The LBT's 8.4 meter mirrors are probably the largest single cast mirrors that will likely ever be built. They were done with a 'spin casting' technique. The kiln was spun as the molten glass was poured......creating a natural paraboloid surface. Much thinner and lighter than the 200" Hale telescope 5 meter mirror......that was poured into a stationary mold and then had to be ground into a paraboloid.
The Keck telescopes have segmented mirrors......as will the larger proposed mirros for the 30-50 and 100 meter scopes that are planned for the future.
Here's another sketch. Believe me, I would much rather be posting real photographs.
The LBT (Large Binocular telescope): twin 8.4m mirrors on a state of the art alt-Az, computer controlled mount. Located on 10,500 ft Mt. Graham in Eastern Arizona.