Observatory Base Structure Page

Base Structure Design

The base structure frame is a 10 sided regular polygon, 1 meter high with a 1.62 meter circumradius. The below left photo shows the first base structure during 2008 constructionThe 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.  The dome is 3.36 meter in diameter and the base structure is slightly less in diameter, allowing 5-6 cm of dome overhang for better rain run off. I originally bent 6 mm plywood around the 10 sided frame, giving a pretty good approximation to a circular base structure. The plywood exterior was treated every year with an exterior oil based water sealant and lasted 8 years. During 2016 I rebuilt the base structure; I upgraded to stronger structural elements to support the heavier bent laminate dome and added vinyl coated metal siding plates (below right photo).  


Base Structure Construction

The top horizontal cross supports (3.8 cm x 5.7 cm x 1 m) form a 72 degree angle with the radius. I set my miter saw at 18 degrees and cut all cross braces and exterior floor joists.  The 10 upright posts (4.5 cm x 9.5 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 sits on the 10 radial floor joists and the 10 perimeter floor joists (below left). I installed two additional concentric rings of floor joists, each set 40 cm on center. The below left and right diagrams show the base structure floor joist plan and side plan, respectively. The base structure is connected with M8 exterior bolts to 4 pressure treated posts, each anchored in concrete and located around the outer perimeter. If I ever want to move the structure, it can then be easily disconnected from the posts.

During 2015 I added a heavier dome, so I rebuilt the base structure with strengthened upright posts (below photos). I added 5 additional 4.5 cm x 9.5 cm x 1 m uprights and inward cross braces to the existing upright posts to support the heavier dome construction (see also the Centering Bearings section- last photo on this webpage). During 2017 I added an additional 5 strengthened uprights and cross braces, so all 10 uprights were strengthened.


Most home observatories have an additional plywood base ring above the top braces to increase stability. I fabricated a 3 layer thick 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.                                                                                                            

Observatory Floor 

The floor required 80 linear meters of 25 mm x 12 cm pine boards and cost about 50% less than 8 m2 of plywood. 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, which gave satisfactory support to the 25 mm pine.  There is 3-4 mm spacing between boards, to allow for expansion. The below left photo shows the original observatory floor during 2008 construction. All surfaces of the floor boards were treated with an exterior oil based water sealant and secured with 5 cm exterior screws. Using screws to secure the flooring makes it easy to remove the floor boards for routine floor joist inspection and water sealant treatment. During 2017 there were problems with burrowing animals mounding too much dirt under the floor boards, so I poured a concrete slab under the floor joists. The slab required about 12 wheelbarrows of concrete and the job only took a single day. I left a small gap between the concrete slab and floor joists for air circulation. The below right photo shows the concrete slab during curing.


Dome Bearings

My first dome was covered with Bituwell plates, which are bitumen impregnated sheets of inorganic-organic fibers (5.4 kg per 0.93 m x 2 m x 3 mm sheet). The first dome was very light and only required 10 dome bearing rollers, each rated for a 25 kg load (250 kg total). During 2015, I installed a heavier dome, and also added 10 larger industrial wheels capable of supporting a total load of 560 kg. The below left photo shows the original small wheels (left) that were later replaced with larger industrial wheels (right). The below right photo shows the new 10 dome bearings and 10 centering wheels.

Centering Bearings

The centering bearings serve two functions: they keep the dome centered on its axis of rotation and prevent the dome from falling off of the dome bearings. The centering bearings used on my first dome (Steel Frame Dome with Bituwell Plates) were constructed from ribbed brackets, small roller wheels, and left over floor joist scraps. The first dome weighed less than 100 kg, so very light weight bearings were adequate. The heavier Bent Wood Laminate Dome (added in 2015) required stronger centering bearings. The below photo shows the new centering bearings, constructed from 10 larger industrial wheels (max. 56 kg load per wheel). 

The new dome is considerably heavier than the first dome and rotation places greater inward force on the centering bearing posts. I decided to strengthen the centering bearing posts by adding 4.5 cm x 9.5 cm x 95 cm inclined supports. Since the observatory was constructed with extra floor area to accommodate future modifications, the added footprint from the inclined supports doesn't give any problems or major inconveniences. The below photo shows the inclined centering bearing supports. The inclined centering bearing supports attach to the 4.5 cm x 9.5 cm x 1 m uprights, which were added to the original posts to support the heavier dome construction; this gives 10 very strong dome and centering bearing supports, measuring 9 cm x 19 cm.