The dome rests on the dome
ring. The dome
ring is 3.31 m outer diameter, 10 cm wide, and
constructed
from overlapping 100-128 cm long arcs of 18 mm thick plywood.
Each
dome ring
layer requires 8.12 arcs (128 cm) and a standard 122 cm x 144
cm
plywood sheet is enough for approximately 20 arcs. Most birdcage dome rings are
constructed from three plywood layers. Because my dome is
constructed of extremely lightweight materials, a two layer dome ring was
sufficient. I
fabricated a template from scrap 6 mm plywood to simplify the
lay out process. I took the swing arm dowel, used for marking
out
the post locations, and nailed it to a board. I drilled a 7 mm diameter
hole at positions 1.55 and 1.65 m from the nail. A pencil was
inserted through the holes to mark out the arcs. I orientated the swing
arm over the center line and drew both arcs. The template and all
plywood arcs were cut out
with a jig saw.
The finished template:
The dome ring
was too large to assemble in my workshop,
so I
test fit it on top of the observatory base structure.
It was next disassembled, all surfaces were water treated,
and
reassembled with lots of galvanized counter sunk 30 mm wood screws
and glue. A photo of the dome ring
construction can be found on the Observatory
Photos web page.
The dome frame supports the sheet materials that clad the outer dome
surface. Most traditional birdcage type
designs use plywood rings,
identical to those used for the dome
ring (3 layers and
2 layers thick for the central
and peripheral
arches, respectively).
The high costs of plywood in Scandinavia made this type
construction too expensive for my project; I also wanted a faster,
simpler construction method opposed to spending days cutting plywood
arches. I required a light weight material, flexible enough to bend,
but rigid enough to support the exterior dome sheeting. It was also
mandatory that the material was inexpensive, easy to work
with, and locally available. I
selected steel
drywall framing supports.
Steel
drywall framing supports are 3 sided bars of 1 mm
sheet
steel, used in the construction of non-load bearing
interior walls. These supports often contain pressed
channels and/or
folded lips that can greatly increase their strength without adding
significant weight. It is
the 3 sided geometry and the pressed structures that
add strength.
This material is difficult to bend without the steel
sides puckering and reducing the structural integrity. I
developed a novel method to bend these supports and retain the
structural integrity, but I will not post details until the observatory
is complete and I have verified the structural integrity. Unfortunately
this is necessary to prevent others from copying an untested dome
design. If my dome survives the local weather conditions
(regular
gale and hurricane strength winds), I will consider posting arch
fabrication details.
I initially planned on building a 3.5 m diameter dome, but this would
give a
structure that was too high to build without a building permit
(local limit is 2.5 m high). I reduced the dome diameter to 3.3 m and
also decided on building slightly less than a perfect half spherical
dome; this resulted in a 2.4 m high structure with only minor
deviations
from a perfect
half spherical dome.
I bolted two 2.7 m long drywall framing
supports together to
form each central
arch. I cut several 1
m wide spacers to ensure a proper dome slit opening
width and clamped the
arches to the dome ring.
I scribed the arch curvature onto a piece of 18
mm plywood and cut 8 identical arch support
sections. Each of
the 4 central arch
supports is made from two 18 mm plywood
sections that are screwed
and glued together (below photo). The central arches
slide
into
the arch supports
and are screwed in place. I also fabricated 10
peripheral arch
supports (below right photo).
Fabricating each peripheral
arch only required about 15 min, which was
much faster than if I had constructed the arches from 10 double layer
plywood rings. Each peripheral
arch was clamped into an arch support,
and then cut to fit (below
photo).
The finished observatory dome frame on top of the base structure
(below). This dome support construction is
extremely light. Had it not been for the size, I could have lifted the
dome ring and
dome structure without help. This structure is very much
like an aluminum frame greenhouse; the metal support can flex back and
forth, but becomes a very stable structure when it is locked together
with a stiff sheeting material.
The dome is covered with Bituwell
plates, which are bitumen
impregnated sheets of inorganic-organic fibers. Bituwell plates
are
very strong yet lightweight (5.4 kg per 0.93 m x 2 m x 3
mm sheet). Since Bituwell plates
are very strong,
they could
be attached directly to the dome support without a plywood
underlay,
reducing costs. The Bituwell
plates cost approximately 25% as much as a
dome covered with a plywood underlay and standard roofing asphalt
sheets.
The Bituwell
plates are corrugated, and this makes them very rigid. I
found that the corrugations could be pressed down by setting the Bituwell plates
in direct sunlight for several days and then walking on them.
Once the corrugations were reduced, the Bituwell plates
could be bent
around the dome support. The Bituwell
plates were easily cut with a utility
knife and attached with self tapping screws and rubberized metal
washers.
The below photo shows a portion of the finished dome (an
additional photo of fitting the Bituwell plates can be found
on
the Observatory
Photos web page). The most time
consuming part is getting the seams to meet and look good. Where two
Bituwell plates
meet, the corrugations seldom line up. I will find a wide
exterior flashing to cover the seams and fill in any gaps with foam
insulation or silicon to get a good tight fit.
