A Concrete Pillar

Here is how I made a concrete pillar for my new Celestron C8 Astronomical telescope.

The project started after some experience last winter using my old Vixen 90mm f/11 refractor on an aluminium tripod. As I am mostly interested in webcam planetary imaging using very long focal lengths (~6000mm), a lightweight tripod is useless as it is incapable of reducing vibrations as required for high magnification photography. Also, the "astronomicial season" at 60 degrees north coincides with the cold season and up to a meter of snow. Try using a tripod under such conditions!

Obviously, something much more solid was required, especially for the more heavy C8 scope. I decided to make a concrete pillar in my backyard. The following illustration shows the basic design:

Below are thumbnails from the actual construction process that took place July 9-11, 2001. Click each thumbnail to see each full size image in a separate browser window. The first few images show various steps in the excavation process. The children are my daughter Elin (8) and my son Andreas ("almost 4"). They were active observers, made numerous comments along the way, and even helped me as you can see!

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The first image show wooden sticks in the ground around the hole. These were used to control the precise location of the pillar. At the stage illustrated in the 4th image, about 40 cm of surprisingly dense, clay-rich soil has been excavated. Another 50cm will be required to avoid the winter frost (and movements created by spring thaw). Actually, theory explains that around 2m depth is required, but I cut that in half as this kind of soil is pretty frost-safe.

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My helpers showed enthusiasm and frustration depending on the progress, or lack of such...

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We finally reached the 90cm design goal!

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A rest is well deserved before constructing the reinforcement of the concrete footer. For all the reinforcement, 12mm bars were used. These were all provided by a friend that collects everything (including over 30 cars). He generously cut them to specified lengths for me. The third image from the left shows the reinforcement bars, and how ropes were used to align them in the hole. The rightmost image shows the footer and bottom column reinforcement in place. The diagonal bars were hammered about 50cm into the ground, a relatively easy task (using the back of an axe) in the clay-rich soil. This pillar is not going to move!

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The left image shows the reinforcement extending from the hole, and the next image shows testing the placement of the concrete casting cylinder ("sonotube"). The tube is 23cm in diameter. The third image shows the support structure to be used while pouring the pillar. It is important that the rebars are placed symetrically inside the tube, and that both the tube and bars are vertical. Cement is heavy, and the wooden structure is designed to provide lateral support while the concrete cures. The fourth image shows the 2 buckets used, one for water and one for concrete. My helpers verified the integrity of the buckets before they were put into action.

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The left image shows a design mistake. I should have designed a square bracing for the upper part of the footer. I had to improvise a little to keep the concrete from sagging too much. The second image shows how I corrected this mistake after initial curing. A simple wooden frame supported by dirt on the outside allows extending the top footer area. The third image illustrates the reinforcement of the pillar before pouring the remaining part of the footer and the pillar itself. The fourth image shows a tired worker being transported to the resting area for feeding.

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Everything has now been poured. All in all, about 9 to 10 bags of pre-mixed concrete was used. Each bag is 40 kg and occupies a 23 liter volume after curing. The left image shows the arrangement for supporting the steel pillar adapter while the concrete cures. The part of the adapter shown here is a circular steel plate welded onto a 50cm steel tube which extends down into the center of the pillar. I designed the adapter and had it made in a local welding shop prior to the actual construction of the pillar. Notice the careful alignment of the adapter wrt. true north indicated by the white rope. Further details are visible in 2nd and 3rd image. The fourth image shows the celebration of the successful pouring of the pillar.

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Final images showing a test setup of the Vixen Super Polaris German Equatorial Mount (GEM) on the pillar, using the custom made pillar adapter. The 3rd image shows whole assembly after removing the sonotube, the hole refilled and the C8 scope mounted. The wooden extension on the counterweight axis is my way of a) reducing costs (importing steel from Japan is expensive), and b) reducing total weight (the capacity of the SP mount is approaching its limits, and a longer counterweight axis reduces required counterweight mass). Final image shows Elin in front of the scope, now fitted with an extra long, home made dew cap. Notice also the stones around the pillar base. All we need now is some dark sky!