The walls of the observatory are made of ¼ inch plywood sheath, mounted to laminations of ¾ inch plywood. The plywood laminates are curved to give the walls a uniform cylindrical shape. 4x8 plywood sheets will be used for the outer sheath, both to conserve work, but more importantly, to insure the structure is orthogonal and uniform. Inner sheath on the walls is optional, but must not be added until the entire structure is otherwise finished. Access to the inside of the wall will be routinely required.
The cylindrical walls must be level, uniform and orthogonal or the dome itself will bind and not rotate.
The diameter of the observatory was selected to be as small as possible, in order to conserve construction costs, and as large as necessary to accommodate the telescope. Details can be found in the file observatory.dxf.
The wall outer diameter is then easy to calculate. We approximate the bending radius of the plywood as its center line. Thus, the outer radius is 1/8” larger than the plywood circumference, and the radius of the plywood laminate formers is 1/8” smaller than the plywood circumference. 2x6 studs were chosen over 2x4 studs, because they were only marginally more expensive, and provides the opportunity for additional insulation.
Given the wall outer radius, we can determine the non-uniformity, and add it to the dome inner sheath radius so that it may overlap without binding. The following spreadsheet can compute some of the important dimensions.
The dome will be stiff enough to be suspended on 3 points. These three points are rollers attached to stepper motors mounted on the walls. The dome must have have an opening for the telescope to peer through; I add 4 inches to each side of the clear aperture. The dome opening must then be 32” + 4” + 4” = 40”. A cover for the opening (shutter) will be a U-shaped section that can be pulled by two ropes for opening or closing. Someday this may be replaced by a motor drive. The shutter will run on two tracks that run almost the complete semi-circumference of the dome, 40 inches wide. The opening will be 16” from the bottom, to 20” past the top.
Ribs of 3/4” plywood again will support the dome sheath. The lower sheath will be 1/4” plywood for stiffness of the base. It is a fine question as to weather the remainder should be 1/4” or 1/.8” ply; but I think 1/4” will be used initially to prevent failure should someone walk across the dome without consideration of the rib locations. Fiberglass tape and epoxy will be used for joints in a manner similar to “stitch-and-tape” boat building.
The bottom 16” section will be built first, and be a stiff base for the rest of the dome to be built upon. This allows the mechanics to be perfected before the full weight of the dome is present, and will allow testing the rigidity of the dome base and mechanics before the design is fully committed.
Once the 16” base section is complete, the opening ribs, spaced 40” apart can be placed on the base. The radius for these two ribs should be 2” larger than all other ribs, so that the extend above the surface of the dome and form tracks for the shutter.
Probably need to do some beam deflection calculations to see that the base of the dome is stiff enough not to sag under load.