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How to Make a Dobson Telescope

In the late 1950s, astronomer John Dobson introduced a low-cost, non-equatorial plywood telescope mount for large homemade reflector telescopes. His invention made it possible for amateur astronomers to spend their money on large, high-quality optics by drastically reducing the cost of a usable mount. Simple, cheap scrounged materials make the easy-to-build Dobson mount the most popular homemade telescope mounting system to this day. The design below is for a 10-inch “Dob”.

Things You'll Need

12 -nch Sonotube 8 feet long (available at a concrete supply store)
10-inch telescope mirror
10-inch mirror mounting cell
10-inch Spider mount for the secondary mirror
Secondary mirror
2-inch focuser
Eyepiece
Drill and bits
Saber Saw
Screwdrivers
4-by 8-foot sheet of half inch plywood
1-inch screws, 1 box
Wood glue
2 old phonograph records
1 half-inch carriage bolt, 3 inches long with nut and flat washer
Felt strips
Scissors

Instructions

The Scope
- 1
  Mount the telescope mirror on the mirror-mounting cell following instructions that came with the cell. Drill holes in the lower end of the Sonotube to match the mounting screws and screw the mount into the end of the tube.
- 2
  Subtract 6 inches plus the length of the focuser from the focal length of the mirror (found in the mirror specifications). Measure this distance from the center of the mirror and mark the side of the Sonotube at that distance. Measure 4 more inches and cut off the rest of the Sonotube. Be careful to hold it upside down while cutting to keep dust off the mirror.
- 3
  Drill a 2-inch hole in the side of the tube where you marked the adjusted focal length. Mount the secondary mirror in the spider mount. Mount the spider across the open upper end of the tube so that the secondary mirror is directly below the 2-inch hole.
- 4
  Screw the focuser assembly into the tube directly over the 2-inch hole. Collimate (align) the optics according to detailed instruction that will come with your optics.
- 5
  Mount the finder scope 1/4 way around the tube from the focuser and calibrate according to instruction that come with the scope. When mounted, add the eyepiece to the end of the focuser to observe.
The Dobson Mount
- 6
  Build an open-ended plywood box with two 12-by-12 and two 12-by-13-inch plywood pieces. The box should be 12 inches square inside. Cut two 10-inch diameter disks from plywood and mount them on either side of the box. Glue felt around the edges of the disks and on the inside of the mounting box.
- 7
  Cut two 16-inch plywood circles. Glue old records on one side of each. Mount them one on top of the other with the records between. Drill a half inch hole through center of the disks and records and bolt together with the half inch carriage bolt with the bolt head on the bottom, recessed below the wood’s surface.
- 8
  Cut two 12-inch wide plywood sides for the mounting pillar that are 26 inches tall. Cut one 13-inch wide plywood side, 18 inches tall. Assemble into a three-sided pillar with the short side in the front. Cut two 6 1/8 inch diameter arcs out of the top of the long sides and line the inside of the cradle formed with felt glued in place.
- 9
  Glue and screw the three-sided mounting pillar into place. The open side allows the tube to swing straight up. Slide the tube into the open-ended mounting box and balance it at the ears of the mounting box. Add felt padding to the scope tube if it needs to fit more tightly in the mounting tube.
- 10
  Set the mounting box and scope into the cradle with the ears resting in the padded cradle. The scope, if balanced should sit at any angle and rotate smoothly, but not slip once in place.

Astronomy