Casting the scope column

Last May I bought an astronomical telescope, and for good observations you need a stable mounting. The scope comes with a pretty massive tripod, but for astrophotography and added comfort you want a real mounting such as a concrete column.

Now, building a concrete column on rented property is usually frowned upon. Luckily Anita found us a Schrebergarten (a 300m² parcel in a garden colony) in April, and that made it possible to implement the plan.



It started with a 80cm deep, square hole (about 60x60cm) in the ground. Why 80cm deep? This is the recommended depth to be frost-safe in Germany. Otherwise ice might form under the foundation, and tilt the construction. Into the hole went three vertical reebars in a triangular configuration, four horizontal rebar mats at regular intervalls, 12 bags of screed cement (40kgs each; I feld like dead afterwards), and lots of brick debris I found in the garden. I cut myself two plywood discs to keep the rebars in the desired position until the concrete set.



Then nothing happened for a couple of months since casting the column required frost free weather. And, yes, the garden gets very wet at times.



A 20cm diameter plastic drain pipe would provide boarding. I cut a screed rebar mat into a rectangle and bent it into a cylinder, using some wire to hold the ends together.



The rebar mat went over the three vertical rebars (that is, the rebars are inside, the mat is wrapped around them on the outside).



Two plastic cable canals went into the center of the column, held in place by wire. One canal will be used to supply power, the other will... be used for something I decide in the future.

How do you keep the boarding away from the rebars (there should be about 2cm of concrete between rebars and the surface)? After trying some stuff that did not work I drilled three holes into the concrete foundation, and put plugs and screws in. These screws kept the plastic pipe were I wanted it. Don't use force when putting on the borading - I broke off one screw, and was not happy about it. No need to care about the vertical rebars since they are already embedded in the foundation.



The last ingredient are three threaded stainless steel rods. The aluminum plate that will later hold the scope mounting is already in place to hold the rods in position when the column is poured. I tied the rods to the rest of the construction with wire.

Tadaa! The concrete is in, with Annie helping a lot. I used a long stick to compact the concrete and drive out bubbles, and used a rubber hammer to tap the boarding, again to remove bubbles. That actually worked quite nicely.

A note on the side: I had bought five sacks of concrete for the column though that seemed a bit much. It turned out I calculated the volume by using the diameter instead of the radius, ending up with a number 4 times to high...

Remember that aluminum plate two images fourther up? That will be the base for the mounting. The three parts in this picture form the counterpart: a ring made from thick plywood (glued together three 4mm layers) acting as a spacer, another aluminum plate, and a fastening screw I welded together from a threaded rod, a nut (both stainless steel), and a cut-off hexagonal spanner ("Inbus-Schlüssel").



This is how it looks put together (from below the base plate). Note the distance between the column and the base plate is quite large, but I can make that smaller if I want to.

The base plate is held in place by three nuts below and three nuts above. After some dickering I got the plate in a nicely horizontal position.



Two weeks later I cut open the boarding (a longish session with a Dremel cutting disk and lots of cursing, but it worked). The column looked great, pretty smooth with only a few bubbles. A hexagonal platform made from Siberian Larch to sit on with a camping chair will be added later (the boards already have been bought and cut).