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Craig Smith, of Firefly Workshop, has been planning a home observatory and is working out the details of easily building a domed roof for it. He writes:
Anybody with a decent telescope knows that a telescope should be cool as the night air to prevent heat radiant distortion. Aside from keeping it out in an unheated shed yet still having to set it up, one will sometimes make an observatory of sorts. Many have built retractable roofed sheds and the like. But my mind kept working at a cheap lightweight dome roof built in the classic style. Anybody with basic carpentry skills can build a cylindrical wall structure or octagon walls, but the dome roof has always been a difficult and expensive build. Here's my prototype made from cardboard, 1/6th scale.
A single 3/4" sheet of 4' X 8' plywood, supplies the material for the framework. The circle makes the base, and circular cutouts outward from that make the frame ribs. Since there is some leftover waste, the framework will be lighter than the full sheet of plywood. But many little galvanized framing brackets used to fasten it together will add some weight, as well as the roofing material. A 24" wide roll of galvanized flashing will be used to make the 10 roof pie-segments and the rear rectangular segment. Put on with small galvanized box nails, each pie panel will overlap the one next to it, sealed waterproof with silver flashing seal caulk during assembly.The retractable watertight sky opening will be tricky. But if all else fails it could be made like a standard flat roof access door only curved. Standard flashing and roofing techniques are a must here to keep an expensive telescope safe and dry.
Have you ever built such a structure? Got any design input for Craig?
You'll want to move the cross brace currently at the top of the dome away from the apex by at least a tube diameter for those occasions when you're viewing straight up. Depending on size, the view slot door could be split or more likely be once piece and slide "up", but you'll have to make the arc a bit bigger than the dome for it to work smoothly.
The observatory I most recently looked at is a robotic observatory that operates a roughly 20ft dome along with the telescope. The view slot is lipped with about a 4" flash, which the doors overlap when closed (with a corresponding ~4" flashing forming a reasonably weather tight cap.
Are you planning to roboticize the dome or manually open/close?
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When I was a teen, I went to one of those gifted kids summer camps where we got to take courses in astronomy, archeology, and other fun stuff. The astronomy teacher had one of those observatories in his backyard that was built from a "hacked" Sears metal garden shed. It was the same basic model that we had, that my dad and I had put together, so it was really cool to look at how this guy had added rails coming down from the roof and the motorized the two roof sections to run down the rails to open up the roof.
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forget the heavy galvanised brackets. glue technology has come a long way since the days of pva wood glue. use a urethane based construction adhesive such as PL Premium : http://www.simplicityboats.com/pl_premium.htm
this stuff is stronger than nails or screws. in fact, if you might ever want to disassemble this structure, *dont* use it.
ive been building large, complex speaker systems for years, and use this stuff in place of screws or nails. its also great because it expands to fill gaps. boatbuilders have been using it for years too.
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This might be great for a kid, if he is only 60 cm tall!? If your one sheet of 1.2 x 2.4 m ply plan pans out, you will only have ~63 cm of height in the center of dome, unless you plan to build a 1.5 m wall around the circumference.
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The center crossbrace is back a bit from center, but you're right, it will need to be farther back for straight up viewing. A structure this small will be easy to hand rotate & open, no need for motorizing here. Where small ribs meet arced beam in the middle at an angle, angled miter-cut blocks will be needed so it screws together solid. I will need your PL glue here. But the best deck frameworks utilize brackets for frame construction.
Observatories generally have stationary walls, a round cylinder like a silo, and a rotating dome roof. Walls are in the project, as paragraph 1 mentions.
I found observatorycentral dot com for serious homebuilt observatories. Wicked!
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This would be a neat way to build a domed roof for my kids' play structure. The top floor is already 7 feet off the ground, so if they ever took an interest in astronomy they'd have their own observatory tower!
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