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Hello Rainsey
Sorry to be jumping in so late but a couple of points / questions
Are you thinking of being in the garage away from the telescope or are you thinking of looking through the scope/
How exposed to wind will the mount / scope be?
Whilst generally our time is cheap ( at least when working on our own projects ) it is limited, the cost of concrete, blocks, steel really isn't all that great, so I would ( and did ) take the view that to spend a bit more on materials and be happy with the outcome is better than saving some money but getting a job that you are unhappy with.
Seems to me you are being hamstrung by the idea of an existing slab of unknown quality. I would remove the slab, either in whole or just enough for a new pier.
My set up is a block of concrete abut 1.5 by 1.2 by 1.2, with a 900mm square " riser " about 800mm tall off it, so all up about 7 tons of concrete, with a 1200mm tall 250mm diameter steel tube attached to the base with 8, 16mm threaded rods welded to the reinforcing steel.
Is it over kill? yes, but I get NO vibration at all, the telescope is protected by a dome that obviously helps.
I was a house builder for 40 odd years so I have some construction experience but the basic premise is anyone can dig a hole, anyone can dump concrete into that hole and anyone who is thinking of doing what you are most likely has the skills to place formwork. I have seen some pretty horrific amateur welds but they rarely fail and youtube can show you the basics, it is unlikely you will need to be able to weld vertically down ����
Good luck with your project
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