I chose to go with a round design, there is a working dairy farm across the road with silos and I kinda like the looks. I'm starting off with a 5' OD and 4' ID by 42" high sewer riser as the main foundation. On there I will build a 96 1/2" diameter deck to support the walls and dome. The inside of the sewer riser will be filled with concrete to be the footing for the pier. The pier needs to be about 12', I'm still thinking about how to construct this.
Below is a cross-section view of the of the observatory. The viewing floor is going to be 2-3 feet above the deck attached to the house.