The large focusing lens found on an old 3M overhead projector can serve as the primary objective lens for a homemade telescope. By using PVC tubes to build the telescope body and ocular support, you can mount the 3M overhead projector lens at one end of the PVC tube and position your ocular at the opposite end. Several short pieces of PCB tube, one placed inside of another, enables you to step-down the large diameter tube holding the projector lens. Doing so allows for the smaller ocular to be held at the focal point and focus the telescope.
Determine the length of telescope tube that you need by finding the focal length of the projector lens. Find the focal length of the lens by holding it above a piece of paper and focusing sunlight through it until you produce the smallest possible point of bright light (Be careful as this point of light will be very hot) and measure the distance between the paper and the lens.
Cut the telescope tube to the length of the projector lens' focal length. Cut the 2-inch length of PVC tube that has an outside diameter equal to the inside diameter of the telescope tube, around the circumference so that you have two, 1-inch lengths of tube (This will be the support for the projector lens). Spray paint the interior of all tubes with matt black paint and allow the paint to dry completely.
Glue the projector lens to one side of the 1-inch length of PVC tube you just cut using silicon glue. Glue the second piece of PVC tube to other side of the lens and fill any gap at the lens edge with silicon glue. When you finish, you will have a lens with a 1-inch piece of PVC tube extending from both sides. Allow the glue to dry.
Slide the PVC tube/projector lens assemble into the telescope tube until one edge of the PVC tube supporting the lens is flush with the edge of the telescope tube (Use light sandpaper to remove small amounts of material so that you get a good fit). You will now have a telescope tube with the projector lens supported within the tube.
Ensure that the lens assembly will fit snugly into the telescope tube, then remove it, apply PVC glue to the outside edge of the lens assembly and insert it back into the telescope tube and allow the glue to dry completely.
Use matt black paint to touch-up any exposed tube so that the interior of the telescope tube does not reflect light. Place your eyepiece into the appropriately sized, 2-inch length of PVC tube (Telescope eyepieces come with outside diameters of 0.965 inches, 1.24 inches and 2 inches).
Start with the tube holding the ocular (eyepiece) and begin adding larger diameter pieces of 2-inch PVC tubing, gluing each in turn (do not glue the tube holding the ocular since you will be moving it in and out to focus the telescope), until you can insert this group of tubes into the large telescope tube (opposite the projector lens end) and have it all fit tightly. Push this group of tubes far enough into the large telescope tube so that the eyepiece is positioned at the focal point of the projector lens, then mark it and glue in place using PVC glue.
Focus the telescope by moving the PVC tube that has the ocular, in or out. Different oculars will require refocusing the telescope when you insert them into the PVC tube.
- "501 Ways to Use the Overhead Projector"; Lee Green and Don Denerink; 1982
- "Amateur Astronomer Handbook"; J.B. Sedgwick; 1971
- "Amatuer Telescope Making"; Stephen Tonkin; 1998
- Drill and tap the tube holding the eyepiece so that a small locking screw can be inserted to hold the eyepiece in place. You may use a PVC tube longer than 2 inches to support the ocular if you need more focus length.
- Do not observe the sun without solar filters because it will cause permanent blindness.