[ChrisM]

My dome has been built for over 5 years now, so I thought it time to update this thread by reflecting on the observatory’s design and how well it has worked.

Firstly, some basic stats on the two-storey 5 metre high obs:

Dome - 3.6 metre diameter, geodesic, 105 off 9 mm plywood triangles
Turntable – rolled 75 x 75 x 6 mm angle iron running on upturned plastic wheels
Walls – 3.0 metres, mini-orb cladding
Foundation: - 2 cubic metre reinforced concrete block
Pier – 2.2 m x 450 mm dia reinforced concrete pier, with an 1100 mm x 275 mm dia steel pier extension.
Mount – AP 1200 GTO
Scopes – M14 SCT & NP 127is
Observatory construction cost - $7k (excluding equipment)


How well has it worked? Overall, very well indeed. To have one’s scope permanently fixed to a mount on a solid pier saves a lot time and makes the whole experience much more satisfying. Set up time is not much more than 5 minutes. The dome provides great protection from the wind and dew, and also from light pollution. The hinged lower slot door rarely needs opening. The observatory has been used mainly for visual work, with over 150 visitors passing through since it was officially opened in 2009. More recently I have experimented with my DSLR camera taking images through both scopes, but mainly through the 14” SCT. I don’t have any guiding (yet) so generally limit exposures to 1 minute at 3.5 metres focal length. I never see any evidence of wind-induced vibration – either in the eyepiece or in images. It seems that the pier is fairly well isolated from the building too.

The two-storey design is neat and compact, but does involve many trips up and down the stairs; this doesn’t worry me at all but does deter some visitors from venturing up the spiral staircase to the observing floor. Last night, we had over a dozen visitors (typically aged over 60) and they all ventured up. If need be, I can rig up a monitor downstairs to show live images.

Issues: Not surprisingly, keeping water out is the key issue.
(a) Water ingress – mainly through penetrations in the dome –eg. Stand-off bolts for the shutter tracks. Also occasionally through an unknown path onto the dome base ring. I am considering hiring a cherry picker so that I can cover the entire dome in a special mesh fabric and then apply a roof sealing paint – industrial quality. This will provide a longer term fix and help protect the timber construction. The top one-third of the dome’s external surface is virtually out of reach – even with a long extension ladder.


(b) The southern side of the dome gradually attracts mould, which can be wiped off with a sponge and some mild bleach.
(c) The bottom plate of the circular building’s timber frame is also timber, and whilst I used a dampcourse layer between it and the slab, it would have been better if I had made it from hardwood or steel. It might be the weak link in the overall design.


(d) Strong winds have not even hinted at moving the dome itself (~250 kg) but have once partly moved the shutter (> 120 kph). It’s now tied down more securely, and it’s best to keep it closed during wind > 25 kph.


(e) Spiders, ants and other insects have chosen the dome as their home. Maybe I need to get a pest exterminator to spray the whole building.


(f) The width of the dome opening is barely 700 mm, which is great for protecting the observers and equipment from the elements, but another 200 – 300 mm would have made the world of difference for ease of keeping the scopes looking through a clear opening. However, the present arrangement is workable, although a 14” scope on an offset GEM is at the limit. (The pier was offset 250 mm North to allow for the original fork-mounted scope.)


If I were to build it again, what might I do differently?
1. Consider making the building only 3.5 m diameter, so that I had the option of using a ready-made fibreglass dome (that could be automated).


2. Use a different material for the building frame’s bottom plate / ring (refer to (c) above)


3. Cut a wider aperture opening – say at least 900 mm wide.


4. Make the dome about 12 mm larger in diameter than the building, so that the weather seal design is much simpler.

Conclusion – The project has been very satisfying to build, own and operate, and has delighted many a visitor. The home-built observatory, which allowed me to upgrade my equipment considerably, does not come without some on-going maintenance, but the rewards are astronomical!

A small selection of pics showing various phases of the project....which of course, will never end.

Chris