Building a 2-metre Octa-Geodesic Dome

by Eric W. Walker

Considerations

Why bother to build?

  • Scottish Highlands weather
    • enough said !
  • Minimise affects from light pollution
    • especially security lighting !
  • Astrophotography
    • can take 60-90 minutes to set up equipment and accurately polar align
  • "Organised" observing
    • all equipment, observing aids, laptop, notebooks, maps at hand
  • Cheaper than purchasing "off-the-shelf" observatory
    • £2,000 to £3,000 for ready-made, delivered, and installed

What do I want to achieve?

  • Protection from the elements
    • can observe in light winds
    • easy shut-down / start-up in e.g. rain showers
    • minimise / eliminate dewing
  • Shaded from light pollution
    • especially security lighting !
  • Permanent set-up
    • always accurately polar-aligned
    • "ready to go" / ready for imaging in <30 minutes
  • "Organised" observing
    • all equipment, observing aids, laptop, notebooks, maps at hand
    • power, internet access
  • Environmental considerations
    • maximise renewable / recycled materials
    • minimise petrochemical derived materials
  • Security
  • Minimal cost
    • £400 - £600

Options & limitations

  • Shed with roll-off roof
    • light pollution / dewing?
  • Rotating dome
    • spherical / hemispherical vs. geodesic
    • square base vs. polygonal base
  • My skills and equipment
    • no workshop and limited toolkit
    • never built anything this scale / complexity
  • Time
    • after work & weekends ..... but not every spare moment !!!
    • spring through summer, ready for autumn observing
  • Appearance
    • significant input from my wife !
  • Minimal cost
    • £400 - £600

Design

I decided on a wooden plinth because it was readily available, is renewable, and would easily cope with the weight and dynamics of my 8" Meade SCT. Note that the final height of the plinth is decided by the actual dimensions of your base floor and preferred observing height, etc.

My wife insisted that the observatory mustn’t look like a boxy wooden shack at the bottom of the garden so she "suggested" an octagonal design would serve the purpose. This base turned out to be an ideal and stable support structure. Make sure you include all the bracing / joists as indicated to support and distribute your weight across the floor. I have included the fundamental dimensions and their arithmetic relationships in order to evaluate various size options.


This simple frame and bracing structure design turned out very firm and stable. I didn't need the "cantilever" upper braces for the dome rail as the 3"x2"s with 8mm carriage bolts and PVA wood glue was so solid.


I also decided to build a geodesic dome and found the following websites indispensible for basic dimensioning and construction advice from which I developed my own ideas and techniques.

http://www.desertdomes.com/rev3calc.html

http://www.orednet.org/~jgarlitz/geodome.htm

http://obs.nineplanets.org/obs/obslist.html

Construction of Plinth and Pier

Every journey starts with the first step ........ or shovel! Little did I know how much work was ahead of me. I should have got a big clue from this starting point of digging a hole (900 x 900 x 700mm) on 14 May 2005.

I used a 2.1 metre length of timber which was originally destined to be sawn and chopped up for firewood. It has made an excellent and stable plinth for the 8" Meade SCT.

The plinth was formed from 0.6 cubic metres of hand-mixed concrete (sand:aggregate:cement 1:1:1). A big tip - use a cement mixer! This took over six hours to complete and was real back-breaking hard work. Another four days for the concrete base to cure and the entire plinth & pier assembly was permanently in place. No going back now!

I used threaded rod screwed about 6cm into the timber pier and fixed securely with a nut and washer. The telescope wedge is mounted on top of a nut & washer assembly which facilitates fine levelling adjustment.

This shows the finished levelling assembly.

A look at the finished plinth, pier, and levelling assembly with wedge attached.

Click here to go to part 2.


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