This article has been accepted for publication in the Amateur Telescope Makers Journal, Issue #16

By Albert Highe

I wanted a telescope I could easily transport to dark skies. It should disassemble into small, lightweight components, and require no tools for quick and easy assembly. About four years ago I was struck by the simplicity of Ron Ravneberg’s portable 8-inch reflector "Alice". Ravneberg had recognized that many portable Newtonian assemblies using the standard eight-member tube truss are actually overbuilt. Instead, he tried two parallel tubes held rigidly at the ends by the mirror box and focuser board. The design turned out more than adequate to resist the force of gravity. His work inspired me to depart from the traditional truss. Since then I’ve built quite a few telescopes using two and three support struts in various configurations, made from aluminum stock in an assortment of cross-sectional shapes. My most recent design, an ultralight 12.5-inch f/5 Dobson Newtonian, shown at right, uses three parallel, cylindrical struts.  It won a Merit Award at the Riverside Telescope Makers Conference 2000. I fashioned all the mechanical assemblies described in this article.

The conventional eight-member truss offers a lightweight, collapsible alternative to the traditional tube used to rigidly support the components of a telescope. Both tubes and trusses successfully resist vertical, lateral, and twisting deformations due to external forces. This resistance to deformation independent of axial orientation is necessary for equatorially mounted or rotating Optical Assemblies (OAs). However, the use of a simple Dobsonian mount restricts the magnitude and direction of forces the OA will experience. Consequently, the support structure for a Dobsonian-mounted OA offers more design options, including simpler truss configuration.

The largest force on an altazimuth-mounted optical assembly is the vertical pull of gravity. The structure must be stiffest in the vertical direction. This is why Ron Ravneberg’s telescope works so well. The construction of two parallel struts is very strong in the vertical plane because the struts are widely separated. The moment of inertia, and consequently the stiffness, increases as the square of the separation of the struts. As fabricated, the individual struts are larger than required for vertical support alone, being sized to also resist lateral deformation caused by moving the telescope in azimuth. Deformation perpendicular to the plane of the two struts tends to be about half the deformation for a single strut.

Instead of using stiffer struts to resist lateral forces, one could add a third strut, creating a second pair of struts perpendicular to the first pair (imagine a long box where struts run along three edges). Alternately, the two struts could be stiffer along one axis, as with channel (C-beam) or I-beam material. For the 12.5-inch f/5 scope, I opted to use three cylindrical struts spaced equally around the OA. Each pair contributes to the stiffness. When symmetrically placed they resist bending equally in all directions.

Optical assemblies made with parallel struts, however, are more susceptible to twisting deformation. For example, the weight of focuser and eyepiece will torque the OA. In practice, the amount of torque is small. Nonetheless, an OA susceptible to twist can vibrate and degrade the image seen at the eyepiece. The diagonal members in a conventional truss resist this twisting as well as the vertical and horizontal deformations. Thus, even though an assembly made with three smaller diameter tubes would have been stiff enough to resist vertical and horizontal deformations, I had to use 1½-inch diameter cylindrical aluminum tubes with 0.065in wall thickness to provide enough stiffness to resist twist and vibration. I used 6061-T6 aluminum throughout the design.

Several benefits came from using three parallel struts: 1) There are only three identical tubes to cut, transport, and assemble, 2) different length tubes can accommodate mirrors with different focal lengths, 3) bearings attached to the parallel struts allow easy and adjustable positioning of the balance point, 4) finders can be attached anywhere along the strut and maintain collimation when reattached, and 5) making clamps to hold cylindrical tubes is straightforward, with no tools required for assembly.

The primary mirror is the heaviest component in a reflector, so I designed the ultralight around an Enterprise Optics 12.5-inch f/5 Pyrex primary only one inch thick. It has an enhanced coating applied by QSP, and weighs about 11 pounds. I was concerned about supporting a thin mirror adequately, and used the program PLOP made available by David Lewis to investigate various mounting cell designs. Surprisingly, a six-point mirror cell with support points arranged at a single radius was predicted to be more than adequate. In fact, using Lewis’ program, the six-point cell turns out marginally better than a conventional nine-point mirror cell, where three support points are at an inner radius, and six support points are at an outer radius. A six-point mirror cell is also easier to make than a nine. Curious to try out the prediction, I made one.

