I 2nd the motion...
Also if you get snowfall in your area make sure the dish is a foot or two above the height of the average snow fall especially if it is motorized. The bottom of my 10' dish is almost 5ft off the ground, but in the winter it looks really low after 3' of snow!
In rocky soil you might be wise to make a wider footing if you can't go deep, but go as deep as you can. This is a great excuse to borrow a tracked excavator, they are lots of fun to play with!