The world’s largest telescope is almost complete

This week, the Steward Observatory Mirror Lab at the University of Arizona began work on the sixth of the seven primary mirror segments for the Giant Magellan Telescope (GMT). On March 1, the lab began heating its one-of-a-kind glass furnace to a temperature of 1,165 degrees Celsius. This in itself was an important step in a manufacturing process known as spin molding. It took about four months to make the mold and another nine hours to coat it with almost 90 tons of rare borosilicate glass.

On Friday, the oven began to spin at about five revolutions per minute. The combination of heat and movement will force the glass out the sides of the mold as it melts, causing it to form a curved surface. Once the 8.4 meter mirror is cast over the weekend, it will go into a month-long “annealing” process that will see the furnace slowly come to a stop as the glass cools. This is done so that the mirror cools evenly, making the end product tough and free from as many blemishes as possible. It will take another month and a half for it to cool to room temperature. This is when the long polishing process can begin.

It takes about four years to manufacture a single segment of glass. In 2019, the university completed its work on GMT second mirror. This is now in stock, awaiting transport to Chile. While the project innovated in 2015, it won’t be complete until later in the decade. But once that is done, the GMT will be able to capture images 10 times clearer than those produced by the Hubble Space Telescope.

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