A cardboard mock-up of the DIY, backyard observatory was made in my garage. An old Meade ETX 90 was used to simulate some of the design elements. The width of the garage doors determined the final observatory outside diameter of 8' 8". The observatory is not large but I wanted to be able to build the silo and dome in the comfort of the garage.
After carefully choosing the observatory site, a piece of metal conduit was driven plumb into the ground and a sleeve attached to the conduit at the final height of the slab. A level was attached to a board which rotates around the conduit. Stakes were driven into the ground so the tops of the stakes were level and equal-distant from the center.
A piece of house siding was bent to fit the inside of the stakes. The siding was persuaded into position with clamps and fastened with screws. To keep water and snow out of the observatory, the slab is 4 inches above grade at the top of the sloping lawn, and about 12" above grade as the lawn slopes away from the slab. The structure was below the square footage requiring a building permit in our jurisdiction.
After the hole was excavated for the pier foundation, a 12" diameter, one foot long metal sleeve was inserted into the hole and leveled with the top of the slab. The pier hole expands towards the bottom and holds about 3/4 tons of concrete. The bottom of the pier foundation is more than 48" below grade, which is deeper than the frost line in Iowa.
Welding the 1/2" reinforcing rod cage that will go from the bottom of the pier foundation through the upper pier.
After inserting the re-rod cage into the bottom of the pier foundation hole, the excavation was filled with concrete. The re-rod cage was centered in the pier hole. Note a plumb bob (some bolts on a string) to make sure the re-rod cage was plumb.
A 12" diameter form was plumbed over the pier foundation and filled with concrete. The total weight of the pier foundation and pier itself is just short of one ton. Vibrations are reduced as mass increases because oscillations dampen through greater internal friction as a function of mass. The height of the pier above the finished slab was chosen to fit an 8" Schmidt-Cassegrain on a Celestron CGEM II mount.
Into the top of the pier wet concrete, a set of three 1/2" bolts, mounted to a temporary form, was inserted. The parts of the bolts inserted into the wet concrete were bent to assure they wouldn't turn. About 3" of each bolt was left exposed on the top side of the temporary form. I should have used stainless steel bolts, but so far, the regular bolts have remained rust-free. The temporary wood disk was removed after the concrete cured.
Before the observatory slab was poured, the pier was wrapped in 1/2" thick rubber to help isolate any vibrations from the slab going into the pier. The slight lip also helped with the screeding of the slab concrete.
PVC electrical conduit was laid in so that wiring from the observatory walls to the telescope pier would be below the slab and not be a tripping hazard in the dark. Wire reinforcing mesh was also added before the concrete pour.
Next, the observatory slab is poured. In addition to the two PVC electrical conduits on the telescope pier, two additional conduits were trenched in to provide 120 volt electrical service and a hard-wired ethernet computer connection.
The observatory silo began by creating a template for the upper and lower rings of the silo. Using a trammel attached to a piece of plywood, a router created a smooth inner and outer edge of the ring template. Each ring is 3.5" across to accommodate the 2 x 4 silo wall studs.
The master template was used to trace more rings on a 4 x 8 sheet of 3/4" treated plywood. The rings were rough-cut with a jig saw, however, the jigsaw left an unacceptably rough edge.
The master template was temporarily attached to each jigsaw-cut ring section. The rough-cut ring was then smoothed out using a router with a bit designed to follow the master template. By this method, each ring section received a very smooth inner and outer surface.
The partial ring sections were glued together to form the lower ring of the observatory silo. Two 3/4"thick sections created a 1.5" thick ring. The individual sections were glued together with waterproof construction adhesive so that none of the joints overlapped. A second ring was created for the top of the observatory silo. The lower rings were made of pressure-treated plywood, since they are in contact with concrete.
The upper and lower silo rings are connected together with 2 x 4 wall studs. The 2 x 4's are spaced such that the corrugated metal wrap will be fastened to the 2 x 4's at the joints of the individual corrugated metal sheets. A wider 24" space was created to accommodate the silo door.
During assembly of the silo, all of the construction was kept in alignment utilizing a simple water level. The silo was easily kept level within 1/16" utilizing the water level.
The door was built from 2 x 6 stock and two 1/2" thick inside panels. The size is 24 x 55, which is about the minimum opening for a person wearing a winter parka.
The upper and lower silo rings are screwed and glued to the 2 x 4 wall studs. The walls were reinforced in the middle of the wall studs with additional 2 x 4's.
Before moving the silo onto the concrete pad outdoors, the base ring of the observatory dome was constructed on top of the silo. The silo was completely level, as assured by the water level. The dome base ring was constructed of 1/2" plywood rings with internal wood blocking at regular intervals. None of the joints in the plywood semicircles overlapped. The result was a very rigid and light-weight dome ring. The dome ring is larger in diameter than the silo to accommodate the dome weather down-skirt.
During construction of the dome base ring, a simple trammel, pivoting from the center of the silo, kept the base rig of the dome circular to within about 1/8". The dome ring was not connected to the silo at this time; the silo was simply used as a level platform on which to construct the dome ring.
