About 3 years ago at one of our monthly meetings we discussed the pros & cons of observing domes, & roll off roof observatories. I tabled some information I had downloaded from the internet, via Bill Arnett's "Amateur Astronomical Observatories" webpage list which can be found Here Among the observatories sites on the "Domed Roof" list was an interesting website belonging to Joe Garlitz. Joe's geodetic dome is made with a foam panel core, re-inforced with a thin concrete overcover. You can find Joe's observatory home page Here .
Barry liked Joe's design & thought he would like to build such an obsevatory. On our trips to Barry's place for meetings, or to observe, we were able to see the progress he made over the next 12 months. The dome and lower wall were both constructed in Barry's sheering shed. The observatory was sited on a small rise about 200 metres from the house and is really excellent. One thing which is a bit disconcerting about the dome, is the striking acoustic stereo effect, which amplifies voices if you happen to be standing in the exact center of the dome and someone talks. The dome is 3 metres in diametre and is slightly over 3 metres high. The following is Barry's story on building his observatory. - Mike Finn
A few enquiries led me to a group of local folk who were keen observers, so I made contact with them and began to go to their meetings. It didn’t take long for me to realised that to do some serious observing I would need a telescope. After a lot of advice and observing through their telescopes, I purchased a 10 inch Dobsonian reflector telescope. As a very amateur observer, I have to say that joining with that group of people has been the best move I could have done. I have learnt so much from the other members, and it is has added a lot of pleasure to my observing experience, particularly when we get together.
To extend my observing at home, I wanted to build some form of protection against the cold night breezes and moisture. At one of our meetings we had a discussion about the pros and cons of a number of observatory designs, and had a look at some of them in more detail. I went home from the meeting with a lot of ideas, but eventually settled on the geodetic dome design by Joe Garltiz. A geodetic dome is constructed with a series of pentagons and hexagons, which are made from 2 different sized triangles. The triangles have a common base dimension, and this allows them to be pieced together to form a sphere, or in this case a half sphere. Soccer balls are often made like this.
Joe’s plan seemed to suit my needs admirably. The first decision I needed to make was what material could I use for the dome framework? Joe had used polystyrene foam for his base. I was giving serious thought on what to make the dome framework (mould) from, when my cousin came up with the solution. He suggested that I use double thickness 2 mm corrugated cardboard for the mould. As my cousin works in the cardboard industry, he arranged to have all the triangles cut to size for me. This was done very accurately by laser machine, which made the assembly later on much easier.
Once the triangles arrived, it was a simple matter to tape them together using 50 mm masking tape, and I soon had my 6 pentagons and 13 hexagons ready to fibreglass (see image1). Having done a fair bit of fibre-glassing in my time, I was convinced this would provide the strength needed, in lieu of Joe’s concrete over coating.
To give the cardboard sufficient strength to make the dome, I used a 4 oz. chopped strand matt and put a layer on each side of the segments.
Because fibreglass shrinks slightly as it cures, it tends to pull the mould slightly out of shape. This can be used to advantage and if you start by glassing the inside of the pentagons and hexagons first, you will end up with a slight curvature of each triangle flat section. This not only improves the visual appearance of the dome, it also improves the air flow around the dome, reducing the prospect of turbulent eddiy currents being generated by windflow around the sharp joints, particularly near the viewing slot. I soon had the inside of each segment done then turned them over and did the outside. This made for very strong segments ready for to assemble (see image 2).
To make the 3 metre diameter timber mounting rings for the dome. These rings are made up from segment strips of plywood which are overlapped an glued and screwed together to form a circle. I started with a 2400mm X1200mm of 12mm thick construction ply. I used a jigsaw on a radius arm to cut the outside radius for the 100 mm wide ring sections first. Once all the ring segments were cut, I set out a circle of wood blocks matching the circumfrence of the dome ring, and ensured they were all perfectly level on the top plane (see image 4). I used a laser level to achieve this as it is most important ; becuase when you glue the segments together, they will hold the shape they are glued in. The rings need to be elevated above the floor so you can use a router to accurately finish the edges to the final shape without disturbing the ring position. My dome ring consisted of 3 ring layers ie 36mm thick.
I decided to make the viewing slot 750mm wide as I thought this gave the best viewing without being to wide. The slot was easy to cut with a small angle grinder. The slide half-rings were then fitted and glassed in using fibreglass bog made up of resin and talc. They were thne glassed in with two layers of fibreglass inside and out.. A skirt made of 2 layers of 6mm ply, 100mm deep, was glued and screwed around the outside edge of the base ring; on top of a 25mm by 6mm strip of MDF, which acts as a spacer so the the dome is free to turn on the wall section ring. The skirt was then covered in fibreglass (see image 6).
The hatches were fabricated from the spare ring, covered with 6mm ply, then glasssed over all external surfaces. The top 2/3rds of the sliding hatch run on 4 small steel ball bearings which run on a 50mm wide x 3 mm strip aluminium that is fastened to the top of the dome slide half-rings. The bottom section of the hatch, approximately 900mm, I manually lift off after unclipping a couple of catches. The top hatch is manually opened using a pole with a hook into an eye fitting on the slide. This seems to work very well, and there is nothing to go wrong or break down. After the dome was completed and before final assembly, all the glass surfaces were painted over using white “flow coat”, a polyester coating that boat manufacturers use to finish boats, so the whole dome is completely waterproof and permanently coloured (see images 7, 8 & 9).
The wall frame for the dome support was constructed using the rings previously made and studs cut from 90x38mm pine. I made the frame height to suit the external wall of an old swimming pool that was to hand, and that just happened to be 1200mm. In hindsight it is a bit high for the Dobsonian telescope, but for anything on a mount it would be fine. The dome is supported on ten small, 75mm diameter, rubber tyred steel needle roller wheels on 12mm axles which are let into the top wall plate. It is held laterally by five nylon wheels, machined to 85mm diameter, running on the inside of the dome skirt. These are running on vertical axles fastened through the top wall plate (see image10).
The floor of the dome is 100mm thick concrete which was poured separately to the telescope base. The base is a block of concrete, approximately 500mm square, and at least 1200mm into the ground. Four 16mm diameter mounting bolts have been set in ready for a pier mount some time in the future. A 90mm plastic pipe was set in under the floor to carry any cables if needed. The whole floor is carpeted at the momentwhich makes itvery quiet
The dome is just over 3 metres in diameter and has ample space for perhaps four or five people to observe comfortably. I had a bench top temporarily set up inside, which restricted the space a bit, so I have put a little addition on the side of the observatory to house the computer and eye pieces out of harms way. One other addition has been to put a spinning ventilator on the highest point of the hatch. I have had quite a bit of trouble with condensation forming inside the dome on cold nights. I don’t know if this has solved my problem, as the weather hasn’t been cold enough yet (see image 13)
The completed dome with the ventilator in place on top.
For any queries about building the dome, please email me Here. Regards Barry Neumann
