A lightweight composite Dob: this 12 1/2-inch Newtonian's great looks are matched by its performance under the stars.
Much of my 121/2-inch scope is made from composite panels--the same stuff used extensively in boat building. Searching the Internet will turn up a wide variety of products. For example, composites can be purchased with a variety of skins and core materials. Lightweight cores include balsa wood, closed-cell PVC foam, and honeycomb structures of polypropylene and impregnated paper. Skins can be made from plywood, plastic laminates, and fiber-reinforced resins.
One of the great attributes of composite panels is that they have high stiffness-to-weight ratios. Because much of a material's resistance to bending forces resides in its surfaces, the substance in between can be replaced by a less dense, weaker core without sacrificing a great deal of stiffness. A composite panel of a given thickness may weigh 25 percent of one made of solid plywood yet retain 70 percent of its stiffness.
Based on cost, weight, availability, and ease of use, I selected composite panels from Tricel Corporation (www.tricelcorp.com). The panels come in 4-by-8-foot sheets and consist of a moisture-resistant paper honeycomb core sandwiched between skins of various types and thicknesses of plywood. I used 1/2-inch thick panels with mahogany plywood skins (known as Tripanel Marine) for the telescope's mirror box. For the rocker box, 3/4-inch and 1-inch thick panels with lauan plywood skins were used for the sides and bottom, respectively.
These composite panels are easy to cut with standard woodworking tools, and their light weight makes them a joy to work with. That said, they do have some drawbacks. For example, the edges of thin veneer skins are easily damaged and are difficult to glue. The thin skins won't hold screws and can be crushed by the force of a tightened bolt. Fortunately, there are ways to solve all these problems.
Any areas where screws or bolts are used must be reinforced. One approach is to drill out the weak core material and insert a solid plug where the screw or bolt is used. This is what I did for the pivot bolt on the rocker box, as shown at right. I used a 1 3/4-inch Forstner bit to drill through one plywood skin and the paper core, down to the inside surface of the second plywood layer. I then glued a plug of solid wood that I cut from a length of 1 3/4-inch hardwood dowel into the hole. After the epoxy had cured, I sanded the plug flush with the surface of the panel, applied the Ebony Star laminate, and finally drilled the hole for the pivot bolt through the solid wood plug.
To protect the fragile veneer from damage and to keep out moisture, I inserted strips of plywood along the panel edges. This edging should be at least as deep as the panel thick. In addition to sealing and reinforcing the edges of the panels, edging allows the use of traditional joining techniques, such as the tongue-and-groove joints. When glued together, such joints will be as strong as if the entire panel were solid wood.
The first step is to cut the panel pieces to the required size. Next, remove the paper core to the depth of the edging with a table saw, which does the job quickly and cleanly. Set the blade height to the desired depth. With the panel on its edge and its face held firmly against the rip fence, make repeated passes, each one offset from the previous one by the thickness of the blade. Be sure to use push blocks and keep your fingers away from the blade. For the 3/4-inch-thick vertical rocker-box panels, I typically used strips 1 inch wide, cut and/or sanded to the thickness necessary to fit the space between the veneers.
One particularly challenging task was to apply solid wood along the curved top edges of the rocker box where the side bearings would rest. The method I settled on was to add an extra-deep piece of edging before cutting the curves. I carefully removed material along the top edges down to a depth of 31/2 inches and glued in a wide strip of plywood. (Great care must be taken when working with a saw blade that has 3 1/2 inches exposed above the table surface!) Last, I used a router to cut the curved sections to a maximum depth of 3 inches from the top of the rocker box. This ensures that there is at least 1/2 inch of solid wood surface along the entire arc.
A Rounded Mirror Box
One of my telescope's most eye-catching aspects is the mirror box, which features gently curved corners--a design arising from the use of composite panels. These panels are built to resist bending; however, if one layer of skin is removed or cut at regular intervals, the panel becomes flexible.
The mirror box consists of seven individual pieces: two U-shaped sections that are joined together by a pair of flat side panels, an internal bulkhead, and recessed top and bottom panels. I first cut the internal bulkhead and the top and bottom panels and drilled the necessary holes. Next, I removed the inside skin and paper core on the four side pieces to a depth of 1/2 inch along the edges that will accept the bottom panel, and I cut a similar ledge to a depth of 1/4 inch along the top edges to accommodate the upper panel. Last, I cut a 1/4-inchwide, 1/4-inch-deep groove on the inside of these panels about 4 inches from the top edge, for the internal bulkhead.
