Okay so the census is towards a concrete pad for a zero maintenance solid foundation; which works as I'll be pouring an isolated concrete pier anyways.
My thought is excavate to level (like 3-4" down from top grade) and add in about 1-2" of gravel as the base (I might poke a few 4" holes a bit deeper to "hold" that wouldn't have gravel).
Lots of work but not a bad idea. Excavating the surface soil will also get rid of any organic material, which is standard practice in construction. You don't want dead plants rotting under your slab, causing voids.
Pour the concrete slab 4" in depth as a hexagon shape so that the inner circumference of the hexagon is bigger than the dome.
4" is plenty thick for strength, but consider thicker to get the top of concrete a few inches above your soil level. That way you'll be above any water that might collect on your property, you'll keep any wood in your observatory dry, and you'll be able to see the tunnels subterranean termites build if they decide to eat your building.
Put Eye-bolts (or threaded achors) at each of the 6 corners, this is so I can do a tarp tie-down over the dome if anything ever happens and I need to cover it.
Anything that protrudes above the top of concrete will be a tripping hazard and you may find yourself cursing them if you trip in the dark.
There will be 1" spacer between the pier and the slab, this will be filled in later with something that won't pass vibrations (Thoughts?)
I simply left a gap between my slab and concrete pier. Friends of mine have filled the space between their piers and slabs with various foams and caulks.
Going with a hexagon shape because:
1. Flat edges are easier to board then round.
Granted. In industrial construction most contractors pour octagonal foundations for round structures because 45o angles are easier to form than 60o angles.
2. Uses less concrete than square.
3. Minimizes exposed pad which could cause water ingress.
If you put a slight slope in your pad (1/8" per foot), with the high point in the center, low point at the edges, you won't have to worry about that.
4. If I ever want to add a Bay to the POD I can just add concrete to one side of the structure as the hexagon will be oriented to match the walls.
The idea that makes my octagon suggestion a poorer choice.
Any concrete tutorials / videos / tips for observatories are appreciated.
This isn't apropos to your project, but if you were building a roll-off roof observatory, I'd suggest putting piers at the corners and at the ends of the gantry. When I dug for my pier, I also used the auger to dig/ drill 12" diameter holes about three feet deep (that's when I hit clay, much more stable than the sand that makes up the surface soil at my site) at those six locations. That way, when my roof is closed or open, its weight is transferred through the corners of my building or the gantry posts to concrete piers rather than my relatively thin slab.