This is my 8 inch F15 unobstructed Yolo telescope. It is similiar to other Yolos except that I added a third flat mirror to bring the eyepiece to a convenient position. Since the focal ratio is long (F15), the primary mirror has a polished in toroid surface and the secondary mirror has a spherical surface. Polishing in the toroid figure, as opposed to bending, is definitely the way to go. Images are very crisp and sharp. With no obstructions in the light path, views of the planets and the moon have very nice contrast.
A black cloth shroud can be placed around the truss structure to further reduce stray light.
The fiberglass rods are 3/8 inch diameter and 4 feet long from Mill's Fleet Farm.(a local farm supply store) Farmers use these rods to put up electric fenses. They were only 67 cents a piece! The structure is glued together with "JB Weld" epoxy. It is extreemly strong and hard when cured.
A special thanks goes out to Greg Gibbons (veteran master optician) for this clever idea:
Supporting the mirrors in the finished telescope. It is generally thought that supporting a mirror at just three points is not the best idea. Since the secondary mirror is facing down, the mirror is held in place at three points around its circumference. I calculate (with the program PLOP) that the secondary mirror sags due to gravity by only about 1/14 wave, when viewing near the zenith. Now the primary mirror is supported in a similiar fashion with three supports strategically located to exactly match the secondary sag. The result of this design is that light striking anywhere on the mirrors undergo the same total path length to reach the focus. Mirror sag due to gravity is not a problem!
If you are considering making one of these and are thinking about bending the glass, read Arthur Leonard's booklet "The Yolo Reflector". I think that his suggestion of bending an oversized secondary makes alot of sense. He suggests that the secondary should have the same diameter as the primary. The bending forces are near the edge of the glass, far enough away from the illuminated area of the secondary, leaving the center area a very nice toriod shape.
Thoughts concerning polishing in the toroid figure:
Polishing in the figure was not as difficult as I had imagined. I used a pitch lap with some of the facets removed to form the toroid. Then I used a full sized lap to smooth the surface. This method was described by Jose Sasian in Aug, 1988, Sky & Telescope. Here I tested the toroid with a conventional Foucault tester instead of the more complicated (dual orthoginal knife edge) tester that Sasian and Arthur Leonard had used. Because of that, I had to walk back and forth a few extra times to rotate the mirror 90 degrees between measurements. The main thing is to get the difference in radii close to ideal, and end up with smooth surfaces as seen in the Foucault tester. Total figuring time was about 10 hours.