How It's Made
Construction of the mold for the casting of an 8.4-m f/1.14 honeycomb blank was completed in the fall of 1996. Furnace modifications were performed to accommodate the geometry of the mold on the furnace hearth. This mirror, cast in January of 1997, is the first of two mirrors for the Large Binocular Telescope Project on Mt. Graham, AZ. The faceplate was remelted in June 1997 to replace a small amount of glass that leaked out during the casting and bring the faceplate back to its optimum thickness.
The image (at left) shows three large honeycomb mirrors in the casting lab at the end of 1997. In the foreground, the first LBT 8.4-m mirror is still in the mold on the furnace. In the center, the second 6.5-m mirror for the Magellan Project hangs in the turning ring. Before polishing the mirrors are lifted with a fixture glued to the front surface with silicone rubber sealant. At the far end of the lab, the first 6.5-m mirror for the MMT is being lifted off of the polishing cell with a vacuum lifting fixture. After polishing the mirror is lifted with vacuum pads to avoid marring the precisely polished surface.
The large mirrors are generated (by machining with diamond grinding tools) on a machine called the Large Optical Generator (LOG) which is a high precision vertical milling machine. Then the mirrors are polished on the same machine by lapping with the stressed-lap polishing tool. The image (at left) shows the second 6.5-m mirror being polished on the polishing machine in 1998 (photo by Peter Wehinger).
The dark reddish frame below the turntable is an air cart used to transport the mirror and its polishing cell between the polishing machine and the test tower.
The image (below left) shows the completed Magellan I 6.5-m mirror in the foreground, and the LBT I 8.4-m mirror in the background under the test tower (photo by Steve Miller).
After polishing has been completed, the mirrors are lifted with a special vacuum- lifting fixture. The lifting fixture is shown in the image (below left) with 36 pads which attach to the mirror surface and three pairs of redundant vacuum pumps. The polished surface of the mirror is coated with a layer of blue plastic (opti-coat) to protect it from scratches during shipping. The frame below the mirror is the base of the transportation box (photo by Peter Wehinger).
A facility for polishing and testing the highly aspheric secondary mirrors required for these various large telescopes is now nearing completion. Stressed-lap polishing and testing with computer-generated holograms will allow the production of secondary mirrors up to 1.8 m in diameter. A 1.2-m secondary has been completed for the wide-field telescope of the Sloan Digital Sky Survey in New Mexico.
Steward Observatory's honeycomb mirrors represent the next technological leap that will allow astronomers to press even farther into space, while keeping down the cost of doing big science.