The cell was made from aluminum, using ½-inch thick plate and 3/8-inch x ½-inch beams. The mounting hardware is the same as that recommended by Bruce Sayre. The beams pivot on stainless steel ¼-inch shoulder bolts. The ¾-inch diameter support pads are stainless "T" (or weld) nuts. I used stainless steel fasteners throughout the design.

The cell completed, the mirror was attached to the support pads using silicone caulk. Such adhesives are very good in resisting shear forces. Even though the total adhesive covers only about seven percent of the mirror area, it is very firmly attached. The soft silicone caulk also accommodates some difference in thermal expansion between mirror and cell. Finally, I placed a stainless spring over each of the  three adjustment screws of the mirror cell and inserted the mirror assembly into the mirror box. The mirror and cell are held in place by three stainless wing nuts, visible from the bottom of the mirror box. In order to keep the center of gravity of the OA as low as possible, the mirror cell was designed to attach directly to the bottom of the mirror box. The distance from the top of the mirror to the bottom of the mirror box is approximately 2-1/2 inches.

The mirror "box" is essentially a fiberglass tube with wooden end rings. It weighs about five pounds without optics or cell, and is very strong and stiff. A layer of fiberglass epoxied within an outer shell of Ebony Star™ laminate makes up the tube. A form, shown at right, was constructed to hold the laminate in the proper shape during fabrication.  I cut the laminate to the proper width and length, allowing approximately 4 inches of overlap. The tube was adhered with epoxy after roughening the overlapping surfaces with sandpaper. Forcing the laminate against the form and clamping the overlap region ensured that the laminate conformed to the right shape. After the overlap region cured, I spread a layer of fresh epoxy on the inside of the tube, laid a fiberglass mat over the fresh epoxy layer, and then applied another coat of epoxy over it. The cured tube held its shape well.

I cut the top and bottom tube rings from ½-inch Finnish Birch plywood. This light, strong material was used for all plywood components of the scope. The top ring has an opening large enough to insert the mirror through it. Because the lower ring supports the mirror cell, I cut a smaller opening in the bottom ring to permit airflow through the mirror box. On the inside surface of each ring, I used a router to cut a 3/16-inch circular groove about 1/8-inch deep with a diameter the same as the fiberglass tube. I then epoxied the end rings to the fiberglass tube.

Note the three small feet on the bottom of the mirror box. These feet keep the mirror box from resting on the adjustment screws while I assemble the telescope and while it is sitting in the rocker box during transport. The rubber feet slide over 5/8-inch diameter dowel rods that are glued into holes drilled approximately three-fourths of the way through the bottom of the mirror box. The three supports are longer than the adjustment screws, but short enough in position so they don’t touch the bottom of the rocker box when the telescope is in use.

The shape of the mirror box is an example of "form follows function." I initially designed the assembly with the struts positioned equidistant around the tube, but the two lower struts would have been a long way from the outside edges of the mirror box. Since I planned to attach the altitude bearings to the struts, I would either have to make the bearings wide and with a small diameter or support them away from the struts. A practical solution was to move the two lower struts outward by widening the end rings in those locations. I hit on the idea of keeping the sides of the end rings straight, from their widest point down to where the struts would be located. The resulting wider separation allowed the use of thinner, larger diameter altitude bearings and stiffens the assembly. Furthermore, the flat sides of the mirror box end-rings overlap the larger altitude bearings, and prevent the bearings from rotating inward. I cut a lower arc just under the struts to maintain the circular design element. Finally, I trimmed about an inch from the bottom end ring to create more clearance between the OA and the rocker box. The completed mirror box with mirror cell and primary weighs 18 lbs.

The struts attach to the mirror box, the bearings, and upper cage assembly with wooden clamps. I made each clamp from 1-inch plywood (two strips of ½-inch, laminated). Although the shape of each clamp is specific to its location on the telescope, each has a 3/8-inch curved section that supplies the spring force and a ¼-28 stainless through-bolt and stainless wingnut. I used nuts with 28 threads per-inch to supply greater mechanical advantage than standard 20 t.p.i.

