My aim is to get something solid that will come in at about 20kg (44lbs). I have a friend who is a carpenter and between us we came up with a design for the tube made up of plywood rings held together with strips of wood and wrapped in a flex-ply sheet. The whole thing will be glued and pinned together. He assures me that it will be very stable. Attached is a picture of the completed frame with me next to it. I'm 5'9", so you will see the tube is going to be big. The obvious bend in the frame from top to bottom is an illusion caused by a wide angle camera phone lens. It's a lot straighter than that.
This is a complete seat of the pants experiment as the build is costing me very little so if the tube doesn't work out, I won't be broken hearted. I'm more than a bit worried about flexure in the tube.
As you will see in the picture, I offset the wooden strips for each section. The strips in each section are 120º apart. I'm thinking that may turn out to be a bad plan but it seemed to make sense at the time. Without any empirical evidence I decided this would spread the strain over the tube better as the sections wouldn't be acting as one unit if that makes sense. Like I said, seat of the pants.
The two centre rings are where the cradle that will hold it to the mount will grip the tube. This will avoid pressure on the ply shell.
The cell is a work in progress. I have a design in mind and if it goes pear shaped, I'll buy one. I'm hoping to build it out of the same 3/4" ply doubled up. Plop is telling me that a 6 point cell would be best but I'm having a hard time believing that for a 16", 1.5" thick mirror. I'll probably address that in a separate thread when I get around to it.
The optics I bought 2 years ago and are a 1/4 wave f5 set.
I'd be very curious to hear what your thoughts are on the project. I'll post here as it progresses. I'm hoping for first light in November as a birthday present to myself