Building my 12.5 inch Dobsonian Telescope—The Design
In scaling up the K/B 8 inch design, I made a few changes that, I hope, will improve the design for 12.5 inch aperture. They use a simple plywood mirror cell, stating that for an 8 inch mirror this cell will be adequate. I decided to order an aluminum mirror cell from University Optics instead so that the mirror could be supported at 9 different points. I also doubled the thickness of the rocker sides (not the front wall) to provide better support for the heavier tube. Thicker walls also required thicker side bearings so that the outside of the bearings would line up with the outside of the rocker sides.
Below are cutting diagrams and diagrams of the rocker and tube box with side bearings. The drawings are only approximately to scale. The rocker sides are each made of two identical pieces of ½ inch plywood glued together, creating two sides one inch thick. Likewise, the side bearings are made of two identical semicircles of 5/8 inch plywood glued together, creating side bearings that are 1 ¼ inches thick.
The dimensions of the Tube Box Sides are based on the outer diameter of my tube—14 3/16. This allows for a small clearance between the outside of the tube and the inside of the tube box (14 3/8). I had initially allowed for more of a gap, but realized once I cut the pieces and held the tube inside them that my gap was too large. The length of the Rocker Front also had to be changed so that the outside edge of the Side Bearings matched the outside edge of the Rocker Sides precisely.
The cutting diagram below shows how I cut the required pieces from two 60 X 60 pieces of Baltic Birch plywood, one ½ inch thick, and one 5/8.
The blue pieces of the rocker box and ground board are arranged as in the diagrams below.
On the bottom of the Bottom board is a ring of Ebony Star laminate that glides over the three teflon pads located directly above the feet on the ground board. The curved sides of the Side Bearings are also covered with strips of Ebony Star that glide on the teflon pads on the curved edges of the Rocker Sides.
In order for the Rocker Box to pivot on the Ground Board, a center pin is placed in a hole at the exact center of the Ground Board and Rocker Bottom. The specific hardware used for this is described on the Building page.
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