I measured the focal length of the mirror at around 29". The time before that I measured the focal length I projected the moon onto a piece of wax paper, and then measured the distance. That wouldn't work this time, as we didn't have a moon in the evening sky. So instead I put a red LED light some distance (d1) away from the mirror surface, focused it on a piece of wax paper, measured the distance of the wax paper from mirror surface (d2) and used the formula 1/d1+1/d2=1/f, where f is the focal length. I did this twice and averaged.
The mirror did have a chip on the back side, and I was worried that this may have set up stresses and distorted things, so I tested it as described in this post, and it was fine. Hurrah.
The first hitch was that the Sonotube that my friend managed to get was only about 0.095" in thickness and very flimsy. Following Cloudy Nights advice, I did a couple of things:
- Cut four plywood rings with a router for outside reinforcement. Currently, one is installed on an end, and I'll put another on the other end, and I don't think more is needed. A pair of plywood rings rigidify a tube by a lot.
- I only needed a 29" tube (that's the focal length; one can't assume the tube length is the same as the focal length, but that's how the calculations worked out for me). Since the Sonotube was 48" long, I had 19" to spare. I cut that into three rings, two 7" wide and one 5" wide. I cut snips out of these so I could roll them into a smaller diamater (snip width: 2(pi)(thickness)) that fits inside the larger tube for reinforcement. I would eventually glued the 7" wide rings at the ends of the 29" tube, and the 5" ring in the middle, but first...
- I impregnated all the cardboard--cardboard reinforcing rings and tube--on both sides, with a 1:1 mix of Titebond II and water. Titebond III would have been better as it's waterproof, but II is water resistant, and it's what I had at home. After impregnating, I inserted the rings inside the tube (the waterly Titebond was good enough for gluing them in place). When I impregnated the insides, I replaced some of the water with DecoArt black acrylic, to make for an initial coat of dark (smudgy blackish gray!) for the inside, which eventually will have to be made much blacker.