The Hale Telescope's 14.5-ton mirror is honeycombed on its underside. The triangular pieces are hollow to reduce the weight of the Pyrex disk. The round areas are the locations of the mirror supports.
The mirror supports are truly remarkable devices. From The Perfect Machine: "The thirty-six supports were precision machines, an assembly of levers, counterweights, gears, and ball bearings like a fine wristwatch, but large enough that it took two men to carry each of them."
"The design of the support mechanisms was clever. The levers of the mechanisms were designed to push up, against the tops of the pockets in the back of the disk, to counteract the force of gravity pulling down on the disk. The supports were four inches behind the actual surface of the disk."
Each of the 36 mechanisms has 33 different places where something can rotate, slide, or pivot (That's 1188 degrees of freedom!) to change the tension on the surface to ideally keep the mirror in its perfect parabolic shape.
All of this happens by gravity alone, there is no active control (Maybe someday, but not now). As the telescope pivots to point to a different location in the sky the pieces within the supports relocate and change their tension on the mirror.
Over the years pieces within the mirror supports can bind up a bit, limiting how easily the parts can move and how effectively the mirror's shape is maintained. During the recent engineering run one of the mirror supports was removed and examined in great detail.
Here is a photo of the mirror cell with the instrument removed from the Cassegrain cage. The big hole at the top is the where the light comes through to the science instrument. The red fans are fans that can blow air on to the backside of the mirror to help the mirror reach the proper temperature. All of the rest of the openings on the mirror cell are locations of mirror supports.
A close-up photo of one of the mirror supports.
More on the mirror supports and something that may surprise you later.