Although the designing of a domed observatory does require a knowledge of geometry, the construction I found is not much more difficult than some other designs. Equipment used consisted of simple tools, such as power and jig saws, reversible drills, hammers, levels, squares and tape measures. If you have and questions on the designing or the building of my observatory, please feel free to contact me for further information.
On October 1, 1991 after finalizing my plans and locating supplies, construction began. First, a piece of 1 x 26 steel plate which was to serve as the upper pier base was drilled with four 3/4 holes for the concrete anchor bolts. Since this plate weighs over 160 lb.. a wood pattern was cut and drilled to match. A back hoe was then hired and a 5x5 ft., 4 ft. deep hole was dug for the lower pier and a power line buried to the site. A 24 diameter sonotube was obtained to use as a form for the lower pier. Inside this was placed 40 ft. of reinforcing rebar. Four additional 10 by 4 ft. deep holes were dug and fitted with rebar loops to serve as building tie downs.
Two yards of concrete was then ordered, around 8000 lb.., the tie down holes and pier form were filled, with the remainder used to fill the 5 ft. hole around the form. The wooden base pattern with anchor bolts attached, was then set and concrete allowed to cure.
Ten pillars of concrete blocks and slabs were set to serve as floor joist supports. Upon this was constructed a subjoist system of paired 2x6s. Above this was built an 18 ft. wide twelve sided floor of 2x6 joists. This was leveled and decked with 3/4 plywood. Twelve sides allows easy 15 and 30 degree angles when building. Next a 28 high wall of 2x4s was built and the entire structure sided with T-111 siding.
The floor is 28 above ground level and the doorway is dug down to allow a 3 step drop to a 49 short, but comfortable doorway. The finished floor is 1 higher then the lower pier so that when the upper pier base was added it forms an even surface. A 1/2 gap was also left between the pier and floor to prevent transferring vibrations from the building to the telescope.
By mid- November the slowest and most tedious stage of construction began, the cutting of the dome rings and ribs. Additional lighting, heat lamps and a portable heater were added to the barn where the cutting was to take place over the next two winter months. As arc cutting table was designed and built. It consisted of a flat 8 ft. wide surface with edge stops to prevent the 3/4 plywood that was used in all the rib and ring frame work from sliding. As eleven foot long arc arm with holes drilled in one end at different radii was used to mark the curves.
For each different size arc, one piece was marked and carefully cut out. These were then used as a master patterns for the remaining arcs. It proved faster to trace the master then to individually mark each piece with the arc arm.
First the base ring was cut, it consists of two layers of plywood 19 ft. in diameter and is 10 wide. These sections of plywood were then placed on the building walls and carefully fitted to within 1/8 of a perfect circle. They were then fastened together and down to the building with drywall screws.
The eight main support wheels were then placed, holes were drilled through the base ring for them and set aside. These wheels are made of hard rubber are 5 in diameter and 2 wide. They have a 650 lb. load capacity and are equipped with grease fittings.
Next the 192 diameter 9 wide dome ring was cut. Two layers of these arcs were laid on top of the base ring, trued up and fastened together. A ring of 1/4 tempered masonite was then cut and fastened to the dome ring smooth side up. This masonite is to serve as the bearing surface for the main support wheels. Tempered masonite although flexible is very hard and resists compression. After years of use it has shown very little wear. The dome ring was then split into two pieces, turned over, rerounded to within 1/8 and fastened together. Finally two additional layers of plywood were added. The now five layer thick 350 lb. dome ring was carefully raised an inch at a time until the eight support wheels could be slipped under it. The wheels were then bolted into place.
Six lateral wheel supports were then built and their 5- 400 lb. capacity wheels placed. The laterals run on the inside surface of the dome ring and prevent the dome from falling off its support wheels. The dome ring was then turned and any needed wheel adjustments made.
A three layer thick slit assembly was constructed and hoisted into position
over a 12 ft. high scaffolding that had been built. The slit opening is
49 wide and passes over the vertical 2 6 to allow access to the zenith.
The slit was carefully centered, adjusted for height and temporarily attached
to the scaffolding to prevent movement.
Starting at the back and working forward on each side a total of 21- 4 wide two layer thick ribs were attached at 26 intervals. They are secured to the dome ring and slit frame with screws and joist hangers. Spacers were added 8 feet up the ribs so that the dome skin could be fastened securely when laid.
