How are the giant mirror segments made?

Mirror fabrication occurs in 3 stages:

1. Cast the mirror blank by melting glass in a rotating mold
2. Perform rough grinding of the front and back surfaces
3. Polish the front surface to optical tolerances

In addition to these steps, the mirror must be installed into its mirror cell. Note that this polished mirror is still just “plain glass” at this point. After it is transported to the mountain-top telescope site, the mirror is given its reflective coating. Finally the mirror cell assembly is mounted on the telescope structure for alignment and testing.

The mirror "blank" is formed by melting glass in a mold formed by high-temperature resistant refractory material.


GMT1 mold is prepared by installation of hexagonal silica fiber cores. Photos by SOML.

The mold consists of a tub made of silicon carbide cement, lined with ceramic fiber. This tub will form the bottom half of a furnace to melt the glass. The mold is filled with about 1,700 alumina-silica fiber hexagonal boxes that form a honeycomb structure. The top of the boxes must follow the aspheric shape of the final mirror surface; no two are identical!


GMT1 mold is loaded with 18 tons of glass in preparation for casting. Photos by SOML.

After the mold is prepared, it is time to load the glass into the mold. Blocks of low expansion glass made by the Ohara Corporation of Japan are inspected and weighed. A total of 18 tons of glass are loaded into the mold, one piece at a time. The furnace lid is placed on top of the mold.


Glass is melted while entire furnace assembly is rotated. Photo by Patrick McCarthy.

The mirror is made using a unique “spin cast” process: the furnace is rotated as the glass melts. This gives the mirror surface a rounded or parabolic shape. The mirror will still need to be shaped further by grinding; however, because the mirror will start off already close it its final shape, the spin-cast process saves several tons of glass and greatly shortens the annealing and grinding time. Heating coils in the walls and lid of the furnace raise the temperature to 1160°C as it spins at 5 rpm. The temperature is held for 4 hours to allow the glass to melt and fill mold. It is then cooled rapidly to 900°C, and then cooled more slowly for 3 months to avoid strains in the final mirror.


The GMT1 segment after successful casting. Photo by SOML.

The successfully cast mirror is lifted out of its mold. When tilted vertically, its rear surface is visible with the mold’s “floor tiles” still attached. The tiles are removed and the fiber boxes are cleaned out with a high-pressure water spray. This leaves behind a pattern of honeycomb-like structure inside the mirror. The glass structure is mostly empty space. This significantly reduces the mirror’s weight, and enable its temperature to stabilize much more rapidly than a solid glass mirror.

The mirror is inverted, and the rear surface and edges are lapped and polished. 165 “loadspreaders” are bonded to distribute the weight of the mirror and provide a permanent attachment points to mount the mirror to its active support system.


The top surface of the mirror is generated by grinding to shape. Photo by SOML.

The mirror is turned face up, and the front surface is ground to its approximate final shape with a series of diamond grinding wheels.

The mirror’s surface is then polished to precise specifications. The polishing system employs a “stressed-lap” polishing tool, which was developed for highly aspheric surfaces. The lap consists of a polishing disk which bends actively to match varying curvature of surface. This provides passive smoothing traditionally associated with spherical surfaces.


GMT1 segment is ground and then polished, using a stressed lap polishing tool. Photos by SOML.

As the mirror is polished, it is tested using multiple, redundant measurements to assure it is precisely figured.  More... GMT1 Metrology Development.