Dome building: Same dome another week.


Sunday morning it cooled off from early warmth to 66F. So I painted the skeleton with Safeway mineral wood preservative. The powder is added to cold water at 5g per liter. The powder does not thicken the water so it goes a long way but can be messy with spray form  brush used with a little too much enthusiasm.

When it started raining quite hard the awning kept the dome completely dry so I went on painting. The dome is now "weathered" just like an abandoned barn, inside and out. It will be painted properly, of course, but the treatment might just prolong its life. The popular GRP covering beloved by amateur dome builders would add considerably to the cost and weight! So I'm hoping that high quality breathable, wood protection paint will do the job instead. I put the panoramic "after" image together using MS ICE.

I asked at a builders merchants what they suggested for sealing my dome panels. Dana MS522 [or 524] was their recommendation which is not very expensive. It has been available for 20 years and the seal around some original window installs, in the harsh conditions in North Jylland, are claimed to be still as good as new. I'm not quite sure that a sealer used around fixed windows is exactly my need. A higher degree of flexibility is desirable where the dome will be buffeted by wind and rotated on a non-perfectly smooth ring. Most materials stiffen with cold and there will be cold aplenty in most Danish winters. Most adhesives/fillers/sealants harden with age. I haven't pinned down any of the latex-silicone-acrylic sealers which are widely available in the US market. Even eBay[UK] dealers list them only from the US.

There was some discussion on the width of the gaps between panels. The smaller the gaps the more flexibility required. I went with edge to edge fitting when cutting the panels on the saw sled. It occurred to me that I could take a small cut across the bottom of each panel. The panels would then all be slid downwards by only a couple of millimeters. This would provide an automatic peripheral gap on all sides without altering the trapezium's other angles.

I thought I'd see whether a boot scraper style, galvanized steel doormat would work as an observatory trapdoor. The reality was the weight rises rapidly with size. Forum members suggested aluminium or fiber grills. Both are available in Denmark but costly in typically large sheet sizes. I only need 50x100cm maximum length. I may be able to source more locally from a user of these products rather than a wholesaler.

I looked at making another larch floorboard trapdoor and reducing the weight by using a hole saw. The maths doesn't add up to a very lightweight trapdoor. Pi.r^2.N but at least it would breathe. An alternative would be a sandwich of polystyrene and plywood. A stiffening wooden frame would be concealed within the footprint. The cosmetic clash with the larch flooring would be rather obvious.

Framing around the small gore beside the slit frame. Not only must the length be perfect, but the ends are compound mitered [all to different angles] The large face bevel must also be made to exactly match the slope of the attached panels. The slit frame will eventually be cut back in straight lines to match the sliding shutters to the panel tiers.

An open grating [trapdoor] would allow free air movement for a chimney effect to clear warm air from the dome. It would be safer than leaving a solid trapdoor open in the dark. Or leaving large doors open in the Western observatory wall. It was suggested I rig up a red light to show the trapdoor is open. A roller micro-switch could do the switching. LEDS can provide low voltage, low level, compact lighting which can be easily concealed from direct view. AC aluminium floor grills aren't so obviously available here compared with the much warmer US.

I ordered another 100 6mm [1/4"] stainless steel screws, nuts and oversized washers to hold the ribs more firmly together. They will help to spread the loads across a much larger area of rib.

Thursday: Spent some hours rebuilding and fitting vertical struts to the narrow gores beside the slit frame. I also reworked the slit frame base to close gaps and square it better against the dome. Rain is forecast for this evening so I have covered the dome with a lightweight tarpaulin. It did rain quite hard so it was well worth covering up the dome.

Click on any image for an enlargement.


No comments: