A Three-Tube Dobsonian

Click on any picture to see larger version (azimuth encoder barely visible in bottom of box) (altitude encoder)

For those of you who don't know me, my name is Bob Smith. I live in Marlboro, Mass, and have been an ATMOB member since May 2001. This is my first attempt at telescope building. I felt that since I belonged to the Amateur Telescope Makers of Boston, that I should at least attempt a scope-building project at some point. This scope started life as a Coulter 13.1", F4.5 Odyssey. I originally obtained the scope from ATMOB club member Dave Aucoin, who had transformed it from the Coulter into a scope optimized for planetary nebula observing. Dave has an excellent article on this transformation on the ATMOB website. Once I used the scope a few times, I realized that it was a little too large and heavy for me to want to take out at a moments notice. As a result, I decided to rebuild the scope into something even smaller and lighter, re-using as many of the parts as possible. I bought "The Dobsonian Telescope", by Dave Kriege and Richard Berry. It provided some valuable insight into what to do and what not to do when building a Dob. I didn't want a scope like all the others, however, so I searched the web for alternatives. I found this scope at http://www.ultralightscopes.com. The design is by Albert Highe, and has all of the features I was looking for. It's simple, lightweight and fairly small. I had been looking at some of the other ultralight designs by guys like Bruce Sayre, but I didn't have the ability to work in metal that easily. Albert's design is a minimalist design in wood, so I decided to go for it and set to work. I used Ѕ inch thick Appleply plywood throughout. I have a friend who kindly loaned me the use of his woodshop to perform all of the woodwork. Since I wanted a lightweight scope, I left off everything I didn't consider essential. That meant that the drive system and battery, dew heater system, and the motor on the focuser had to go. The optical tube assembly consists of a single ring of Ѕ inch Appleply plywood, with the Astrosystems spider with 2.6 inch secondary and the Moonlite crayford focuser attached to it. I also have a bracket that holds a Rigel Systems QuikFinder 1x finder and the Sky Commander digital setting circles computer/control box. The Sky Commander allows you to "zero in" on any of the 9000+ objects in its database. I was able to find any object I desired the first night using this system. The mirror box is a little unusual. It uses two pieces of Ѕ inch Appleply, one for the base and mount for the mirror cell, and a ring above it. I bought the mirror cell from Ultralight Scopes. It's a triangular piece of cast aluminum and uses six pads (T-nuts) in a circle to support the mirror. I know this is a deviation from the "recommended" 9-point support system, but it was designed using PLOP, and it works great for this size mirror. The mirror is held in place with silicone caulk. There are no clips or slings to hold the mirror in place. The original design uses a fiberglass tube to connect the mirror base plate to the top ring to form the mirror box. I tried to make a fiberglass tube, but failed miserably. I decided to build the mirror box like a standard secondary cage instead, using aluminum tubes with push-in inserts. I used Kydex to keep out stray light. The rest of the OTA consists of three parallel aluminum tubes. They're 1 Ѕ inch diameter, .065" thick. The two lower tubes hold the altitude bearings, held in place with two, 1-inch thick Appleply clamps. The altitude bearings can slide up and down the tubes about an inch as needed to compensate for balance. The upper tube holds the 10x50 right-angle viewfinder. I made a mounting bracket out of an aluminum conduit connector and used thumbscrews to hold it in place. All three tubes are retained at the mirror box and the secondary ring using nine, 1-inch thick Appleply clamps, three at the secondary and six on the mirror box. I had to make 13 clamps in all. The rocker box consists of 1-inch Appleply sides and bottom with a 1 inch thick ground board. All of the bearing surfaces used by the scope are the standard Teflon on Ebony Star laminate. This scope has proven easy to balance and use and weighs a total of about 50 pounds. I hope more people will become interested in this design and in ultralight Dobs in general. There are many excellent designs out there. This is only one. A lighter scope is a lot more likely to be used than a heavier one, which is the goal after all, isn't it? Bob Smith

Click on any picture to see larger version

Click on any picture to see larger version

Click on any picture to see larger version