How to Build a Galileo Telescope

Calculate the length of the tube by adding the focal length of both lenses, since the lens set you purchase may have a different focal length from the ones suggested here. Remember that the concave lens will have a negative focal length so you will, in effect, subtract the focal length of the eyepiece from the objective lens. Add 12 millimeters or about half an inch to the combined focal lengths and you have the total length you need for the outer tube. Cut the outer tube to that length with a coping saw.

Cut the inner tube to about half the length of the outer tube. Spray-paint the inside of both tubes flat black to reduce light bounce inside the tube.

Line the lower end of the outer tube on the inside with a layer of flocked paper about eight inches deep into the tube. Glue the flocked paper in place. This should provide just the right amount of friction for the inner tube to slide in and out and remain snugly in place when set.

Shellac the outside of both tubes to make them hard, stiff and water-resistant. Shellac works better than varnish because it won't overly wet and distort the tube shape.

Cut three half-inch-deep pieces off the leftover piece of the inner tube with the coping saw. These act as spacers to hold the lenses in place. Glue a half-inch spacer ring inside the outer tube one inch from the end of the end that will point at the stars. Let it dry.

Hold the tube vertically and slip the objective lens into the tube to rest on top of the spacer. The curved face of the lens should be turned up or outward. Put a light coat of glue on another half-inch cardboard spacer and slide it inside the tube so it presses the objective lens between it and the first spacer. Allow the glue to set and dry.

Cut a hole in one of the cardboard end caps that came with the mailing tubes. Make the hole an inch across and centered on the end of the cap. Turn the cap upside down and lay the eyepiece lens, flat side down, in the open cap.

Glue the outside of the third half-inch cardboard spacer. Don't use much, to avoid getting glue on the lens. Slide the spacer into the cap and press it tightly against the eyepiece lens to hold it in place. Allow the glue to dry.

Glue the completed tube cap over the end of the inner tube and let it set till dry. Test the telescope by sliding the inner tube in and out of the outer tube until a clear image is formed in the eyepiece.

Decorate the outer tube of the telescope by wrapping it in decorative paper. In Galileo's day, no instrument worth making was left unadorned. Use the telescope as a hand-held instrument or buy a mount for it for improved astronomical viewing.