These are the easiest of all to build. You will find binoculars for less than five dollars at every flea market
and many of the garage tag sales you go to. They have usually been dropped and are so far out of alignment that
they are useless. Unscrew the objective (leave the lens in the holder), take out the eyepieces, and keep the prisms
to play with (they are wonderful for demonstrating how to break up a beam of light into its consituent colors).
Throw the rest away. With a few pieces of plastic pipe and some black plastic tape to make the parts fit snugly,
you can build two finders from each pair of binoculars. The part that holds the eyepiece tube can be made from wood,
with a piece of felt glued in to adjust the tension. Focusing is done by twisting the eyepiecetube in and out.
(Even finderscopes that you buy don't have rack and pinion focusing.)The mounting brackets can be cut from wood or plastic.
I have started using nylon screws instead of metal ones to prevent scratching up the plastic.
A guidescope for astrophotography can
be made from an old Tasco 60mm telescope. I removed the focuser and added a simpler eyepiece holder that will take 1 and 1/4 inch
eyepieces. This eyepiece has a crosshair reticle that can be focused.