wpo - building a small domed observatory

My observatory and its construction can also be found in Small Astronomical Observatories; Patrick Moore [editor]; Springer-Verlag 1996; chpt. 6 - ISBN 3-540-19913-6

  my LX200 pier    detail photos  1  2 3  4  5 6  7  8 octagonal dome schematicdome plans   cross section    Ron's 'octagonal' dome build

Effects of material mass and reflectivity on solar heat gain - thin bright ali sheet gets hot whilst providing shade but cools rapidly at sundown.  Timber walls have low conductivity eg low heat uptake unlike brickwork.   If brickwork used then external shade materials like wall planting/ timber trellis preferable to white [titanium] paints.  Internal insulation tends to delay heat dissipation prolonging spoilt seeing throughout the evening!

The observatory's diminutive size proves no disadvantage - from 2009 I've used the LX200 almost exclusively as an astrograph with CCD camera permanently attached and controlled by a laptop within the observatory. 

A permanently installed telescope within an observatory greatly improves the enjoyment of our hobby - being instantly ready for action with the minimum fuss.

My preference is for a revolving dome with observing slot which gives complete protection for observer and equipment.  A dome minimizes local light pollution and prevents dewing which plague the run-off-roof design.   Contrary to popular opinion spheres and cylindrical forms are not difficult to build.  They are inherently rigid and can therefore be made lightweight. The observatory was built with hand tools plus electric drill and jigsaw.  The cost of building materials was about the same overall as a prefabricated garden shed of similar size.

Planning: Any observatory design starts with the telescope - these photos and notes describe my 1650mm diameter [5'-6"] observatory housing a 12-inch Meade LX200 SCT.  Using a star-diagonal the entire sky, except for the less interesting northern quadrant, can be viewed from a centrally placed stool. Two observers can just squeeze in.    The observatory - raised on stilts to satisfy an adjacent roofline - was fabricated top downwards prior to preparation of foundations and erection.


The Dome is 1750mm [5'-10"] in diameter [100mm greater than the walls] by 950mm high i.e. 60% of a sphere. The eight-sided ply base-ring had eight smaller fillet pieces to make-up the required diameter each set at an angle of 22.5o to its neighbour i.e. 16 x 22.5o = 360o.  The 50mm wide upright ribs, cut from ply with a jigsaw [using a rod, pencil and nail as a compass] were clad in 1mm thick bright aluminium sheet curved as partial cylinders over the frame, drilled and nailed in place at 100mm intervals.  The aluminium sheet is easily trimmed with tin-snips. One section of the dome formed the 450mm wide shutter opening [i.e. 50% greater than the telescope aperture] with telescope access from horizon to beyond the zenith.

A 550mm wide unframed section of aluminium sheet forms the shutter that seals against a continuous section of garden hose and is held in place with clips formed in the sheet short edges [centre-right below].   The lightweight flexible shutter is lifted off with a boat-hook and set aside internally against the walls.  All roof joints sealed with a narrow strip of self-adhesive Flashband.

The walls form a cylinder or drum in four equal panels.  The 1500mm height is set by the uncut length of feather-edge boarding from the local timber merchant.   The door height is equally generous, and aided by access from steps below, avoids the bent back posture common on entry to some amateur observatories. The door, formed within a panel, is curved like the walls. The vertical framing is lightweight 38mm x 18mm tiling batten and the structure was quite flimsy until clad externally with feather-edge boarding which makes the building completely rigid.  A layer of roofing felt under the boarding keeps out draughts and protects structural framing timbers from rainwater penetration and potential rot.

Floor & foundations : the plywood observatory floor is raised 1m off the ground and supported on timber joists.  The foundations are minimal.   The load is carried by two posts and the existing masonry building.  The foundations for the independent telescope pier penetrate 1.2m into clay subsoil and is offset 350mm south from the observatory centreline to allow for the telescope overhang.  The pier, built from high density concrete blocks, is capped in concrete - the telescope is supported on a steel column and wedge.   The 10mm gap between the pier and observatory floor is packed with fibreglass quilt.


images & text [c] Maurice Gavin 1995/2003/August 2011