Observatory Base Structure and Pier Page

## 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.

## Telescope Pier Foundation

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.

## Telescope Pier

The telescope pier foundation was originally built as a low platform to accommodate my homebuilt 10" GoTo Newtonian telescope. I later purchased a Celestron C8 and retrofitted a taller pier; this pier was a gift from my neighbor and constructed from 6" diameter steel pipe welded to  steel top and base plates (below photo)

Because the pier foundation was not designed to accommodate a tall metal pier, there were stability problems. The pier acted like a long pendulum and vibrated too much. I filled the pier with sand and this partially dampened the vibrations, but not to an acceptable level. I was planning to weld triangular struts between the base plate and tube, but eventually realized there was another problem: the base plate had bent.

I encased the pier bottom in a large concrete block with molded holes to accept the pier foundation bolts. This firmly locked the metal pier to the concrete pier foundation and strengthened the pier bottom against bending. The concrete mold was a Styrofoam cooler (below photo). I used wood dowels to form the channels for the pier foundation screws and coated the Styrofoam and wood dowels with cooking oil (raps oil) as a mold release agent.

The mold and metal pier were leveled, the Styrofoam mold was filled with concrete,  and a few sections of rebar were placed around the metal pier. I used an old paint bucket to mold the top cylindrical concrete section. The paint bucket was also coated with raps oil and the rebar extended up into the cylindrical concrete section (below photo). I used a slightly dryer concrete mix for the cylindrical section to reduce leaking between the paint bucket and rectangular concrete sections.

The concrete was left to set overnight, removed from the mold, and the oil coated wood dowels were easily hammered out of the concrete.  Below are two photos of the finished pier in my observatory. The concrete pier base has significantly strengthened the pier and greatly reduced vibrations.

## 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.

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 mof 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.

## 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).

## Base Structure Wind Skirt

The above photo shows that there is a small gap between the observatory bottom and the ground, allowing wind to blow under the observatory and up around the telescope pier. I added a 15 cm wide polyethylene wind barrier to the base structure (below photo). The polyethylene film is tacked to the exterior siding and secured at the top with a strip of plastic floor molding and silicone caulking.

## Stone Rain Ring

After installation of the base structure wind skirt, I  added a ring of masonry block (below photo). The masonry block serves several functions: securing the wind barrier, making it more difficult for rodents to tear through the polyethylene wind barrier, and to absorb the impact from rain running off the dome overhang. All masonry blocks were angled to channel rain run off away from the observatory.

## Dome Bearings

The dome ring will sit on 10 dome bearings that can be independently adjusted (raised or lowered). I purchased a single roller and made a prototype (below photo). I drilled out the mounting bracket holes to 6 mm and connected two M6 threaded rods. The M6 bolts and washers allow the roller to be raised or lowered relative to the base structure. This roller is rated for a 20 kg load, giving a maximum limit of 200 kg for the dome.

I eventually found a larger wheel that had a M6 hole and was rated for 25 kg. It is important that the dome bearings are placed tangent to the base structure and dome ring. Because I had installed corner supports, this simplified the layout procedure. Since each corner support is fitted to be tangent to the base structure, I could use this to aline the dome bearings. I positioned the dome bearings where I wanted them and used a carpenter's square to aline the wheel bracket tangent to the base structure and parallel to the corner support (below left photo). The below right photo shows an installed dome bearing. There are M6 washers and bolts below and above the wheel bracket. This allows each wheel to be independently raised or lowered to level the dome ring. An additional photo of the dome ring sitting on a dome bearing can be found on the  Observatory Photos web page

## Centering Bearings

The centering bearings serve two functions: they keep the dome centered on its axis of rotation and prevent the dome from lifting off of the dome bearings. The prototype centering bearings were constructed from steel plates and brackets (below photo). The idea was for the steel plates to flex as the dome rotated, however they were too weak and bent inwards; otherwise the basic bearing design worked fine.

The final design (below photo) just replaced the steel plate with left over floor joist scraps (4.5 cm x 9.5 cm).

The below photo shows the final centering bearing; note the hole that was cut in the arch support to create a lip and remove obstructions to the top bracket with the small roller.

All original graphics, photographs, content, and writings are copywrited © 2007 by the author and all rights are reserved. Do not copy or reproduce in any form without prior written consent.