The dome isn't completely finished, but I could not resist placing it
on the frame (below photo). I still need to do some trimming along the
central arches
and get the Bituwell
plate tops smoothed and fastened down, but
its starting to take shape. Even with the Bituwell plates
attached, two
persons can easily lift the dome-its very light but solid.
The dome slit
cover will fit inside the dome slit and slide up
and
over the top of the dome. I am planning on placing small
wheels
on the bottom of the dome slit cover frame, that will ride inside the
corrugations in the Bituwell
plates. I scribed the
dome
curvature onto a scrap of plywood and cut 8 arches (10 cm wide) from 6
mm plywood. Each door frame side is two arches long
and two
plywood layers thick (12 mm total). I cut 4 cross pieces from scraps of
steel
drywall frame (below photo). The dome slit cover
is 90 cm wide,
giving 5 cm of clearance on each side (when centered in the 1 m wide
dome slit opening).
Since I spend lots of rainy weekend days working on my
telescope,
I wanted the option for natural lighting without having to open the
dome slit cover;
I decided to use clear plastic for the dome slit cover.
This
will give lots of natural light for daytime work and I will fabricate a
removable inner cover that can be installed to block sunlight.
The
below left photo showes the dome
slit cover. The clear plastic
is 90 cm wide, just covering the door frame. I used scraps of Bituwell plates
for the side and top skirts. The below right photo shows the top
skirt. This will be bent to arc backwards over the top of the dome slit opening.
The below left and right photos show the dome slit cover
frame test fit in the dome slit opening
and the nearly finished dome, respectively.
I had intended to cover the Bituwell
plate seams with flashing, but after
some trimming, I found this unnecessary. The below photo shows the
finished dome.
I installed wheels on the bottom of the dome slit cover
and
it is
quite easy to slide it up and over the top of the dome (an
additional photo of the slit cover and wheels can be found on
the
Observatory
Photos web page). The wheels just
ride in the Bituwell
plate corrugations. I will fabricate a hook
mechanism to lock the opened/closed cover in place to prevent wind
damage. Back
To
Top
Dome Rain Skirts
The dome rain skirts cover
the opening between the base structure
and the dome ring,
sealing the observatory from wind and rain. The
prototype skirts used 8 cm wide strips of scrap Bituwell plates,
installed underneath the dome covering (below left photo).
These
simple rain skirts
gave problems, they touched the siding and caused
dome rotation problems. This arose because the original plan
was
for a faceted, 10 sided structure. When I sided the structure, I found
that the 6 mm plywood could be bent to a circular structure; this
looked better but decreased the dome overhang. The solution was to
install 10 cm wide rain
skirts (6 mm plywood) spaced 2 cm from the dome ring (below
right photo).
The dome rain skirts attach
to 2 cm thick spacers with 5 cm long
exterior screws (below left photo). The below right photo shows the
rain skirt,
that takes run off from the dome over the rain skirt. I
wedged a strip of Bituwell
plate between the wood rain
skirt and
the corrugated dome exterior, and then sealed it all with silicone. The
corrugated voids were filled in with a lot of silicon; it took 4-5
tubes to seal the entire rain
skirt. All silicone surfaces were angled
to prevent rain water from pooling. The clear silicone appears
white in the below photo, because it isn't totally cured.
The dome slit
cover slides up over the top of the dome. It was necessary to
fabricate a latching mechanism to keep the opened dome slit cover
from
rolling completely off of the dome. I fabricated two hooks (below left
photo) from aluminum square bar and several old kitchen cabinet
handles. These hooks attach to the bottom of the dome slit cover
and
hook onto the top of the dome
slit opening, causing the dome slit cover
to hang off
the back of the observatory dome. Unfortunately the aluminum
was
too weak and the hooks failed (below right photo).
I fabricated a new set of hooks from M6 threaded rod and small scraps
of oak square stock (below left photo). The below right photo
shows the hooks installed on the inside of the dome slit cover.
The below left photo shows how the hooks latch onto the top of the dome slit opening
to stop the dome
slit cover from falling off. I added a steel angle
iron on each side to prevent the wheels and hooks from damaging the
dome exterior. The below right photo shows how the opened dome slit cover
hangs off the back of the dome. The dome still rotates easily with the
dome slit cover
open.
It was necessary to
fabricate a locking mechanism to keep the closed dome slit cover
from being blown off of the observatory dome (my location regularly
experiences gale and hurricane force winds in the late winter/early
spring). I took the simplest approach (KIS) and cut a M8
threaded rod into 4 sections. I installed threaded M8 sleves in wood
blocks that are screwed into the dome slit cover
sides. The basic idea is to just have a large screw (M8 threaded rod)
that screws through the side of thedome slit cover
and into the central
arches or central arch
supports. The dome locking screws on the dome slit cover
bottom and top are shown below (left and right, respectively).
The below left photo shows how the hooks latch onto the top of the 
