Telescope Pier and Observatory Base Structure
Observatory Design Pier/Base Structure Dome Observatory Photos
Base Structure Layout
The base structure is a 10 sided regular polygon (decagon), 1 meter high. The dome will be 3.3 meter in diameter and the base structure will be slightly less in diameter, allowing dome overhang for better rain run off. A 10 sided regular polygon with 1 meter sides has a 1.62 meter circumradius (Regular Polygon Calculator). This gives 3-4 cm of dome overhang per side and also simplifies construction, since most European exterior sheet materials come in 1x2 meter sections. The below drawing is a prototype drawing, showing how the finished observatory will look.
The observatory will be located in the back corner of my yard. I fabricated a simple swing arm to simplify staking out the footprint. I took a 3 meter wood dowel and drilled a hole 1.62 meters from the end. I nailed this to a wood stake. The wood stake is driven into the ground in the center of the observatory. Staking out the footprint then requires only about 10 min. The idea is to place a stake at the end of the swing arm, then rotate the swing arm and measure out a 1 meter chord. The intersection of the chord and swing arm gives the next stake location. The below figure shows the layout procedure. The stone ring marks an unused concrete lined pond; the telescope pier will be integrated into this concrete structure to give greater stability. The base structure is orientated so that all the framing contacts the ground outside of the concrete pond, so vibrations won't be transferred to the telescope pier.
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Telescope Pier
The above photo shows the outline of a concrete lined goldfish pond, which I had previously filled in with dirt. I decided to remove the dirt and cement the telescope pier into and through this concrete structure. I also filled in the structure with as many large stones as I could find, to increase its stability. The below photo shows the concrete pond before pouring the pier. I chiseled a hole through the concrete. Several 1 meter threaded M12 rods were inserted through the left hole, and concrete was poured into the large right hole and into the bottom of the concrete pond. This locked the pier into both sides of the concrete pond liner, creating a very stabile pier foundation.
I built a simple jig to hold the threaded M12 rods parallel, inserted them into and through the hole in the concrete pond, and leveled the jig (below left photo). I poured the concrete base and then placed a 30 cm x 39 cm form over the rods to form the pier top. The form was just a plastic storage container, which was very smooth plastic that easily slipped free of the dried concrete. The plastic form was filled with concrete and allowed to dry (below right photo).
After sitting for 1 day at 30 c, the plastic form was removed (below photos). I will cover the pier with a thin layer of concrete to fill in any voids.
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Base Structure Construction
The base structure is a 10 sided regular polygon with 1 meter sides; this will give a pretty good approximation of a circle when it's clad with exterior siding. The base structure is built around the telescope pier, without direct contact between the pier and observatory. The basic design was modeled after a photo on Clement's Observatory Website.I filled in the space around the concrete pier with sand and encircled the pier with a ring of masonry blocks (below photo). The floor joists will rest on the masonry blocks and the sand will hopefully help to isolate the pier from any vibrations.
After leveling the masonary ring, I dug holes where the 10 polygon sides intersect (previously staked out). These holes were filled with sand and a masonry block was added to support the framing. I dry fit the floor joists and adjusted all masonry blocks to level the joists (below photo). Additional framing photos can be found on the Observatory Photos web page.
Framing in progress (below photo). The top horizontal cross supports (3.8 cm x 5.7 cm x 1 m) form a 72 degree angle with the radius (calculated using the Regular Polygon Calculator). I set my miter saw at 18 degrees and cut all cross braces and exterior floor joists. The 10 upright posts (7.2 cm x 3.6 cm x 1 m) and all floor joists (4.5 cm x 9.5 cm) rest on the masonry blocks. The cross bracing between the 10 upright posts and the perimeter floor joists are 3.8 cm x 5.7 cm x 55 cm, with a 45 degree bottom miter and a 45 degree x 18 degree compound top miter.
The floor will sit on the 10 radial floor joists and the 10 perimeter floor joists (shown in the above photo). I installed two additional concentric rings of floor joists, each set 40 cm on center. The following top and bottom drawings show the base structure floor joist plan and side plan, respectively. Back To Top
The base structure was connected to 4 pressure treated posts, each anchored in concrete and located around the outer perimeter (below photo). If I ever want to move the structure, it can then be easily disconnected from the posts without removing the flooring. I replaced the temporary screws with M8 exterior bolts when I sided the base structure.
Most home observatories have an additional plywood base ring above the top braces to increase stability. I will fabricate a plywood ring to support the dome (dome ring), but I wanted to eliminate using an additional plywood base ring on top of the top braces. My solution was to strengthen the top braces by adding corner supports at all 10 uprights. The corner supports are just scraps of 3.8 cm x 5.7 cm wood, cut to a 144 deg. angle, that fit into the internal angles where the top braces meet (below photo). This greatly strengthened the top braces and eliminated the need for an additional plywood base ring.
Floor
The original plan was to use heavy plywood for the flooring, but this would cost too much; a quick calculation showed that 8 m2 of 20 mm thick plywood would cost in excess of $200. This clashed with my design theme of keeping everything simple and inexpensive. I determined that 80 linear meters of 25 mm x 12 cm pine boards would cost about 50% as much as 8 m2 of plywood. I purchased 9 linear meters of 25 mm x 10 cm pine boards for a test fabrication, as shown in the below photo.
25 mm pine was thick enough for most of the flooring, with the exception of the three longest boards at the outer perimeter. I installed 10 additional cross joists perpendicular to the outer perimeter (marked with arrows in the above photo), which gave satisfactory support to the 25 mm pine. There is 3-4 mm spacing between boards, to allow for expansion. The below photo shows the finished observatory floor. The floor boards were treated with an exterior oil based water sealant and secured with 55 mm galvanized nails.
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Exterior Siding
The exterior siding is 6 mm thick plywood (122 cm x 244 cm sheeting). I treated the plywood with an oil based water sealant and let it dry overnight. The 6 mm plywood easily bends around the 10 sided frame, giving a pretty good approximation to a circular base structure. The plywood is fastened with galvanized screws .
I added a rain skirt above the door and around the door lock to keep the door weathertight (below photo).
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