Once the pieces were cut to size it was time to form the two U-shaped parts. It is straightforward to calculate the spacing and minimum number of cuts necessary to make a curve of a given radius, knowing the panel thickness and blade width. To determine the minimum number of cuts required for a 90[degrees] bend, multiply the thickness of the panel by [pi]/2 (1.57) and divide this by the width of the saw blade. Round up to the next whole number and you have the minimum number of cuts needed. To determine how far apart to space the cuts, multiply 1.57 times the desired outer radius and divide this by the number of cuts minus 1. For example, for a 1/2-inch-thick panel and a saw blade width of 3/32 inch, 1.57 x 1/2 + 3/32 = 8.4. The next whole number is 9, the minimum number of cuts. For a curve with an outer radius of 3.8 inches: 1.57 x 3.8 + (9 - 1) = 0.75 inch, the center-to-center spacing between cuts. For my telescope, I actually used 13 cuts spaced on 1/2inch centers (a value equal to the thickness of the panels)--considerably more than the minimum required. This was to ensure a smoother curve, but the result was that when I bent the panel 90[degrees], the gaps between cuts didn't close completely. To ensure maximum strength, I filled the gaps with epoxy mixed with a thickening agent called Cabosil. This gives the epoxy the consistency of peanut butter and ensures that it stays in place without running while it cures.
With the pieces cut, it was time for assembly. The parts are joined together with epoxy, but be sure to test-fit everything first. I began by sliding the interior bulkhead into the groove cut into one of the curved pieces. The primary purpose of this bulkhead is to prevent the struts from accidentally striking the primary mirror when they are inserted through the top panel. In addition, it stiffens the structure and provides a light baffle for the primary mirror.
Next, I joined the two curved sections via the flat side panels. I removed the core along the adjoining edges so that I could glue a solid wood spline into place. The spline creates a strong joint and provides sturdy material for attaching the side bearings later. Finally, I epoxied the top and bottom pieces to the structure.
Upper Ring and Struts
The rest of the scope is a minimalist design that uses a single upper-ring secondary cage to hold the focuser and secondary mirror. The traditional approach to joining the mirror box and secondary cage is to use eight angled struts to form a truss. That would work well, but I chose to use three parallel struts, similar to designs I had used successfully before. The square mirror box suggests placing a strut in each of the four corners; however, symmetry isn't necessary since three points define a plane.
On my other scopes I used wooden clamps to attach the struts, but these are time-consuming and difficult to make. For this scope I opted for a simpler attachment method. I placed threaded inserts inside both ends of the three 1 1/2inch-diameter (0.049-inch wall thickness), 56 3/4-inch-long struts. At the mirror-box end, the poles pass through holes in three of the corners of the top and the internal bulkhead before threading onto 1/4-20 screws protruding through the bottom of the mirror box. These screws are permanently held in place with T-nuts mounted on the inside of the mirror box. Similarly, three 1/4-20 screws pass through the upper ring and thread into the tops of the struts.
The spacing of the struts on the upper ring is different than on the mirror box, which causes the struts to bend inward. As a result they are forced against the edges of the holes on the top of the mirror box. This shortens the effective length of the poles to the distance between the top of the mirror box and the bottom of the secondary cage. Since stiffness is proportional to the cube of the inverse length, decreasing the effective length of the poles results in a substantially more rigid structure.
This scope uses a JMI Telescopes RCF-mini1 focuser, which allowed me to orient the focuser exactly how I wanted it. The entire focusing mechanism attaches to its mounting plate with only two screws. I removed and discarded the mounting plate and attached the focusing mechanism directly to my homebuilt mounting bracket. By doing so I was able to rotate the focuser 45[degrees] so that the focus knob cleared the strut, while I placed the focuser at approximately 25[degrees] from vertical--providing a convenient eyepiece viewing angle for most elevations. The eyepiece height at the zenith is a comfortable 60 inches.
When I create a new design there is always the risk that something won't work as planned, but I'm happy to report that this scope looks as good and performs as well as I had hoped. I've received numerous compliments at star parties, especially about the curved mahogany mirror box, which attracts a lot of attention. I particularly appreciate the scope's light weight when I'm packing up after a night of observing. Also, the design presents a small profile to the wind, so when other scopes are being pushed around like wind vanes, this scope remains steady.
Is it possible to build a lighter scope? Yes, but I think the labor and materials costs soon reach a point of diminishing returns. This scope is now light enough to take as checked baggage to Australia--a trip I will be making soon.
For 20 years Albert Highe worked in the research and development of industrial products using unique materials. Now retired, he applies his considerable experience to telescope design and construction. He can be contacted at firstname.lastname@example.org.
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|Title Annotation:||amateur telescope making|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2005|
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