Plexiglas telescope
Using the primary mirror and it's holder, the secondary mirror, and the eyepiece of an old 3.5-inch Newtonian telescope, I used Plexiglas to create the remainder of the telescope and the altitude-azimuth mount to hold it. I use it as a demonstration device to show the path of light rays inside a reflecting telescope. To run the demonstration, I put smoke from a lit cigarette in the tube and shine a laser down the length of the tube. In an otherwise darkened room, the internal reflections are very evident. The telescope works as an optical device as well, but, of course, the transparent tube admits all stray light and produces an optical image that has very little contrast.