March 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - March 2001 Issue

The Telescope Nut
by Jeff Baldwin

40" f/3.6 Telescope Progress

Glen Youman has been a very busy man. He builds telescopes for schools, museums, friends, as well as for enjoyment. Have I mentioned that he is building about 99% of the 40" telescope?

Glen's latest achievement is the secondary cage for the 40" scope. As you can see in this photo, it is beyond huge. Up close it is a beautiful piece of work. The three rings are Baltic (Russian) Birch, perfectly cut and assembled, sporting a fine coat of flat polyurethane. There are three rings because of the Kriege style of building large Dobsonians. Two rings are for the cage itself, while the third ring connects the poles to the cage. When the telescope will be assembled, the third ring will attach to the truss poles. Then a brick will be attached to the ring to hold it horizontally while the cage is attached to this third ring. That will keep us from climbing up a ladder with a priceless secondary cage and then dropping it. It's heavy, 46 pounds, and we'll be way the heck up a tall ladder.

The wall of the cage is a plastic called Kydex. I got it from Interstate Plastics. They make two sizes, 0.028" and 0.040" thick. I used the thinner material on the 24" and it wrinkled after about a week. I used the thicker stuff on this and am happy with it at this time. Being thick is the first reason why I don't think it will wrinkle. The second reason is that Glen installed it with holes larger than the screws so that it can slip around a little rather than being fit tightly. This stuff is 42% heavier than the thin stuff, but worth it for large scopes.

The poles are 2" diameter with .065" thick walls, the same as the truss poles. There are 7 of them, which sounds weird, but makes sense. There would be 8 of them, mounted in 45° intervals, but the focuser is mounted where the 8^th pole would have been. Glen powder-coated them black, and they're awesome. We couldn't come up with good umbrella clips to hold the tubes to nuts on the rings, so Glen mig welded aluminum disks into the tubes and tapped them with thread. We tried to break one, but the bolt broke before the device.

The third ring has alignment studs to ensure that the cage mounts exactly the same way each time. The truss pole brackets were custom-made by Glen for this telescope with unusually large poles. The spider and secondary holder are also custom-made for this scope. The secondary mirror is going to be quite large, m.a. = 7", and the holder is large enough to accommodate this. The mirror will be silicone-glued to the holder, and a strap will be attached to it in case the glue fails. Don Taylor has come up with a 7" pipe that I will be able to use to saw out the flat when I finish polishing it. 7" is an unusual size and I'm lucky he was able to find this. Lots of friends make telescope projects happen.

I thought that I would use 48" Sonotube for the 'hatbox', a drum-like container to keep the cage in so that it will be protected. The cage is 48" in diameter, but the hardware on it protrudes beyond 48" . I may still be able to use 48" Sonotube, but will probably have to slot parts of it. My school is building me a trailer to haul and store the telescope, and the front compartment will be rigged to hold the cage. I still think I would like to have it boxed up somehow.

I will try to bring the cage to the March meeting, as well as the Tri-Valley club meeting the next night, weather permitting.

One last note -- notice that the picture is of my wife and the cage in the living room. She's really nice. Can you imagine letting your spouse keep an object like this in your living room?

(Note from Glenda: The deal is that the cage is out of the living room as soon as the trailer is ready!)

Clear Skies...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.