General
The Dobsonian mount is probably the simplest mount available. It consist of a flat, horizontal platform (A) with a axis (H) in its center. Mounted on this axis is a box (B) that is open to one side and the top. In this box the telescope tube (C) is mounted, such that the circular plates (D) that are attached to the side of the tube rest in openings on the top of the two opposing sides of the box. The bearings (F) are made of pieces of Teflon. The focuser is at (E).
The horizontal movement of the scope is made by the box (B) around axis (H) relative to the platform (A). The vertical movement is made by rotation of the plates (D) relative to the box (B).
The Dobsonian mount is named after John Dobson. Using the known alt/az design, he perfected it for use as a low cost telescope mount.
Advantages
The Dobson is extremely easy to make. It takes only basic woodworking skills. And it requires no high-tech parts. Most material needed is readily available at little or no costs. Its usage is very intuitive, just push it around. No polar alignment necessary. It is also ultimately portable since the platform (A), the box (B) and the tube (C) can be assembled and taken apart without tools.
Disadvantages
Being such an incredibly simple design, there are of course some disadvantages. It is only usable for visual observing. At higher magnifications, it can get a bit difficult to find and track objects. Also at higher magnifications the mount can introduce vibrations if the box is under-dimensioned. When motorizing the mount, an additional computer is needed to control the motors.
Design Issues
The following points should be considered when designing a Dobson mount;
- To prevent vibration and backlash, make the box from thick plywood.
- The friction between the platform (A) and the box (B) should be as low as possible. Then you can place the bearings as far out as possible, thereby increasing stability.
- The tube should be balanced, be sure to include eyepiece and finder when balancing.
- The friction between the box (B) and the plates (D) should be just right. It is just right when the tube can be easily moved (without backlash) and stays in position even when eyepieces are changed. The friction can be increased by moving the teflon pieces (F, between B and D) farther from each other.
- The plates (D) should be as large as possible. They are often made too small.
If the tube moves when changing eyepieces, add a counterweight.