This is what the 3.5 meter mirror looks like from underneath. It is being inspected after having been washed, prior to re-assembly in to the telescope. The mirror itself is a little over 1" thick, the honeycomb hollow cells are approximately 8" tall. The mirror weighs approximately 4500 lbs. and was built at the Kitt Peak/University of Arizona mirror lab beneath the U of A football stadium.
A procedure was developed so that the mirror could be washed without removing it from the telescope, greatly improving the safety of the mirror from being destroyed in a lift accident. Now the only time the mirror is removed is when it is taken for re-aluminization every few years back to the mirror lab.