How are GMT Mirror Segments Made?
The GMT mirrors are made at The Mirror Lab in Tucson, AZ.
Mirror fabrication occurs in 3 stages:
GMT1 mold is prepared by installation of hexagonal silica fiber cores.
GMT1 mold is loaded with 18 tons of glass in preparation for casting.
Glass is melted while entire furnace assembly is rotated.
[Photo: Pat McCarthy]
The bottom side mirror blank is cleaned after successful casting.
"Load spreaders" are added on bottom side to distribute weight.
The top surface of the mirror is slowly polished into optical precision.
A computer controlled "stressed lap" tool is used to polish GMT's aspheric surfaces. [Photo: SOML]
In addition to these steps, the polished glass must be installed into its mirror cell. After it is transported to the mountain-top telescope site, the glass is given its reflective aluminum coating to make it a mirror. 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.
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!
After the mold is prepared, the glass is placed inside 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. Finally, the furnace lid is placed on top of the mold.
The mirror is made using a unique “spin cast” process whereby the furnace is rotated as the glass melts. This gives the mirror surface a rounded, or parabolic shape. The mirror will still require additional shaping by grinding to achieve optical tolerances. However, this process saves several tons of glass and significantly shortens the annealing and grinding time because the glass is already in a parabolic shape. Heating coils in the walls and lid of the furnace raise the temperature to 1160°C (or 2120°F) as it spins at 5 rpm. The temperature is maintained for four hours to allow the glass to melt and fill the mold. The glass is then cooled rapidly to 900°C, and then cooled more slowly for three months to avoid strains in the final mirror.
When the glass has cooled, the successfully cast mirror is lifted out of its mold. When tilted vertically, the mold's attached "floor tiles" are visible on the rear surface of the glass. The tiles are removed and a high-pressure water spray is used to clean out the fiber boxes. This leaves behind a piece of glass with a honeycomb-like structure; the mirror is mostly empty space. This significantly reduces the mirror's weight and enables 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 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.
As the mirror is polished, it is tested using multiple, redundant measurements to assure it is precisely figured. This phase is called Optical Metrology and it is the most demanding part of all.