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Chrys, do you plan to do astrophotography or only visual observing?
If you will be doing AP then your pier foundation will need to be more solid.
I don't have the answer for you, but my thoughts are:
1. The packing sand probably wouldn't provide much support so you might have to plan for a pier that is effectively 400-500 mm taller than you actually have. In other words, the real foundation is 400-500 mm below ground level.
2. I'm pretty sure that clay soils 'move' with the season, so that might indicate more mass in the foundation, and perhaps spread out over a wider base.
3. I think it makes sense to build the foundation with a flange to which you can connect your 1200 mm pier. Changing or removing the pier in the future will then be easier.
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