13" Truss Telescope
A friend had an older home made truss dobsonian telescope. He had built it back in the 80's using 3/4" plywood. It was solid as a rock, but it weighed like one too. Although this 13" truss dobsonian was his favorite telescope it was getting too heavy for him to manage by himself. He loved to observe but rarely dragged it out of the garage to his favorite dark spot any more.
I had just finished my first truss dob. Mine was built around a 16" primary, yet weighed less than 80lbs total including the 26 lb mirror. The mirror box which was the heaviest component was about 50 lbs. He and I figured that if we re-scaled my recent design down to fit his 13" primary, we could reduce the weight of his dob by half. I started drafting up plans for a 13" version. You can view and download a PDF copy of those plans here if you like. [You will note that we changed a few of the details including the altitude bearings during this built but I did not as-built the plans to match.]
Following is a brief description of the re-build of my friend's 13" Truss Dob.
|One of the major advantages of investing the time making a good set of plans, is that the time spend in the shop is greatly reduced. All of the major plywood components were cut on the table saw in only an hour or two.|
|The truss pole clamps were made by cutting small blocks from 2x4 stock. 1" diameter holes were drilled in these blocks with a spade bit using a drill press. The 1/4" hole for the clamping bolts was drilled before the blocks were cut in half, end to end. The matching halves were match marked so the pairs could be re-assembled later after sanding and finishing.|
|The sides of the altitude bearing box were carefully cut using a saber saw. Both sides of the box were cut together to assure they matched each other. The sides are only 1/2" plywood. [Mission: Keep it lightweight.]|
|Next the 1/4" thick top cage rings were cut using a saber saw. Many ATMers like to cut these out with a router on a radius arm. I am fairly good with the saber saw. These came out nice and round. Like the side boards, they were cut out together so they would match. Notice the notches cut on all four sides to accept the cage column boards later. Keeping the top cage light is essential to maintaining a low center of gravity for the assembled truss telescope.||N|
|It was raining the day I wanted to assemble the altitude bearing box and my Patio Telescope Shop was closed. I spread some plastic on the dining room floor to protect the carpet. I used wood glue and finishing nails to assemble the plywood parts.|
|Next I assembled the mirror box. I held the the mirror box square with a string while the glue dried. The mirror support/cover deck was inserted as a place holder just to assure a good fit later.|
|I wasn't long before the summer sun was back. I built a 6 point cell. I used the "tee" support beam configuration again. I used "top side" collimation again too.|
|The photo to the right shows how simple the cell really is. This mirror box deck served as cell support and mirror cover too. It was very light and very strong once installed in the mirror box.|
|To the left is a front view of the mirror cover and cell support deck. Notice the three "top-side" collimation adjustment locations.|
|The altitude bearing box was reinforced along the bottom edge and at the bearing locations using scrapes 1/4" plywood.|
|The ground board was made from 1/4" thick plywood. The ground board does NOT need to be thick if the load from the telescope support feet passes directly through the telfon bearings to the bottom of the altitude bearing box. The lazy-susan bearing does NOT carry the load of the telescope, the telfon bearing and feet do. The lazy-susan is simply a centering device, like a centering bolt. [Many truss dobsonians use a centering bolt instead.]|
|The ground board was trial fit on the bottom of the altitude bearinfg box. Later the bottom of this box was covered with a sheet of formica laminate. The telfon bearings on laminate surface provide just the right combination of materials for smooth "push-to" action.|
|The ground board has a 1" diameter hole to allow final attachment of the lazy-susan bearing to the underside of the altitude bearing box.|
|The altitude bearings were cut from 1/2" plywood. The center spokes were added because the owner wanted to add Digital Setting Circles later. The outer edge of the bearings was doubled-up with another layer of 1/2" plywood. The outside of the bearings were also covered with Formica laminate later, like the bottom of the altitude bearing box was.|
|The parts were trial fit. With lots of sanding and finishing to be completed, it is looking very good. Note the mirror cover is completed too. This one is going to be tough to give away.|
|The top cage is assembled wood glued and finish nailed together. The cage column slats are reinforced using 3/4" square stock. Attachment points at the top and bottom rings are also reinforced. The assembly is very light but is also stiff enough to carry the secondary mirror and its spider without deflection.|
|Installation of the internal plastic light shield, boards for the focuser and telrad really stiffen up the final top cage assembly. Remember the top cage should be light but also should not deflect easily.|
Final 13" Truss Dobsonion with telrad finder. The owner purchased a very nice two speed MoonLite focuser at NEAF. This photo shows his Televue Paracor and one of his very nice 2" eyepieces in the new focuser. Later his wife made a nice black light shroud for him.
Mission accomplished. The heaviest piece, the mirror box, weighed only 25 lbs. My buddy was able to get out there again. After this photo, he added his DSC's which worked perfectly.
He and I had a good time at Coyle field running rings around the dark sky with his DSC's.