Big Telescope Mirrors | Spin Casting

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.

GMTO Corporation 251 S. Lake Avenue, Suite 300 Pasadena, CA

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 Tub

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.

Stewart Observatory Mirror Lab – SOML

Hexagonal Forms

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.

Melinda & Dean Ketelsen

Melinda & Dean Ketelsen

Melinda & Dean Ketelsen

Melinda & Dean Ketelsen

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.

SOML – Ohara Optical

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.

Melinda & Dean Ketelsen

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.

Completing GMT

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.


18 thoughts on “Big Telescope Mirrors | Spin Casting

    • I agree. It would be so awesome to be part of that process and make a mirror half as wide as my house. I’d love to see the entire telescope when it is completed with 7 of those mirrors working together.

      Thanks for your visit and comment.


      • Right. The process of watching the mirrors be put in place would be a true sight. Let alone building the telescope around them. So fascinating, and it was my pleasure. Thanks for posting.


  1. This is fascinating! I have a friend in Hawaii who, along with his partner, at times will get a job to manufacture mirrors for telescopes in their shop, though I don’t think it’s on this scale! He’s tried to explain the process to me, but I now have a much better understanding….and this means I have like a million questions for him!


    • That’s good. Glad to stimulate some questions. I have never ground my own mirror. It takes a long time. This spin casting process cuts it down a lot.

      Thanks for stopping by, Alex.


  2. Jim in IA, I notice in the sketch that there are no barrel enclosures shown to protect the mirrors. Will there be such? Or is the building itself sufficient to exclude contamination from the elements? I thought about birds too, but a little research shows that, while they are capable of flying at 8,500 feet and that migratory birds fly well over that, they are unlikely to be a problem. I wonder if there’s some built-in mechanism for cleaning? Wiper fluid, anyone?


    • Here is a document that answers the first part of your question regarding the enclosure building. Maintaining as little temperature range as possible is a challenge. The astronomers do not want to be waiting for cool down.

      As to your cleaning question, there will be special handling and cleaning equipment for that job. Dust and grime will coat the surface over time. Here is a video from the Very Large Array in Chile showing how the mirror is removed and taken to a different building for cleaning and re-coating. How often likely depends on several factors.

      Thanks for those good questions.


    • The deeper I dug into the process, the more complexity I found. That seems a common theme. It makes it all the more interesting to find out the amazing skills and talents of people.

      Thanks for coming by for a look.


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