Steel Frame Observatory Dome Page

Steel Frame and Bituwell Plate Observatory Dome

This webpage documents my first dome, which was an inexpensive and very simple to build dome construced from steel arches and roofing sheets. The dome is covered with Bituwell plates, which are bitumen impregnated sheets of inorganic-organic fibers. Since Bituwell plates are very strong, they could be attached directly to the dome support without a plywood underlay, reducing costs. The Bituwell plates cost approximately 25% as much as a dome covered with a plywood underlay and standard roofing asphalt sheets. I built this dome in 2008 and it survived 7 years of rain, gale and hurricane force winds, snow, and summers. This is a very lightweight dome that was very easy to construct at a low cost, so it fit the budget astronomer.

 

Dome Ring

The dome rests on the dome ring. The dome ring is 3.31 m outer diameter, 10 cm wide, and constructed from overlapping 100-128 cm long arcs of 18 mm thick plywood. Each dome ring layer requires 8.12 arcs (128 cm) and a standard 122 cm x 144 cm plywood sheet is enough for approximately 20 arcs. Most birdcage dome rings are constructed from three plywood layers. Because my dome is constructed of extremely lightweight materials, a two layer dome ring was sufficient. I fabricated a template from scrap 6 mm plywood to simplify the lay out process. I took the swing arm dowel, used for marking out the post locations, and nailed it to a board. I drilled a 7 mm diameter hole at positions 1.55 and 1.65 m from the nail. A pencil was inserted through the holes to mark out the arcs. I orientated the swing arm over the center line and drew both arcs (below left photos). The template and all plywood arcs were cut out with a jig saw (below right photos). 

  
The dome ring was too large to assemble in my workshop, so I test fit it on top of the observatory base structure. It was next disassembled,  all surfaces were water treated, and reassembled with lots of galvanized counter sunk 30 mm wood screws and glue. 

               

Dome Frame

The dome diameter is 3.3 m and slightly less than a perfect half spherical dome; this resulted in a 2.4 m high structure with only minor deviations from a perfect half spherical dome. The birdcage dome design usually uses a total of 10 peripheral arches, with 5 peripheral arches equally spaced between each of the two central arches. Viewed from above, each peripheral arch starts on the dome ring and extends toward the dome ring center. The central arches are actually chords across the dome ring, and are less than the dome ring diameter.

The finished observatory dome frame on top of the base structure (below).  This dome support construction is extremely light. Had it not been for the size, I could have lifted the dome ring and dome structure without help. This structure is very much like an aluminum frame greenhouse; the metal support can flex back and forth, but becomes a very stable structure when it is locked together with a stiff sheeting material.


 


Dome Exterior

The dome is covered with Bituwell plates, which are bitumen impregnated sheets of inorganic-organic fibers. Bituwell plates are very strong yet lightweight (5.4 kg per 0.93 m x 2 m x 3 mm sheet).  Since Bituwell plates are very strong, they could be attached directly to the dome support without a plywood underlay, reducing costs. The Bituwell plates cost approximately 25% as much as a dome covered with a plywood underlay and standard roofing asphalt sheets. The Bituwell plates are corrugated, and this makes them very rigid. I found that the corrugations could be pressed down by setting the Bituwell plates in direct sunlight for several days and then walking on them.  Once the corrugations were reduced, the Bituwell plates were easily cut with a utility knife and attached with self tapping screws and rubberized metal washers. The most time consuming part is getting the seams to meet and look good. With the Bituwell plates attached, two persons can easily lift the dome-its very light but solid.
     

Dome Slit Cover

The dome slit cover fits inside the dome slit and slides up and over the top of the dome.  I placed small wheels on the bottom of the dome slit cover frame, that ride inside the corrugations in the Bituwell plates. The dome slit cover is 90 cm wide, giving 5 cm of clearance on each side (when centered in the 1 m wide dome slit opening).  I wanted the option for natural lighting without having to open the dome slit cover; I decided to use clear plastic for the dome slit cover. I installed wheels on the bottom of the dome slit cover and it is quite easy to slide it up and over the top of the dome. The wheels just ride in the Bituwell plate corrugations. The dome slit cover slides up over the top of the dome and hooks onto the top of the dome slit opening.  

Dome Status 

Since this dome was an experimental design, it was unknown how the Bituwell plates would hold up to the local weather. As of spring 2015, the dome was seven years old and still weather tight without any significant problems; it has survived seven years of pounding rain, gale and hurricane force winds, ice, snow, and summers. All in all, the dome design has proved itself and I am very pleased with the results.  I replaced this dome during 2015 with a more solid designed dome (see the Bent Wood Laminate Dome webpage). Below are several photos during dome disassembly. Note that this was a strong dome construction, but it broke down into very little material.

 
                                                                                                                                              
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