6" F/5 Dobsonian: Primary and tube

I've been missing making a telescope, but didn't need another one.  So I persuaded a colleague to make one with me.  The optics were cheap--a 6" F/5 mirror (with a 1.5" secondary) from a Cloudy Nights seller for about $60.  In classic style, this was going to be mounted in a Sonotube, with most of the rest made of 1/2" Baltic Birch plywood.

Before I even got my hands on the mirror, I cut altitude bearings from 3/4" red oak.  They are slightly more than semicircles.  It's traditional for altitude bearings to be circular, but using semicircles (or slightly more), the radius could be made larger for semicircular bearings without wasting money on a wider board, thereby making for greater stability.  Moreover, it's easier to cut less than full circles on my fixed-base router.  And I cut lightening holes with a hole saw.

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.

The result is a very solid and strong tube, though right now I only have one plywood ring in place.  (One problem: the inner diameter of the rings is too small by about 1/16", and it took about 1.5 hours of sanding to get the one that I installed the right size.)