By Dave Scherping
Until recently, the Prairie Astronomy Club, in Lincoln, Nebraska, had only one club telescope, a 12.5″ Newtonian on an equatorial mount. When a 13.1″ f4.5 primary mirror and 2.5″ secondary mirror were donated to the club by Tom Miller, one of our members, we decided it was time to design and build a portable club scope. A project team was formed and began meeting on a weekly basis.
With the requirements that the scope had to be portable, compact, and of “professional” quality, we concluded that a truss-tube Dobsonian would best meet the club’s needs and expectations. Soon after the project was initiated, a Telrad, a Tectron 2″/1.25″ low-profile focuser, truss tubes, and several other components were donated. The project team was given a budget of $350 to complete the scope.
The design, which began in July, 1994, was laid-out in a 1/4 scale assembly drawing with accompanying detail views. This is an important step that is often overlooked in amateur projects. Accurate, detailed drawings are an asset in evaluating the design and coordinating construction. Our design was highly influenced by Obsession Telescopes ad discussed in Dave Kriege’s articles in Telescope Making #35 & 44.
Although we wanted to build as much of the scope as possible, we chose to purchase a 9-point primary mirror cell and a secondary holder from Astro-Systems. This was done primarily to save time and meet the high quality criteria. We also purchased Kydex and Ebony Star formica from Astro-Systems, although these could have been obtained locally.
Prior to construction, the design was checked dimensionally and preliminary balance calculations were made. This required weight estimates for all components. Weights were measured directly for on-hand components and estimated for all other components. An estimated density of 35 lbs/cubic foot was used to calculate the weights of wood components. The design was then modified slightly so that the centers of the altitude bearings were at the theoretical balance point.
For most of the wood components, we chose 9-ply 3/4″ birch plywood, which was obtained locally. All of the components were carefully laid out on the sheet of plywood prior to making any cuts, so as to conserve material. This allowed us to retain an excess of approximately 5 square feet in case we needed to redo a piece or two. The initial cuts were made with a saber-saw. To make the final cuts, we used a radial arm saw for straight edges and a milling machine for circular components. Solid maple was used for several other components, such as the split-blocks, focuser board, and Telrad mounting board.
The rocker box and mirror box were assembled using glue and 2-1/2″ drywall screws. The screw-heads were countersunk and hidden with dowel plugs. Prior to assembly, the sides and the bottom of the rocker box were clamped in place and checked for squareness. Additional spacers were used to ensure it remained square until after the glue dried, which is critical to ensure the mirror box moves freely inside the rocker box. The mirror box was clamped with the primary mirror cell in place and also checked for squareness prior to screw insertion. The primary mirror cell was later attached with carriage bolts after painting the inside of the box flat black. A 4″ 12 Volt DC muffin fan was installed for cooling the primary mirror.
Square corners and edges of the plywood were beveled with a file to prevent delamination. Gaps and voids were filled with wood putty and the surfaces were sanded with a finishing sander. After staining, we applied several coats of ultra-gloss spar polyurethane finish to all wood surfaces except the insides of the lower tube assembly and rocker box, which were painted flat black.
The scope was designed for 10″ radius altitude bearings to allow for a low profile rocker box. This maximizes stability and minimizes vibration. The bearings were made from a laminated piece of 3/4″ & 1/4″ birch plywood and were cut on a milling machine. Relief areas were removed to allow clearance for truss tubes and split-blocks.
The bearing surfaces of the altitude bearings and the bottom of the rocker box were covered with Ebony Star formica. These surfaces ride against Teflon pads attached to the rocker box and the ground board, providing incredibly smooth motion in all directions. Felt pads were used to eliminate any slop between the mirror box and the rocker box.
Obsession style split-blocks were chosen to secure the lower ends of the truss tubes. To make the split-blocks, we first glued together two pieces of 3″x2.5″x3/4″ maple for each block. After squaring the edges on a belt sander, we used a drill press to drill the necessary holes and a band saw to cut the slots. A milling machine was used to cut the relief grooves on the backs of the blocks. The upper ends of the truss tubes are secured to the upper clamp plates with threaded knobs.
To simplify assembly, the telescope was designed so that the eight truss tubes are of identical length so as to be interchangeable. 1″ diameter seamless aluminum tubing, with 1/16″ wall thickness, was used for the truss tubes and the upper ends were flattened in a vise to allow attachment to the upper tube assembly. While flattening, the ends of the tubes were heated with a torch to prevent cracking. I recommend using a large vise, since the handle of my vise was shaped like a horse-shoe by the time we were finished. The flattened tube ends were then painted with black enamel and a coat of clear polyurethane for durability and the remaining lengths of tubes were covered with foam pipe insulation. By using split-blocks to secure the tubes to the mirror box, we eliminated the need to flatten both ends of the tubes and the problem of keeping the two flattened ends parallel.
