This telescope is a remake of an existing telescope, most of which I scrapped..The two design criteria were, it had to fit through a 30" door and track stars. . The ATM archives, Mel Bartels webpage links and Richard Berry's and Dave Kriege' s excellent book, The Dobsoinian Telescope served as the main inspirations.What emerged was a lighter version of a Kieige/ Berry style of telescope blended with an ultralight weight telescope ala Mel Bartels. I thought this would yield a nice blend of lightness and durability. The entire scope was designed on a Drawingboard CAD program.
Main point of interest
- Top cage with laminated thick bottom ring and very thin top ring (see below)
- Mortices in the top cage to hold the poles
- Paired sets of poles to facilitate assembly with interesting folding joint
- unique mirror box with large cut outs and inner stiffening ring
- unique pole clamps
- use of a full size plot to make acurate welding jig
- hybrid steel and wood rocker box weighing to save weight
- Dob Driver II installation
This Telescope has a long focal length of 110 inches so I really concentrated on making the top cage light. Making the bottom ring of the cage very stiff meant I could skimp on the thickness of the top ring. The bottom ring of the top cage is made of 7/8" balsa cut into eight segments with tongue and groove ends. After the sections were glued together I cut out a circular ring which I laminated a 1/16"maple strip to the inside and outside of the ring to protect the ring from dents and dings .The Balsa ring was then laminated between pieces of 1/8" plywood in a vacuum bag. This whole process ended in a very stiff bottom ring weighing less than two pounds. I ended up using a 3/8" top ring which works well and shows no sign of distortion from the spider vane tension.
Truss poles and connectors
To make the scope easy to take down and setup I connected the truss poles together in pairs. The way they connect at top and bottom is unique and I think very effective. The top of the poles are connected with a machined UHMW piece that holds the poles together and registers the position of the pole to the upper ring. It has a machined square tennon at the top that inserts into a matching mortise in the top ring. It also has a groove in the bottom that allows it to fold flat. The truss poles fit into the UHMW at the correct angle with respect to the mirror box. I used aluminum bolts to hold the poles and UHMW piece together to save weight. The truss tubes are held to the bottom ring of the cage by aluminum rod threaded for a knob at one end and with a hole at the other end for a clevis pin. This system holds collimation very well. I used picture hanging wire to hold the lose pieces together so they won't drop on the mirror. I may change this wire to a nice string for more flexibility.
The bottom of the truss tubes are held on the mirror box with Kindorf aluminum conduit hanger with homemade nuts. I made the stops for the tubes out of UHMW . This system works very well with quick precise setup and take down. I was concerned about crushing the ends of the tubes when tightening them so I epoxied an extra 6" of tubing into the inside end of the tubes.The tubes need to point inward slightly toward the upper cage so I made wooden wedges that fit under the clamps. To connect the truss poles to the mirror box you merely insert the nut into the conduit hanger and turn. Reverse to release. The knob and nut assembly stays on the poles permanently.
Contact: raynalds@spiritone.com
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