The world’s largest telescope is edged closer to completion

This week, the University of Arizona’s Steward Observatory Mirror Lab began work on the sixth of seven primary mirror segments for the Giant Magellan Telescope (GMT). On March 1, the laboratory began heating its unique glass furnace to a temperature of 1,165 degrees Celsius. That in itself was an important milestone in a manufacturing process known as spin-casting. It took about four months to manufacture the mold and another nine hours to cover it in nearly 90 tons of rare borosilicate glass.

On Friday, the oven began rotating about five revolutions per minute. The combination of heat and movement will force the glass up 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 sees the furnace slowly stop while the glass cools. This is done so that the mirror cools uniformly, making the final product tough and free of as many imperfections as possible. It takes another month and a half before it cools down to room temperature. It is at this point that the long process of polishing can begin.

It takes about four years to manufacture a single glass segment. In 2019, the university completed work on GMT’s second mirror. One is now in stock waiting to be shipped to Chile. While the project broke ground in 2015, it is not complete until later in the decade. But once done, GMT will be able to take images that are 10 times clearer than those produced by the Hubble Space Telescope.