My pal and I actually have a patent on a cute method of doing this--you use a hard-coated plastic sphere about 2 feet in diameter, full of water, with a flat mirror shaped like a pagoda, so as to compensate for the horrible spherical aberration of the sphere.
The tracker was my contribution. It's an all-passive system--no motors, no electricity--based on buoyancy, and (I calculated) could easily achieve accurate enough tracking for 2000 suns concentration. (Without being too mysterious, it combined the principles of the Crookes radiometer, the Edmund Scientific drinking bird, and the coffee percolator.)
I demoed the coarse tracking servo, but I left IBM before we had a chance to build a whole system. It's a pretty cool gizmo--the water is the lens, cooling system, and the mechanical stabilization system all in one. You just dump 'em off the back of a truck, hook up the wires, and drain the Colorado River to fill them up. ;) Then they Just Work.
Cheers
Phil Hobbs