The brand new plywood cladding boards can be given the mineral powder treatment. Hopefully to darken the surface to a weathered finish. Like the shed, but without the very long wait.
An octagonal roof would need some overhang to ensure no rain can "run down the observatory's collar." An octagon has a smaller radius at the center of its straight edges but overhangs considerably more at the "points." There will be a series of eight rotation points where the relative points and straight edges overlap with large or small overhangs.
I like the idea of an octagonal, double pitched roof for another reason. The panels which form its upper triangles and lower trapeziums are of manageable proportions in a 10' nominal diameter. The roof can thus be safely clad by a person working alone. Assuming, of course, still conditions are chosen to perform the task.
Here I have modified the roof of a gorgeous, old, wooden barn to an equal length, double pitched octagon. How can I simulate small "tiles" or "shingles" without adding massively to the roof's rotating weight or badly increasing solar heat gain?