Back to Carl's Telescope Home Page
Carl's Telescope:
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.
My optical design.
My WinSpot design.
How to make the toroid mirror.
My collimation procedure.
e-mail Carl Anderson
Here I am at the eyepiece.  The scope comes apart in two main sections for transporting.

A black cloth shroud can be placed around the truss structure to further reduce stray light.
To maintain high contrast performance, the mirrors must be kept clean.  This dust cap was made from a plastic water bottle.
The mirror cells are plywood rings glued together.  They clamp onto the structure for observing.  For storage, the cells are placed in a clean plastic box to remain dust free.

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.
       Primary Mirror                                    Secondary Mirror

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!
This metal alignment cross is temporarily placed at the secondary mirror to align the primary tilt.  When the cross is centered in the focuser view (no eyepiece installed) the tilt of the primary is right.
This bending bracket was used to bend the primary mirror into a toroid shape.  It worked fairly well, but I thought at high magnification, the focusing could be better.  It was made of 1/2 inch steel tubing with aluminum plates.  These were glued onto the back of the mirror with silicone rubber gasket adhesive.  The rubber is about 1/32 inch thick.  This method eliminates the troublesome pressure points that I experienced an earlier bracket.  The force of the compressed spring is about 55 to 60 pounds. 
An amazing view with a Denkmeier binoviewer made the Moon and planets appear 3d!
Thoughts concerning bending:
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.