The previous post Big Telescopes | Examples of Three Types, describes the refractor, Newtonian reflector, and Cassegrain reflector. Of these, the Cassegrain design is built into the largest diameter and most powerful telescopes in the world. In this post, I focus on the process used to form the mirrors of the Giant Magellan Telescope scheduled for completion in 2019. Each mirror is 8.4 meters across. Those seven mirrors will form a telescope objective of 24.5 meters, or 80 feet. Note the human figure at the lower left in this artistic rendering.
The mirrors need to be as light as possible. They cannot be made of a thick piece of solid glass. Instead, they are built in a honeycomb hollowed out structure. It is a remarkable story of engineering and technology.
This four minute video from the Univ. of AZ Mirror Lab explains some of the rationale for this telescope and the large mirrors being cast for it. More details are found below.
Spin Casting Fabrication
The Mirror Lab at the University of Arizona developed the current spin casting technique to make the world’s largest mirrors. Work done today at the Steward Observatory Mirror Lab started 1980 with an experiment by Dr. Roger Angel, founder and scientific director. By 1985, with financial help from the US Air Force, the National Science Foundation, and the University of Arizona, the current facility was installed under the UA football stadium.
Past mirror making systems produced a flat surfaced mirror which needed to be ground out in the center in order to make a parabolic curvature. It was then polished and coated with a reflective material. These mirrors were very thick and heavy, even with their honeycombed internal structure.
This new process uses of a large rotating oven to contain the mold and glass, heat it to melting, let it cool for several months, all while spinning at a constant rate. They can form a mirror of 8.4 meters in diameter. This short video will demonstrate the principle of spinning a liquid to make a parabolic curved surface. Credit to the Univ. of Nebraska-Lincoln Astronomy Education Group.
The start of the process is a large circular tub made of silicon carbide cement. Ceramic fiber lines this tub. The tub forms the bottom part of a furnace in which the glass is melted.
The tub is filled with about 1,700 alumina-silica fiber hexagonal cores that form a honeycomb structure. This loading process takes several weeks. Alignment and spacing is critical. There must be uniform distance between each core to allow the proper thickness of glass to melt into it. Each core has a cover. The cores grow taller the farther from the center they are positioned. Their tops must follow the parabolic shape of the final mirror surface.
Placing Glass in the Mold
About 20 tons of glass pieces are placed by hand into the curved mold. It comes from Ohara Optical in Japan. The glass is very similar to the Pyrex used for cooking at home. It has very little expansion or contraction with temperature changes. It becomes runny enough when melted to flow into the spaces in the mold and fill them with no air gaps. The company makes it in one ton batches, breaks it into these chunks, and ships the boxes to the Mirror Lab. The pieces are about the size of a fist.
A scaffold allows the workers to reach down and place the chunks close together. The circular mold is slowly turned to allow them to continue adding glass safely. Other workers bring the boxes of glass to the scaffold and remove the empty boxes. The glass loading process to the proper depth takes a full day. This is for the center mirror of the GMT.
The workers have nearly completed the loading process for this mirror. This example has no center hole. It will be one of the 6 mirrors around the outside of the GMT. Only the center mirror has a hole in the center. The process is the same either way.
Cover and Melt the Glass
A cover is lowered over the tub. Heating coils in the walls and lid of the circular oven raise the temperature to 1160°C (2120°F). It takes several days to raise the temperature to the desired level. When the glass begins to soften, rotation of the oven is started at 5 rpm. The desired temperature is maintained for several hours to make certain the glass melts and fills the entire mold with no air pockets. The glass is cooled slowly for three months. More rapid cooling would put strains in the glass and possibly cause fractures in the future.
Once cooled, the mirror casting is removed from the oven and tilted up on edge in a special frame. Workers use a high pressure water jet spray directed through holes in the mirror bottom to remove the white hexagonal cores from within the honeycomb structure of the glass.
Grinding, Polishing, and Coating
The mirror casting is then ready for grinding the surface to the proper parabolic curvature for polishing and the addition of a reflective coating. The following brief video shows the rotating oven and some views of finished castings. It also shows a brief part of the grinding and polishing work. That work takes many months to complete.
The GMT is scheduled for completion in 2019. It will be on a peak in the Andes Mountains 8,500 feet above sea level. It will be located near several other telescopes at Las Campanas, Chile. For more detailed information, please follow this link to other documents.