8" f/6 Truss Dobsonian


Design Considerations

The internet hosts a number of interesting sites created by telescope makers who describe their ATM projects. After receiving an eight inch mirror for Christmas, (I was a good boy in 2002), I began looking for designs to inspire my first ATM project.

Several features and characteristics were important:
dobsonian
minimalist
compact

The right design would be easy to construct and a welcomed addition to my living room decor were it would be stored when not in use. After collecting a long list of bookmarks I found Barry Leger's web site featuring his December 2000 Sky & Telescope article "A Simple Truss-Tube Dobsonian". The article provided cutting diagrams and a good description of the project.

 

 

 

Construction

Using the cutting diagram from Barry's article I drew the various parts on the mahogany. A Plexiglas template was created to define the shape of the altitude bearings that make up the sides of the mirror box and position the dowel and truss tube hardware holes. Parts were positioned to use the square edge of the board as reference points for the various half circles in the ground board and mirror box. Some consideration was also give to the grain in various areas of the board, saving the best parts of the board for the outside surfaces of the rocker and mirror boxes.

After the parts were drawn a table saw was used to cut the board into more manageable pieces. All parts were trimmed near to the scribe lines of each piece with a rotary cutting tool, the Ryobi version of the RotoZip. The pieces were sanded to their final shape and size with a combination of belt and hand / finishing sanders. The altitude bearings were trimmed to final size with a straight router bit riding on the template as it was affixed to the piece.

The various parts were joined with wood dowels and glue. Most holes were drilled with a hand drill at the start of the project. As the project progressed a bench top drill press was used to keep the dowel holes in position and square.

The upper ring is a variation on Barry's design. It was easier for me to make a circle. The tops of the truss tubes / channel material are bolted to four blocks. The finder shelf was fashioned from a scrap created while cutting one of the rocker box sides.

The mirror cell matches the diameter of the mirror and has three vent holes cut in the center to expose the back of the mirror to the air. Three bolts were glued below the surface of the cell. A nine point glue pattern was used to affix the mirror to the cell. Three plastic clips were attached with screws to the cell to prevent the mirror from sliding laterally. Wing nuts, washers, and springs hold the cell in the mirror box and allow for collimation adjustment.

The original design called for 3/4 inch dowels for the truss tubes. It was difficult to find enough straight dowels so alternative materials were considered. Aluminum 1/4 inch channel material was light, strong, straight, and available in sufficient quantities at my local ACE hardware.

The rocker box and ground board are separated by 8 round 1 inch furniture pads glued in a circle around the center bolt of the ground board. These provide a smooth movement and have performed well in various humidity and temperature conditions.

All the parts were dry fit, finish sanded,and stained in preparation for final glue up. Star testing, collimation, and first light were accomplished prior to final glue up of the mirror box. The Ebony Star was glued to the altitude bearings and a laminate cutter was used to trim the material to the final width.

As the parts came together the need for counter weight became apparent. Lead fishing weights totaling 4 pounds were used to compensate for the weight of the upper ring and its component parts. The counter weights hang from eye hooks below the back cross member and rest on the cross member that supports the mirror cell.

A three hole eyepiece tray was the final part was added to the project after final staining and glue up. The finished project has an overall height of 51 inches and total weight of 25 pounds with two eyepieces and counter weights in place.

Cost and Suppliers

The wood for the project was purchased at a lumber company that specializes in fine grades of wood. Reel Lumber in Riverside CA. had 8 board feet of 3/4 inch, 20 inch wide Philippine Mahogany. Cost $55.00.

The focuser, finder, and secondary were purchased from Orion Telescopes. Cost $85.00.

Aluminum channel material, bolts, and wing nuts were purchased from ACE Hardware. Cost $45.00.

The primary mirror, teflon pads, and Ebony Star laminate were purchased from Meridian Telescopes. Cost $195.00.

Total cost for the project with various other materials was less than $400.

 

First Light

As the project was nearing completion a significant event was approaching, Mars was at its closest point to the earth on the morning of 8/27/03. My goal was to time first light of the scope during that event. On the evening of 8/26/03 Mars was successfully brought into focus.

Shortly thereafter at a Mars viewing outreach the scope has its first public "performance". The Riverside Astronomical Society hosts monthly public outreach activities and a monthly Star Party at our dark sky site in Landers California.

Reactions

Part of the fun of a scope of this design is the reaction to the telescope at outreach events. The design challenges what many people think of as a telescope.

The three most frequently asked questions:
Q: "What is it?
A: An 8" f6, newtonian, truss tube, minimalist, dobsonian telescope.

Q: "Where is the tube?"
A: The open truss design allows you to see the component parts of the telescope.

Q: "How much did it cost"?
A: Total cost, less than $400. But the "wow's" are priceless.

My favorite reaction to date:
"It's a sculpture!"

 

Modifications Completed and Planned

The project included many challenges and changes of plan.

The original spider design was easy to create but lacked stability. Collimation was easy to achieve without tools but didn't stay in position after transport or rough use. The second generation design placed a small block of wood with collimation screws passing through it. It was somewhat more stable but variations in the spider arms placed the secondary just off center.

The final version of the secondary holder is a more conventional design. The key here was to bend the spider arms together for precise placement in the center of the upper ring.

The focuser block started as two blocks that resembled the posts connecting the upper ring to the tops of the truss tubes. A single block was easier to position square to the center of the upper ring and provides a more rigid platform for the focuser.

The most challenging aspect of construction was the position and squareness of dowel and hardware holes. Hand drilling was not as precise as needed. The entire project was more successful after the purchase of a small bench top drill press.

While the project is generally complete several additional features and parts would make observing easier. They include a small base for dark site observations and a shroud. The base would raise the ground board about 18 to 20 inches and would include a small slide out chart table. A shroud would improve image contrast during suburban observations.

 

Other Leger Inspired Scopes

www.webjones.net/sky/

www.cloudynights.com/howto/atm.htm

www.dbpeckham.com/Telescope/Scope/Index.htm

www.starcrwzr.com/truss.htm

 

Questions, comments?

Contact: info@fesunoff.com