The Pad
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The grass and topsoil were removed from a 10' diameter circle around the concrete pier.
The top of the pier foundation was then isolated with 1/2" of styrofoam. Roofing felt was placed on top of the styrofoam to keep the hardcore from getting embedded into it. This is intended to keep any vibrations from walking around the observatory from affecting the pier (and therefore scope). |
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Several tons of hardcore where then poured into the hole, raked out, and whacked. The hardcore provides both a stable platform for the concrete and more importantly, drainage of any ground or surface water from under the pad. The whacker is required to get a nice, compacted layer.
You can also see the plastic pipe running from the outside, under the hardcore and up. This will be used as a cable race to feed the power line into the observatory. |
Finally, a vapor barrier (polythene) was laid over the hardcore, a timber frame added to form the edge of the pad, and steel reinforcement cut to fit.
(Don't cut the steel over the vapor barrier. The flying sparks will melt lots of little holes in the plastic. Trust me, I know.) |
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First order of business was to place mounds of concrete strategically under the steel to support it in the middle of what will be the finish depth. Then the rest of the concrete was mixed, barrowed in, and screeded.
This was a very long day (even with my buddy Gerry helping out again).... |
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And the finished result.
We had cattle in the field at the time, so the fence went up to prevent a Hollywood-style hoof-printing session by the cows. They had spent several hours earlier licking the windshield of my car, which I thought was pretty funny until I realized that I had 1/2" of caked cow-snot stuck to my window. |
Framing it Out
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The main framing will consist of two plywood rings top and bottom, with 3x2 studding in between. Cross-braces between the studs will support the floor joists.
The ring sections were marked out on sheets of 1/2" WBP plywood, and then cut out with a jig-saw. We needed enough sections to make 4 complete circles. |
The sections were then laminated into two rings, with two layers to each ring. This allowed us to stagger all the joints. The ring layers were glued and screwed to form a nice, cohesive unit.
A gap will be cut in the bottom ring for the door, but we left it whole for now. |
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We're constructing in the middle of the wet season in Ireland, and a round building is a bit fiddly. We therefore made the (very prudent, in hindsight) decision to do a test erection under a roof. Fortunately, I have a space big enough for this.
First order of business was to lay out several sheets of Sterling board (OSB on the other side of the pond) and use one of the rings to rough-cut the flooring pieces. |
Next we cut all the floor joists to length. There are two principal holes in the floor -- one for the telescope pier and one for the stairs. The stairs hole (in the foreground from center) will be covered with a trapdoor when not in use.
We perhaps got a little carried away and mortise-and-tenoned all the flooring joints. But hey, this is a hobby, right? |
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I agonized at some length over how one gets a flat door into a round building. In the end, I chose to model the "vestibule" loosely on the Grubb 10" dome at Armagh Observatory.
The roof over the vestibule was quite difficult to make as the back edge is a compound curve. But after considerable fiddling we managed it. |
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The stringers were marked and cut against the pre-assembled floor joists, and were then rebated for the treads.
The front of the treads were rounded over, and then screwed and glued into the stringers. |
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