Lars, you can see my observatory build thread here:
http://www.iceinspace.com.au/forum/s...ht=Observatory
Our environment is very different to yours, and so my observatory is built on treated pine posts ( insert and termite damage is a consideration). Some use concrete slab bases. My obs has a separate pier footing of course. I am originally from Canada, lived there for 30 years, so i and am very familiar with the granite of the Canadian Shield, which is very similiar to your bedrock.