observatorydome

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. The template and all plywood arcs were cut out with a jig saw.

The finished template:

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. A photo of the dome ring construction can be found on the Observatory Photos web page.

Dome Frame Material

The dome frame supports the sheet materials that clad the outer dome surface. Most traditional birdcage type designs use plywood rings, identical to those used for the dome ring (3 layers and 2 layers thick for the central and peripheral arches, respectively). The high costs of plywood in Scandinavia made this type construction too expensive for my project; I also wanted a faster, simpler construction method opposed to spending days cutting plywood arches. I required a light weight material, flexible enough to bend, but rigid enough to support the exterior dome sheeting. It was also mandatory that the material was inexpensive, easy to work with, and locally available. I selected steel drywall framing supports.

Steel drywall framing supports are 3 sided bars of 1 mm sheet steel, used in the construction of non-load bearing interior walls. These supports often contain pressed channels and/or folded lips that can greatly increase their strength without adding significant weight. It is the 3 sided geometry and the pressed structures that add strength.

This material is difficult to bend without the steel sides puckering and reducing the structural integrity. I developed a novel method to bend these supports and retain the structural integrity, but I will not post details until the observatory is complete and I have verified the structural integrity. Unfortunately this is necessary to prevent others from copying an untested dome design. If my dome survives the local weather conditions (regular gale and hurricane strength winds), I will consider posting arch fabrication details.

Central Arches

I initially planned on building a 3.5 m diameter dome, but this would give a structure that was too high to build without a building permit (local limit is 2.5 m high). I reduced the dome diameter to 3.3 m and also decided on building 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.

I bolted two 2.7 m long drywall framing supports together to form each central arch. I cut several 1 m wide spacers to ensure a proper dome slit opening width and clamped the arches to the dome ring. I scribed the arch curvature onto a piece of 18 mm plywood and cut 8 identical arch support sections. Each of the 4 central arch supports is made from two 18 mm plywood sections that are screwed and glued together (below photo). The central arches slide into the arch supports and are screwed in place. I also fabricated 10 peripheral arch supports (below right photo).

Peripheral Arches

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 arche 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. As with the central arces, I bent a peripheral arch, clamped it onto the dome ring, scribed and fabricated 10 peripheral arch supports, and positioned 5 supports equally between each central arch.

Fabricating each peripheral arch only required about 15 min, which was much faster than if I had constructed the arches from 10 double layer plywood rings. Each peripheral arch was clamped into an arch support, and then cut to fit (below photo).

Connecting the peripheral arches to the central arches was very simple. A tab was cut on each peripheral arch and it was bolted into the central arch with a M6 bolt (below photo). When peripheral and central arches intersected at other than right angles, the sides were simply bent to fit.

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 Bituwell plates were easily cut with a utility knife and attached with self tapping screws and rubberized metal washers.

The below photo shows a portion of the finished dome (an additional photo of fitting the Bituwell plates can be found on the Observatory Photos web page). The most time consuming part is getting the seams to meet and look good. Where two Bituwell plates meet, the corrugations seldom line up. I will find a wide exterior flashing to cover the seams and fill in any gaps with foam insulation or silicon to get a good tight fit.

The dome isn't completely finished, but I could not resist placing it on the frame (below photo). I still need to do some trimming along the central arches and get the Bituwell plate tops smoothed and fastened down, but its starting to take shape. Even with the Bituwell plates attached, two persons can easily lift the dome-its very light but solid.

Dome Slit Cover

The dome slit cover will fit inside the dome slit and slide up and over the top of the dome. I am planning on placing small wheels on the bottom of the dome slit cover frame, that will ride inside the corrugations in the Bituwell plates. I scribed the dome curvature onto a scrap of plywood and cut 8 arches (10 cm wide) from 6 mm plywood. Each door frame side is two arches long and two plywood layers thick (12 mm total). I cut 4 cross pieces from scraps of steel drywall frame (below photo). 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).

