Dome building: Framing the observation slit.

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Having cut the ribs, to frame out the observation slit, I laid it over the dome's skeleton. Cutting back two of the three, full height gores allowed the frame to rest on the last gore opposite the slit.

The frame still looked too tall as if projecting too much above the dome. Nor did the remaining gores want to match the internal radius of all the ribs. There should be an unobstructed, hemispherical space inside the containing woodwork. Any projection might spell disaster for the enclosed telescopes or mounting drives in a collision.

In an attempt to stabilize the slit framework I used some battens and boards to support it from the ground. Further marking and sawing produced more 16mm thick arcs to reinforce the slit frame. Choosing where to break the arcs to avoid local weakness of the necessary joints needs some thought. The arcs remain lightly screwed and clamped together while I decide how best to proceed.

Fortunately the forecast remains dry so I have no real need to cover the skeleton. Nor its foolishly projecting timber props. I'd rather not cut them off unless absolutely necessary. The horizontal bar half way up the slit is only temporary and designed to allow me to duck easily inside the dome. It is important that the slit frame is parallel, symmetrical and square while the dome's skeleton is joined onto it.  

If I can arrange the observing slit frame at a suitable height I can start tying in the gore ribs by adding extensions. I need to bring back the strength missing from "cutting out" a huge hole in the dome just to be able to see out. Horizontal struts will need to be tied into the nearest ribs once they have been doubled for strength. Then vertical struts will be added to provide a supporting surface for the trapezoid cladding sheets.

The dome is already remarkably strong considering it is simply clamped together. I was easily able to climb up the horizontal braces to place a builder's level across the top batten of the slit frame. Once the ribs are glued and screwed together it will gain remarkably in strength. Then come the trapezoid panels, glued and screwed on top. These will add a stressed skin effect to resist the skeleton from twisting.

Saturday, I spent some time cutting out 16mm arcs for the slit frame. Still two, half arcs short of a set. I just found a 5'x5' of 16mm birch in my plywood stock which I had completely forgotten about. It has survived the winter with some darkening along one edge. Fortunately I needed only two shorter arcs so I could avoid the staining.

Sunday, I doubled the arcs with offset joints and 2"x6s" top and bottom. Now firmly propped, independent of the dome to ensure the slit frame  is central, the correct height and level. Rain and showers are forecast so I have covered the dome in lightweight tarpaulins to keep the worst off.

Monday, I cut six horizontal struts to join the slit arcs to the nearest [doubled] ribs. This was quite time consuming because the compound angles changed with height. Lots of to-ing and fro-ing between the miter saw and the dome. Taking small cuts allowed me to sneak up on the required length. I am still using up 40x40mm batten off-cuts from earlier in the build. So wastage has been minimized.

Tuesday offers gales and rain. I may be able to work on the dome when it brightens up later.

 Click on any image for an enlargement.

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