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What is your soil type? If it is clay soils then you will have a degree of swell and shrinkage – your footings will have to be larger in order to stabilise the pier?
Is there rock in the area? You could tie in the pier structure to this for stability
How heavy does the mount + weights+ tube weigh now? Might have to do some equilibrium calcs?
You will probably have to submit a DA to the local council? Get the neighbours on side first by taking the plan to them personally and getting them to sign a copy and leave them with one. Send in the signed copies to the council with your application. Some people mightn’t like the idea of you being able to peek into their yards?
As for the base, the pier will have to be stabilised by mass pour and have enough reo in it to integrate into a column pour. You might want to search out some other observatories built OS that have the design you are doing to see what they have? I have seen a column built within a house 450 dia going to a second story landing then the mount placed on top.
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