Construction of a round observatory 2005

Finished observatory with Milky Way panorama

Photo: Jeremy McInnes

My childhood dream came true this year: the round observatory was built entirely of wood - with a rotating dome. It has a diameter of 3.20m. First the pit was dug. This took a week of fast work and then the first base ring was finished. For this we had made a template as a semicircle. Two more will follow.

The 1st base ring was painted today in good weather and then mounted on 16 shoes. The shoes will later be set in screed and are then firmly connected to the floor. At the same time, the shoes were screwed tight with one through bolt per shoe. At the same time, the steel beam that previously supported my 6-inch refractor was brought into position. That means it was set to the middle and then correctly balanced and aligned. tomorrow we will finally start with the concrete underneath.

Then the first base ring of the observatory was set in concrete. You wouldn't believe how much concrete fits into a circle 3.20 metres in diameter. We have already shovelled in 3 cubic metres and about 2 cubic metres will probably still fit. The first base ring with the concrete shoes was levelled and moved into the final position by means of a 4 metre long lath. Then the shoes were concreted in.

For the final layer, more cement was added to the concrete. The concrete was drawn off from the inside with a drawing rod set to size. Care was taken to ensure that there was a slight slope from the centre to the outside. Then after the concrete was set, a sieve was used to finely distribute the cement on the surface and then it was removed very cleanly with a large trowel.
All in all, 68 bags of cement weighing 25 kilos each and 5 cubic metres of gravel fitted into the excavated pit. the pit is 60 cm deep and the outer diameter of the pit is 3.20 metres, so the ring has an outer diameter of exactly 3 metres.

Now the construction of the dome began. First the frames for the gap closure were built. The gap width is 80 cm. It was chosen so that it would be possible to observe comfortably with a 14 inch telescope later on and so that the dome would not have to be adjusted too often for long exposures. Since the frames are 3-layered, they had to be nailed slightly offset. This continues the rounding of the secondary frames. The gap closure was joined together at 120 degrees. The 120 degrees must be so that one can also observe comfortably in the zenith. Then a plumb bob was placed in the gap closure at exactly 90 degrees. This is used to align the secondary frames. The secondary frames were then made. A total of 18 side frames are set. The setting of the secondary frames proves to be quite difficult, but it can be done. They are aligned by means of plumb bobs, then the frame is placed on the gap closure. This way you always get the exact alignment cut. This is different for each rib. Half of the secondary ribs are finished and screwed in place. The rest is prefabricated and marked and then put on later. Otherwise the dome will be too heavy and can no longer be moved. Then we sawed the squared timbers to 1.70 metres and screwed them to the first base ring using a spirit level. Then the second ring with the running rail could be placed on the squared timbers. The height of the squared timbers was calculated beforehand so that it was possible to observe close to the horizon.

After the side frames were complete, the dome was placed on the ring with 4 helpers. 9 of a total of 18 frames were not mounted until later because the weight would have been too high. then the topping-out ceremony was celebrated....................... cheers !!!!!!!!!
Then we slowly started to make the segments for planking and nailing them on. This is an extremely time-consuming job because each segment has to be measured with an adjustable angle. In addition, the segments have to sit exactly in the middle of the frames so that the uniform spherical shape is maintained. After about 20 hours we had worked ourselves in. In addition, the vertical squared timbers were joined again with squared timbers. This increases the stiffness even more.
After the dome was assembled and placed on the base frame, we started to clad the observatory. Timber with tongue and groove was used. These were then screwed to the lower and middle base rings. Finally, the cladding was painted. After this was done, the rings for the slit closure were built. These were pre-bent from flat steel by a locksmith's. Then they were put on the dome and brackets, also from flat steel, were built and welded. Then the next flat steel (which was bent on edge and forms the outer closure) was welded onto the lower one. A narrower ring was welded onto the inside.
The slit closure then runs in this guide. This will act as a blind and when it is opened, it will be guided backwards on the outside. This will be a total of about 30 boards that lie next to each other, run on rollers and are connected with hinges. This works in much the same way as an electric garage door. These are then nailed to the outside with 30 zinc sheets individually on the boards. At the same time, they overlap so that no water can penetrate. Then, between the third and second base ring, an aluminium cladding was attached. After the two rings for the gap closure had been put on, the roofing could be started. For this, plain tile shingles from the DIY store were used. These had to be cut individually, the more you got to the top, because they overlap more and more towards the top. After this was completed, the gap closure could be continued. The blind, which opens and closes in one piece, consists of 35 individual boards. These are connected to each other with hinges. They also run on rollers, which allows them to move along the rails. The boards were then covered with aluminium sheet. These overlap each other so that the rainwater can run off. After this was done, the round door was built and painted and the observatory is now ready for use.