inches, from my experience.
I used enhanced reflective coatings on my personal scopes.
I made my own finders from surplus optics, and PVC pipe fittings, turned on the mini-lathe. The finder bracket is a commercial dovetail bracket.
My prototypes used an automotive-type paint, sprayed on with a pressurized paint gun, in a professional booth. You can get a really nice, glossy paint job from a spray can, though, if you take your time.
I would like to thank my wife Julie, and daughter Whitney, for putting up with my telescope activities while I ignored household chores, Gary Seronik and Sky & Telescope for publishing the design, my boss, Al Tiley, for allowing me to use company facilities occasionally, you, the reader, for your interest, and all those companies who throw telescope-making raw materials into their dumpsters each day.
July 1999
Don't forget to request the sketches.
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Woodware Designs, Woodware@woodwaredesigns.com