To construct the upper tube assembly, two 14.5″IDx16.5″OD rings were cut from 3/4″ birch plywood and were connected with four 1″OD x11″ aluminum tubes inserted into 1/4″ deep recesses in the rings. These tubes were made from excess truss tube material. A 7/8″ diameter dowel was inserted in each tube, allowing us to attach them at the ends with drywall screws and drill holes through them to attach the spider. We considered using the threaded inserts mentioned in Dave Kriege’s article, but they were hard to find locally. A sheet of Kydex was glued to the inside of the upper tube assembly to act as a light shroud. The focuser & Telrad mounting boards were made from 1/2″ maple and were attached with finishing nails. It’s important to ensure the focuser board is sturdy and well secured to prevent flexing and possible breakage. The focuser board is the only thing preventing someone’s $300 Nagler from hitting the pavement! The 4-vane spider was made from threaded rod, 1.25″ aluminum channel, and 1.25″ wide x .03″ thick steel banding strap.
We intended to optimize the lengths of the truss tubes by first cutting them a little longer than needed and then shortening them to the desired length based on actual focusing results. However, we accidentally cut one of the tubes 3/4″ too short. This was easily remedied by shortening the remaining tubes and slightly modifying the design of the upper tube clamp plates. It seems that no matter how careful you are, there’s always at least one surprise in every project.
To install the split-blocks, they were first attached with only the clamping bolt to allow them to pivot. The truss tubes were then installed and connected to the upper tube assembly. With the split-blocks now in their final positions, they were secured to the mirror box with additional screws. It was now time to attach the altitude bearings. These were first held in place with C-clamps to ensure the scope would move properly when placed on the rocker box and were then secured to the mirror box with drywall screws.
A string was attached the back of the secondary mirror with silicone and the other end was tied to the spider to prevent the secondary from accidentally falling onto the primary mirror. I recommend that this be done with all reflectors. To protect the primary mirror during transporting and storing, a dust cover was constructed out of 1/4″ birch plywood with maple ribs and weather-stripping around the perimeter. Matching brass handles were attached to the sides of the rocker box, mirror box, and dust cover.
With the scope assembled, it was checked for balance and found to need additional weight in the lower end of the mirror box. We used 1″ channel with lead shot and placed 5 pounds on the high side and 3 pounds on the low side behind the primary mirror cell. This provided perfect balance when using any eyepiece with the scope pointed at any angle. Note that if all of the weight was placed on the high side (or on the low side), the scope would not have balanced near the zenith.
Overall, construction of the scope went fairly smooth however it became a much larger project than originally anticipated. We attribute the lack of “surprises” to the thorough design and pre-construction work. We also benefited from experience, and mistakes, of several members who have built their own scopes. The key to a successful project is thorough planning. Also, if you don’t have the right tool for the job, find someone who does, and pay close attention to small details, such as countersinking, sanding, and finishing. Building a telescope deserves just as much care as finishing a fine piece of furniture.
Total cost of the PAC Telescope was nearly right on budget, at $360 (not including donated items). If all of the donated components were purchased, the cost would have been approximately $1000. The cost could have been reduced if we had built items such as the primary mirror cell and secondary holder. On the other hand, many of the components we built could have been purchased, such as the split-blocks, spider, altitude bearings, and upper tube assembly. If purchased, a scope of similar type, size, and quality could easily run $2000.
The finished telescope saw “first light” at the PAC meeting on December 27, 1994. Everyone was amazed at its exceptional appearance and performance. The telescope is extremely stable, moves very smoothly, and has virtually no vibration. After presenting the new scope, it wasn’t too difficult convincing club to allocate money to purchase eyepieces. We purchased two Meade Series 4000 plossls and a Tele-Vue plossl through Starry messenger for $150, bringing the total investment to $510.
In closing, the project turned out to be a very rewarding experience for everyone involved. Being a fairly large club with over 100 members, there are always members who don’t own scopes and those who would like a larger scope. The club’s investment was well spent, for now we have a great scope that any member may use. We now check the scope out on a weekly basis, with the stipulation that the scope must be available for our regularly scheduled observing sessions and for the Nebraska Star Party, which we coordinate. I highly recommend that other organizations consider projects such as this. It’s fun and exciting, gets people involved, and leaves the organization with a valuable, useful asset.