Telescope 18-Point Floating Mirror Cell (whippletree)

This is an 18-point floating mirror cell to hold a 14" lightweight sandwich mirror for a reflecting telescope utilizing the whippletree concept.

1. Aluminum channel
2. Aluminum flat stock (1/8 to 3/8 inch)
3. Hex screws
4. Screws and bolts
5. Delrin stock
6. 12 Small bearings (recovered from old hard drives- see solar projects)
7. 3 small radial-spherical bearings
8. 3 medium radial-spherical bearings

Tools

1. Hacksaw
2. Scroll Saw
3. Plasma Cutter
4. Drill Bits
5. Step Drill Bit
6. Blade Drill Bits
7. Taps for threads
8. Allen Wrenches
9. Combination Wrenches
10. Drill Press
11. Portable Drill
12. Bench Vise

I did a lot of research and found a lot of design options. My design requires no advanced tools or welding. I purchased a 14" lightweight sandwich mirror from Hubble Optics in 2008 and it sat in the closet for over a decade before I started on this telescope project. I build this Mirror Cell in 2021. You can see the telescope tube here at my Plywood Tubes Instructable.

A Whipple-What? According to Wikipedia, it is a mechanism to distribute force evenly through linkages. Large telescope mirrors are subject to more optical distortions due to uneven thermals, and gravity during different viewing alignments. This requires the mirror to be supported in a 'floating' manner so it has the capability to equalize the pressures of it's changing environment.

Know your mirror size, weight, and focal length. Layout your desired support points with PLate OPtimizer (PLOP). Once I had my PLOP, I went into 3D modeling to design the rest of it. Since my mirror is a sandwich, with a gap in between the front and back glass, that will be a good area for a mirror retainer. This will keep the mirror in place in case it is ever aligned below the horizon (meaning it could fall out of the mirror cell if leaned forward). 3/4" U-channel will makeup a triangle base, holding three moving levers, with six three-point contacts (18 points).

I got my first plasma cutter and have limited experience with one. I thought I could just create a template for the tip to follow and cut out some pieces from 3/8" stock. I created a two part template from MDF in my CNC. For five minutes of work, they came out pretty messy. So I went back to the standard scroll saw and metal blade and took 10 minutes each to cut them out by hand. I needed six.

I cut a length of 1/8"x3" flat stock into six triangles for my supports. These triangles will be on a swivel bearing (radial spherical), meaning they will have some orbital sway. I did not have the correct drill bits to make a proper holding clamp for the bearings, so I just got close enough with a flat blade into the aluminum. Then a back plate with the same hole to complete the swivel bearing mount.

I cut 3 sections of U-channel for the main frame, held together by the corners from the first steps. I cut 3 pieces of aluminum bar for the balance levers to hold the triangles. I used my CNC to cut some sliders from Delrin for the 18 mount points. I attached these to the triangles with screws. Multiple iterations of assembly, disassembly, adjustments, and reassembly to get things in order.

Instead of a sling, I opted for a 6-point support around the mirror using levers again (whippletree!). They will have a bearing in the center and two mounted radially on the ends for the mirror to glide against. To mount the bearings on the ends of the sling levers, I had to build a jig to hold them a the correct angle for drilling and tapping. These bearings will only engage the lower plate of the sandwich mirror.

On the set of 3 balancers, I added additional holes to make them lighter. The bearings are held in place by two hex set screws. With everything moving as it should, the triangles were actually moving where they shouldn't. So I used some office rubber bands over the mount points to keep them in the general direction.

Somewhere around 2017, I had designed the worm gear set for the Alt-Az design. I had cut a 360-tooth worm gear as 16 identical pieces that I fit together. Some of those scraps were saved and utilized here to retain the Mirror in the mirror cell. Delrin is slippery like Teflon, but also durable and stiff. By adding them to the top of the sling lever bearings, between the layers of the mirror glass, they hold the mirror in place. 3D printed bolt knobs added for collimation and collimation locks. Laser assisted collimation in progress (the tube is still not flocked).

At my current rate of progress, I think this scope should be completed by 2030.