Four more arches each 3 layers thick and over 18 ft. long were made for the shutter doors. Two are 4 wide and the others have two layers 4 wide with the third layer is 8 wide to provide an overlap with the slit for a weather seal.
A total of 195 arcs of various sizes where cut resulting in three burned out jig saws and about 4 dozen dull or broken saw blades.
A 16 wide dome skirt was cut out of 1/4 tempered masonite which was fastened to the lower half of the dome ring. This skirt extends 4 below the base ring to provide a weather seal.
Up until now all of the work had been done by myself, my very understanding wife and ever helpful 2 year old daughter. However, it was now time to add the dome skin and I needed manpower. A request for help from my follow members of the Cheyenne Astronomical Society quickly produced some eager volunteers.
In late January, on a very cold weekend, my hardy crew gathered, we hauled the no longer needed arc table out to the dome to use as a work platform.
The spaces between the ribs, although looking like a triangle, is actually a section of a hemisphere and as a result has two curved sides. To mark these sections the 1/4 masonite which I used for the dome skin, was laid in place and marked along the ribs from the inside. Cutting these with a jig saw proved rather slow work and produced some sloppy cuts, however one of the crew suggested cutting it with the circular power saw. The saw proved capable of making the gradual curves and produced quick smooth cuts, this greatly accelerated the process. With one person on the saw, and three of us marking and securing the sections to the ribs, a total of nearly twelve hours was required over two days to finish the dome skin.
Two 10 ft. long shutter tracks were constructed out of 1 x 2 channel iron and bolted into place. Then Four shutter wheel assemblies were built with two 3 -150 lb. capacity wheels on each. Two assemblies, two shutter arches and four 2x 4 cross braces were combined to form each door frame. The frames were then covered with masonite. Off the side of each door a pair of track covers extends. These boxes enclose the tracks and prevent rain, snow and ice from accumulating.
With eight months of snow and freezing temperatures this is of major importance for keeping the observatory doors operational year around. The boxes also can stop the wind from lifting the shutter doors off and have allowed observing even in 30 mph winds. A cable opening system as described in Amateur Telescope Making vol.2 was added to aid in the opening these 130 lb. doors.
The 220 volt, 30 amp power line that had been laid, was fed into a breaker box and split into two circuits. One circuit serves the dome drive and lights, while the other handles the telescope clock drive and outlets for any future electronics. This prevents any voltage drop that might occur when the dome drive is running from affecting the telescope tracking rate. The dome is turned by a 3/4 h.p. reversible motor run through a pulley reduction and a 1 to 40 gear box. This feeds the dome drive wheel which is a 10 go-cart wheel turning on the inside edge of the dome ring. The finished 1700 lb. dome can make a complete revolution in about one minute.
In early March temperatures warmed, the seams between the dome skin sections were sealed with acrylic latex caulk. The dome was primed and painted white with exterior latex. Gray paint was applied to the floor with the remainder of the interior painted black.
Next my telescopes pier was welded to the base plate, eased into position on the lower pier and bolted down, a perfect fit. My telescope was quickly reassembled and on March 25, 1992 we had first light . The observatory was now operational and has been used on nearly every clear night since.
Final weather sealing consisted of a 3 masonite over lap and two foam seals between the shutter doors. A 4 strip of rubber was also stapled to the inside edge of the shutter slit overlap and seals against the dome surface. A large clamp keeps the doors held tightly shut in bad weather. Last a 4 strip of rubber was stapled under the outer edge of the base ring and was bent down to ride against the inside of the dome skirt.
This arrangement has proven to be very effective against blowing snow, yet rain leaks began to appear. Close examination revealed tiny cracks appearing between the caulk and masonite after the observatory had been used on cold nights. Apparently there is enough flex in the dome to cause the caulk to pull loose when it stiffened in the cold weather. Recaulking and repainting the seams slowed, but did not halt the problem.
There is a product on the market know as elastomeric roof coating or rubber paint. Since it is available in white and white seam tape for it is also available, I decided to give it a try. First a coat of roof coating was painted onto the seams, then the seam tape was laid atop the seams and smoothed out. Another coat was then applied to seal the tape down. After drying, two more coats were applied onto the seams and the entire dome covered with rubber paint. This has proved to be very effective, because the interior has remained dry ever since.
Although building the observatory required a considerable investment in time. the end result has more the lived up to my expectations. With a permanent site sheltered from the elements, full of charts and catalogs that can be readied for use in just a few minutes, my observing time has more then tripled. The large size of the dome permits room for guests and expansion for larger equipment.
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