I'm lost trying to figure out how you're doing a concrete "Pad" UNDER the deck of the observatory...
Heres a thought... Pour the pad for the Dob BIGGER... Like about 1/4 the observatory (That whole corner)... When you lay your joists in place, you can place those "Dec-Blocks" under the joists all over that area to remove any flex in the deck. That should be plenty for the Dob...