Sewer Pipe Mount

After rebuilding the telescope tube I needed a new mount. In my quest to find 7 inch aluminum tubing I had seen piles of 8" plastic sewer pipe at the local junk yard. It occurred to me that a short length of tubing, if cut properly, could be made into a rocker box for a dobsonian mount. I bought a 6 foot piece of 8 inch tubing for $20. This was enough tubing that I could afford to make at least a couple prototypes before my final design.

Sewer Pipe Rocker Box and Trunnions

Rocker Box

Rocker Bottom	Tom-Kap

The image on the left shows my completed rocker box. The telescopes trunnions fit into the small U-Shaped cuts on either side of the rocker box. The trunnions are made from two 3 inch ABS Tom-Kap sewer plugs. To mate the trunnions with the side of the scope tube I placed very rough sand paper on the tube and sanded the plugs until they exactly matched the tube's shape. The U-shaped cuts in the rocker box were made with a 3 inch circular saw. The circular saw hole cuts must be made before the tube is cut perpendicular to its length! I found that it was best to cut the plastic pipe with a hack saw blade; wood saw blades tend to grab tightly to the plastic tube and are consequently difficult to control. For the large circular depression I had to determine the formula for two intersecting cylinders (a fun little puzzle).

The Tom-Kap plugs are designed to fit snuggly into a 3" tube. As a result the circular holes for the trunnions are a little to small. This is in fact advantageous. A little careful carving with a sharp knife was all that was neccessary to make them fit perfectly. After carving, the tops of the U's touch the Tom-Kaps while the bottoms of the U's are free.

Now that I know the tricks to fabricating a Sewer pipe rocker box I could probably make another in about an hour! Tricks include:

• Use hack saw blades to cut PVC
• Use large sheets of rectangular paper to mark lines on the tube
• Drill the trunnion holes BEFORE cutting the tube.

Base Board

In a classical Dobsonian telescope the bottom of the rocker box is covered with Formica and it rests on a base board with three teflon pads. In my design the bottom of the plastic sewer pipe rests on a base board covered with high density polyethylene (HDP). The HDP is essentially impossible to glue so it is screwed into a plywood base. The pipe riding over the HDP creates a smooth action. I picked up a sheet of the HDP at a junk yard for a few dollars. In the pictures shown here the base is bolted onto a Celeston tripod. The scope can also be used without the tripod.