I started with a couple of fairly large pieces of the one-inch plywood. I drew the shape of each clamp on the pieces and bored all the 1-1/2-inch diameter holes with a Forstner bit. The larger pieces are easier to hold when boring. I then cut out the rough shape of each clamp with my table saw. I shaped the outer surfaces with a belt sander and smoothed the internal surfaces with a one-inch diameter drum sander, held in a drill press. I then used my table saw to cut each ring along one side. Prior to installation on the mirror box and upper cage assembly, I shaved approximately 1/16-inch from the articulating section of the clamps. This allows each clamp to move freely after it is glued and screwed into place.

Most portable Dobsonians have upper "cage" assemblies that resemble the upper section of a telescope tube. Since my mirror box and its components are so light, it was a greater challenge to make an upper cage assembly light enough to maintain a low center of gravity for the OA. I designed the upper assembly following the examples demonstrated by Mel Bartels and Bruce Sayre. The upper "cage" consists of a single ring that I cut from ½-inch plywood. It has the same shape as the mirror box end rings.

The focuser is an NGF-1 model by JMI, with black anodized finish and flat base. I attached the focuser to the upper ring using an aluminum angle bracket cut and shaped from a piece of large aluminum channel. Its finished dimensions are approximately 1-1/2 x 4 x 4 x ¼ inches thick. The thick aluminum was drilled and tapped to attach the focuser to the bracket and ring without using any additional nuts. To keep the focuser bracket as short as possible, I attached the spider above the plane of the ring and mounted the secondary mirror as close to the plane of the ring as possible. The spider brackets are made from aluminum channel and fastened with "T"-nuts inserted under the ring. The focuser is positioned 45 degrees from vertical, which permits comfortable viewing at all angles. With this focal length and focuser position, anyone over about 5’8" tall can view anywhere in the sky while standing on the ground.

I made the baffle opposite the focuser out of Kydex™ plastic. To prevent the Kydex from deflecting into the light path, I created a permanent set in the material before I cut it. I wrapped a sheet of Kydex together with some laminate (think of a jelly roll) to form a tube that fits the inside diameter of the plywood ring. I tied the tube with twine and placed it in a convection oven at 200°F for about 20 minutes. The temperature is critical. At higher temperatures, Kydex becomes too soft and begins to degrade; at lower temperatures it won’t take a uniform set. After the Kydex cooled, I cut slots in it to fit over two spider vanes, and large enough so I can only see the baffle through the focuser. The baffle behaves as if it were cut from a rigid tube, and doesn’t sag or deflect into the light path. Two small strips of Velcro™ hold it on, permitting easy installation and removal. The complete upper optical assembly, with baffle, weighs 4.2 lb.

In order to keep the focuser bracket as short as possible, I needed to make my own secondary holder. I cut a short section of 1-1/2-inch aluminum rod stock at a 45-degree angle. I then milled slight flats into the block for the vise to grip. In order to reduce weight, I milled out a one-inch cylinder through the center of the piece to within 1/8-inch of the bottom. I then drilled and tapped three equally spaced holes to accept the three alignment screws. At the center of the bottom I milled a spherical hole to accept a brass acorn nut. The nut sits on the end of a threaded rod that I inserted through a circular 1/8-inch thick plate and fixed in place using a locking nut. The three alignment screws pass through three holes in the circular plate and screw into the bottom of the aluminum block. This construction is similar to many commercial diagonal holders, but the diagonal is held in place by silicone caulk. I used a 2.6-inch diagonal mirror with an enhanced aluminum coating. I painted the secondary holder and the edges of the diagonal flat black to reduce reflections.

The large 18" diameter altitude bearings overlap the sides of the mirror box, preventing the bearings from twisting inward. The altitude bearings were made by gluing two 18 1/2-inch disks of ½-inch plywood together. This one-inch laminate was then routed down to 18-inch diameter, with the bit at ¾-inch depth. This created a ¼ x ¼-inch lip to overlap the Teflon™ pads, keeping the OA centered in the rocker box. Before cutting the disk into two, I epoxied a ¾-inch wide strip of Ebony Star laminate to the bearing surface. This is made easier by inducing a "set" in the laminate by wrapping it into a circle and letting it sit under heat for a while. After the epoxy cured, I routed out arcs to make the internal bearing faces. I then cut the disk in half, and routed out the remaining internal material. The combined weight of the bearings is 3.4 lb.

