How to Build Dobsonian Telescopes

Measure the optical mirror. Choose a tube that is about 2 inches larger in diameter than the mirror. Hardware stores sell a wide range of cardboard tubes that are used to form concrete pillars.These sturdy tubes make great telescopes.

Remove the plastic film from the inside of the tube. If you plan to paint the outside, sand the outside of the tube lightly.

Cut one end of the dowel at a 45-degree angle. An old closet rod works great. Cut the rod 3 inches from the shortest point. Cut three slits in the side of the dowel equally spaced around the side.

Slide a cedar shingle into each slit in the dowel. Glue the shingles in place with the silicone adhesive. Paint the entire assembly black.

Cut three 1/2-inch pieces of leather scraps. An old belt or even an old shoe top will work fine. Glue a piece of leather over each of the slits on the angled end of the dowel. Glue the secondary objective mirror to the leather pads. Prop the entire piece, known as the spider, so that the mirror dries level.

Cut a 1 1/2-inch circle in the masonry board. Glue the cardboard tube to the hole cut in the board. Allow the glue to fully dry.

Figure out the placement of your eyepiece. This will depend on the focal length of the objective mirror. Add the diameter of the tube to the length of the eyepiece and the focal length of the mirror. Add any space needed to mount the mirror. Subtract this total from the length of the cardboard tube.

Using the sum, measure the distance from the end of the tube to the center of the eyepiece. Cut a 1 1/2-inch hole in the side of the tube. Slide the cardboard tube through the hole and secure the masonry board in place with the sheet metal screws.

Paint the inside of the tube black. This prevents ambient light from reflecting into the eyepiece.

Insert the brass eyepiece in the cardboard tube. It should fit snugly, but still slide freely. Wrap the hose clamp around the outside of the cardboard tube. Tighten the clamp to hold the eyepiece securely.

Place the spider inside the tube so that the angled mirror is directly below the eyepiece hole. Look through the eyepiece to assure that the mirror is aligned properly. You should be able to clearly see the end of the tube. When the spider is aligned, glue the shingles in place around the inside of the tube. allow the glue to dry completely before moving.

Cut a square of plywood that has the same width as the diameter of the objective mirror. Draw two lines connecting opposite corners. Where they cross is the center of the square. Use a compass to draw a circle the diameter of the inside of the tube. Lop off the corners that extend outside the circle.

Draw a second circle 2 inches smaller than the diameter of the objective mirror. Drill holes equally spaced around this circle. Thread bolts through these holes. Cut a triangular piece of scrap cardboard large enough to cover the bolts and glue onto the plywood so that the cardboard covers the head of the bolts.

Nail four rubber furniture to the inside of the tube just in front of mirror's resting place. Carefully place the mirror into the tube. Screw small scraps of plywood into place behind the mirror to secure it in the tube. Place the plywood square with the cardboard facing the mirror, onto the back of the telescope. Slide an L-bracket onto each of the exposed bolts and secure the other side of the L-bracket to the inside of the tube.