I’ve been helping students at the First Light Saturday science school at the Carnegie Institution for Science for several years now. We’ve done a variety of activities, from making small generators and exploring water power; building and programming robots; measuring the chemical content of foods; growing plants under various conditions (including simulated zero-gravity); and this year, experimenting with light, including building their own small telescopes.
They so far have made three such telescopes: a Galilean, a Keplerian, and a more modern achromatic refractor. Here is what they used to make them. The lenses, all from Surplus Shed, cost a grand total of Five dollars per set. The PVC was a bit less, I think.
Because of bad weather, our winter term was somewhat shortened. Here is one example of what they will finish – a small refractor on a tripod! (They’ll need to supply their own cat, though…)