Two clamps with tightening knobs hold each of the bearings in place, and permit the scope to be easily balanced. This is one of the features I like most about the three-strut design. When I want to alter the balance point, I simply loosen the knobs, nudge the bearings to a new position, and then re-tighten the knobs. The finished bearings ride on Teflon pads screwed into the rocker box. As stated above, the lip of each bearing also rides against the pads and keeps the OA centered in the rocker box.

In order to keep the rocker box as light as possible, I made it out of ½-inch plywood. All joints are tongue-and-groove, adhered with Gorilla Glue™. This is a great adhesive. It expands as it dries, forcing itself into the grain, and makes a very strong bond. It is also waterproof when cured. Although most of the rocker box is made from ½-inch plywood, it has five features that contribute to its stiffness. First, the sides could be made short because I used large altitude bearings. The second, and probably most important feature, are the gussets extending along the backs of the two sides to the widened base. The wider base also adds stability. Third is a lip around the backside. It stiffens the bottom board and makes the gussets more effective. The fourth feature is a one-inch thick bottom board (two ½-inch pieces laminated) within the walls of the box. This resists twist. Finally, I use an eyepiece shelf within the rocker box, which stiffens it and provides ready access to eyepieces during observing sessions. The eyepiece shelf is ¼-inch plywood, glued into grooves routed on the inside of the rocker box. It has space for two 2.0-inch, and four 1.25-inch eyepieces.

A sheet of Ebony Star laminate covers the bottom of the rocker box. The box rides on three Teflon pads placed at the vertices of a triangular ground board. The ground board sits on three rubber feet. Similar to the rocker box, the rubber feet slide over one-inch diameter dowel rods that I glued into holes drilled about three-quarters of the way through the bottom of the ground board. The rocker box pivots on a ½-inch diameter stainless bolt passing through a brass bearing. At 18 1/2 lb., the rocker box is the heaviest component.

When observing, I place a second Kydex baffle over the top of the mirror box. This provides a light baffle for the primary mirror and minimizes reflections from the top of the mirror box, improving contrast.

The telescope balances nicely with a 50mm finder, but a larger 80mm right angle finder suits me better. It is positioned so I can star hop comfortably while sitting down. A mounting bracket of aluminum "C"-channel attaches to the top strut using a stainless hose clamp. The channel aligns itself with the cylindrical strut, maintaining alignment upon reassembly.

Even with the 80mm finder positioned halfway along the length of the OA, I needed to add some counterweights. I inserted a one-pound weight into the bottom of each of the three struts. These cannot be seen during use.

There is no displacement or sag of the components when I move the scope around. I see little or no change in collimation when viewing the mirror alignment with a Cheshire tool at different altitudes. The scope also maintains collimation very well when reassembled. I only need to tweak the primary adjustment screws to bring it back into good alignment. Vibrations during observation are small, and damp out quickly. Under dark skies, the views of objects show nice, high contrast. I can see the spiral arms of Messier 51, and the galaxies of Stephen’s Quintet are clear in the eyepiece.

The telescope cools down quickly. With no tube, there are no internal currents to plague seeing. The exposed optics in the open structure have resisted dew even while dew was collecting on the rest of the telescope. However, I plan to add a dew heater to the secondary and a fan to the bottom of the mirror box.

The components are all light and easy to move around. The optical assembly  weighs 30.8 lb., including the altitude bearings. The 80mm finder and mounting bracket add another 3.5 lb. With the rocker box and three pounds of counterweight, the total weight of the telescope in use is 55.9 lb. Assembly or disassembly of the scope in the dark only takes ten minutes. No tools are required. The disassembled scope is compact. During transport, short lengths of 1-1/2-inch aluminum tube substitute for the long struts and securely support the upper ring.

Bottom line? My favorite telescope is this ultralight portable. While very short focal ratios show too much off-axis coma for my taste, at f/5 the effect is reduced to a threshold that is visually pleasing. The configuration allows me to stand on the ground while observing. The scope gives good images and is very convenient to use.