Since I spend lots of rainy weekend days working on my telescope, 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. This will give lots of natural light for daytime work and I will fabricate a removable inner cover that can be installed to block sunlight. The below left photo showes the dome slit cover. The clear plastic is 90 cm wide, just covering the door frame. I used scraps of Bituwell plates for the side and top skirts. The below right photo shows the top skirt. This will be bent to arc backwards over the top of the dome slit opening.

The below left and right photos show the dome slit cover frame test fit in the dome slit opening and the nearly finished dome, respectively.

I had intended to cover the Bituwell plate seams with flashing, but after some trimming, I found this unnecessary. The below photo shows the finished dome.

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 (an additional photo of the slit cover and wheels can be found on the Observatory Photos web page). The wheels just ride in the Bituwell plate corrugations. I will fabricate a hook mechanism to lock the opened/closed cover in place to prevent wind damage.

Dome Rain Skirts

The dome rain skirts cover the opening between the base structure and the dome ring, sealing the observatory from wind and rain. The prototype skirts used 8 cm wide strips of scrap Bituwell plates, installed underneath the dome covering (below left photo). These simple rain skirts gave problems, they touched the siding and caused dome rotation problems. This arose because the original plan was for a faceted, 10 sided structure. When I sided the structure, I found that the 6 mm plywood could be bent to a circular structure; this looked better but decreased the dome overhang. The solution was to install 10 cm wide rain skirts (6 mm plywood) spaced 2 cm from the dome ring (below right photo).

The dome rain skirts attach to 2 cm thick spacers with 5 cm long exterior screws (below left photo). The below right photo shows the rain skirt, that takes run off from the dome over the rain skirt. I wedged a strip of Bituwell plate between the wood rain skirt and the corrugated dome exterior, and then sealed it all with silicone. The corrugated voids were filled in with a lot of silicon; it took 4-5 tubes to seal the entire rain skirt. All silicone surfaces were angled to prevent rain water from pooling. The clear silicone appears white in the below photo, because it isn't totally cured. I later replaced the clear silicone with black sanitary silicone, containg a fungacide.

Slit Cover Latching Hooks

The dome slit cover slides up over the top of the dome. It was necessary to fabricate a latching mechanism to keep the opened dome slit cover from rolling completely off of the dome. I fabricated two hooks (below left photo) from aluminum square bar and several old kitchen cabinet handles. These hooks attach to the bottom of the dome slit cover and hook onto the top of the dome slit opening, causing the dome slit cover to hang off the back of the observatory dome. Unfortunately the aluminum was too weak and the hooks failed (below right photo).

I fabricated a new set of hooks from M6 threaded rod and small scraps of oak square stock (below left photo). The below right photo shows the hooks installed on the inside of the dome slit cover.

The below left photo shows how the hooks latch onto the top of the dome slit opening to stop the dome slit cover from falling off. I added a steel angle iron on each side to prevent the wheels and hooks from damaging the dome exterior. The below right photo shows how the opened dome slit cover hangs off the back of the dome. The dome still rotates easily with the dome slit cover open.

Slit Cover Locking Screws

It was necessary to fabricate a locking mechanism to keep the closed dome slit cover from being blown off of the observatory dome (my location regularly experiences gale and hurricane force winds in the late winter/early spring). I took the simplest approach (KIS) and cut a M8 threaded rod into 4 sections. I installed threaded M8 sleves in wood blocks that are screwed into the dome slit cover sides. The basic idea is to just have a large screw (M8 threaded rod) that screws through the side of the dome slit cover and into the central arches or central arch supports. The dome locking screws on the dome slit cover bottom and top are shown below (left and right, respectively).

Dome Status and Maintenance

Since this dome was an experimental design, it was unknown how the Bituwell plates would hold up to the local weather. As of fall 2011, the dome was three years old and still weather tight without any significant problems; it has survived lots of pounding rain, gale and hurricane force winds, ice, snow, and three summers. All in all, the dome design has proved itself and I am very pleased with the results. Recently I noticed that the Bituwell plates are appearing dry, as if some of the bitumen impregnation has weathered away. In order to keep the dome as weather tight as possible, I decided to treat the dome exterior with roofing asphalt. Below are three photos of the dome: before (left), during asphalting (middle), and the treated dome (right):