The dome ring was skinned with 1/4" plywood on the outside and 1/8" hardboard on the inside. The skins were attached with waterproof wood glue and nails. The finished ring is very lightweight and extremely rigid. The ring resists twisting, warping, bending and bucking. The design is that of a box beam where the flanges are the upper and lower plywood rings, the webs are the inside/outside skins and the stiffeners are the internal pine boards.
The silo was placed over the slab. Before the silo was attached to the slab with anchor bolts, the silo was leveled with thin shims so that the top ring (on which the dome wheels will be attached) was level to within about 1/16" (notice the bubble level spanning from the silo top ring to a level board above the pier.)
The silo was skinned with corrugated metal and attached with washer-screws at each joint of the metal pieces as well as several places between the metal joints. The door was installed. Note that the metal siding is higher than the wood frame. This forms the up-skirt. The silo up-skirt together with the dome down-skirt will keep out rain and snow.
The lower edge of the corrugated sheet metal was caulked to the concrete slab with Vulkem 116 commercial polyurethane sealant. The structure was added to the home-owner insurance policy.
The doorway was fairly complicated. Note the aluminum threshold with a top rubber gasket resting on a treated plywood block, weather stripping around the door frame and "F" channel metal to transition from the door opening to the corrugated metal siding. On the inside of the door, at the bottom, is a rubber door sweep.
The PVC electrical conduits were positioned on the concrete slab so that they would enter through the center of the silo sill plate. A pattern of the conduits was made on a piece of cardboard and the holes drilled in the sill plate before the silo was lowered onto the slab. A GFCI circuit protector feeds the observatory at the breaker box. Note one of the many 1/2" anchor bolts to attach the sill plate to the slab and the hurricane ties to connect the sill plate to the wall studs. The observatory has no insulation so it can quickly reach temperature equilibrium with the outdoor air. The silo has very low thermal mass because of the thin wall construction. The concrete floor will be covered with rubber matting to help isolate that thermal mass (and help protect any dropped objects from breaking).
The finished dome base ring is surprisingly rigid and lightweight.
Scale drawing of DIY backyard observatory.
Cross-section of DIY backyard observatory.
Arcs for the dome were assembled on the flat surface of the dome ring, since the dome ring was flat and level. Wax paper was used under the glue joints of the arc sections so that the arcs wouldn't stick to the dome ring. Pine boards were used for the arcs rather than plywood to provide a more rigid assembly.
Individual arc sections were assembled with simple spline joints. Spline joints are very strong and allow several short arc sections to be joined into full arcs. Biscuit joints would also be very strong but spline joints are pretty easy to make with a table saw.
The two shutter ribs are attached to the dome ring. The shutter ribs extend about 2" beyond the dome ring. All other ribs are flush with the dome ring.
Some things learned so far:
1. The summer heat was quite oppressive at times. We erected a tent canopy over the observatory construction to keep out the direct sunshine but the tent quickly became a favorite home for wasps. Eventually, the tent was broken twice by high winds and we had to abandon that idea.
2. Our lawn was dry and rock-hard for most of the summer, so we decided to have the concrete truck drive to the site and pour the observatory slab. The day before the concrete was to arrive, we had a downpour and the concrete truck left some deep divots in the lawn. We filled the divots with soil and seed but this, plus a couple weeks of watering, took a lot of extra time.
3. The house siding used to form the slab form was just barely able to make the proper diameter without cracking. Additional stakes were driven next to the circular form to keep the siding from cracking. The additional stakes were a blessing, because the thick side of the slab needed extra reinforcement to prevent a concrete blow-out.
4. We began the electrical conduit trenching by hand, but quickly discovered that digging in clay 24" deep and thirty feet long was not going to work. A gas-powered trencher saved the day and fortunately a neighbor also needed some trenching.
5. The work needed between various steps of this project was significant. Clean up of tools and materials at the end of the day, took much more time than expected. Even though we tried to plan each step carefully, many extra trips to the hardware store were needed.
6. My wife and I have a pretty solid knowledge in math and geometry but I would not advise starting a project like this without it (or a close friend who can readily help).
7. My wife was hugely supportive in this project. She was encouraging and physically helpful at many points in the construction. She often had excellent ideas when I did not. She also readily supported cutting down some trees on our acreage to allow for better telescope viewing.
8. Special thanks to the Des Moines Astronomical Society and particularly Norm Van Klompenberg who provided his time, inspiration and advice in the building of this backyard observatory.
8. Clamps: I regularly used up to 14 clamps. You can never have enough!
9. Our home-owner's insurance requires notification within 90 days of erecting a new structure. They didn't have experience with observatory insurance, but we quickly came to an agreement. The additional insurance was less than $20/year.
10. Although we didn't need a building permit for a structure under 200 square feet in our county, your city or county may have different requirements (or even neighborhood covenants). We've tried to meet all the building codes, regardless of the need to do that.
(more photos will be posted as the construction continues)
Cut-away view of the silo, wheels and dome ring.
One quarter inch per square.