Building a 2-metre Octa-Geodesic Dome: part 3
Construction of Geodesic Dome
75 triangles of the correct dimensions were cut out of 5mm plywood sheeting. They were glued together using Pink Grip grab adhesive into combinations of pentagons, hexagons, and half-hexagons.
This was the exciting part - would all the pieces fit together and make the desired dome shape? The individual shapes were "tacked" together using duct tape then glued with Pink Grip. This job required two people and also some spring clamps to hold things together during assembly. One of us had to enter the dome and work inside while the other glued the final pieces from the outside.
The dome was covered with plastic sheeting for a week to protect it from the weather and let the Pink Grip adhesive cure and form a strong bond.
The dome frame was covered with a layer of papier-mâché (shredded newspaper, hand-pulped in a mixture of 1:10 PVA adhesive:water). 60-70 litres of papier-mâché was needed. The dome took ages to dry as the weather turned cooler and cloudy and I was concerned it would turn mouldy. I had to cover it with a tent made from the upper part of a cheap gazebo and sides formed using plastic dust sheeting. I used a 2kW fan heater on and off for a week to assist the drying process.
The dome was painted with two coats of red mahogany satin finish wood preserver.
The papier-mache formed a good bond with the plywood frame. The dome was securely attached to the dome base ring by wedge-shaped blocks of wood screwed and glued to both the base and frame. The frame was also glued all the way round with another strong grab adhesive which cured over a period of days.
I cut out a 50cm wide hatch and extended past the top centre point of the dome to ensure I could observe the zenith when the observatory was finished.
The cut out piece would be used to make the observatory hatch lid.
The weather was turning against us and we had to take the dome assembly indoors in order to add a papier-mâché weather-tight lip to the hatch. Note the "cling-film" to prevent the papier-mâché adhering to the dome surface.
In order to keep the dome running on track I attached a skirt made of 6mm rubber compound. It is quite stiff but with enough flex to "ride" the guide rollers. It was screwed on using 4cm "easy fixings" (for chipboard & plasterboard) and the tops later painted with black enamel paint for cosmetic purposes. The dome was further weather-proofed with two coats of yacht varnish.
Roofing shingles were attached to the watershed and the whole assembly sealed with silicone sealant ensured a pleasing appearance and a functionally weather and watertight structure.
Hatch off and stowed inside, scope up and ready for viewing after five months work - a really great feeling!
Inside the Dome
The hatch is very easy to open by angling it then taking it inside the dome and simply reversing the operation to re-attach it. The thin rubber weather seal on the inside surface of the lip and the door-stop wedges between the support blocks and adjacent handles ensure a weather and watertight seal.
The working area of the dome is quite spacious and the image below shows how it is all laid out. There is a workstation with foldaway shelf, whiteboard and pinboard for skymaps, notes, data, etc., and an adjustable height stool for use either at the workstation or scope. The laptop is plugged into my home network and internet via an ethernet socket. You can see the red-white changeover lighting at opposite sides of the observatory. The hatch fits snugly against the inside base wall so I devised a simple way of stowing it by hanging it on two S-hooks suspended from two of the dome support braces. It prevents the hatch being damaged, is easy to attach and remove, and doesn't present a tripping hazard when observing. A radio-controlled clock is tuned into the Greenwich signal and the internal temperature is also displayed. The handles to assist with rotating the dome can be seen on the dome base ring. In addition to the generous floor space you can see that there is also plenty of headroom.
The following image shows the foldaway worktop & shelving, thermostat for controlling internal temperature via 120W tube heater or 700W oil-filled heater (frost & moisture protection - 700W heater required for very cold periods), red-white changeover lighting (on-off-on switch at left of image), ethernet socket (slightly hidden by shelf & laptop), 240V 13A mains socket outlet. Not shown is a local alarm system which was amazingly easy to link into the house alarm system.
Summary
This has been a very successful and satisfying project. All my original objectives were met, including the cost (£598). The observatory has proved indespensible and has allowed me to work in comfort and also take the limited weather-window opportunities which present themselves in this part of the world.
This is not a project to enter without some serious consideration and planning but I have no hesitation in recommending it to the serious amateur enthusiast who wants to spend more time at the scope rather than looking forlornly at the bands of clouds approaching on the horizon.
More Information
More information, construction details, and images can be found on the author's website Eric's Astronomy Blog.
Clear Skies
Eric
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