GMT Casts a New Mirror

You are getting a first look at the newly cast 3.8 meter (over 12 feet) diameter "Fold Sphere" mirror. This mirror, which will be used for testing the accuracy of the 8.4 meter primary mirror segments, is itself larger than the Hubble Space Telescope's 2.4 m primary mirror.

GMT CASTS A NEW MIRROR - Tucson, AZ. This new mirror will be mounted near the top of a tower used for testing the accuracy of the primary mirror segments, see illustration at right. The tower, currently under construction, will be over 28 meters (92 feet) high.

The mirror, called a "fold sphere", is 3.8 meters (over 12 feet) in diameter. As a reminder of the scale of the Giant Magellan Telescope, this mirror, which is only used for testing the GMT, is itself larger than the Hubble Space Telescope's 2.4 m primary mirror. This picture was taken at the University of Arizona's Steward Observatory Mirror Lab.

The Giant Magellan Telescope's Primary Mirror is comprised of seven large mirror segments, each 8.4 meters (27.6 feet) in diameter. In order for the seven mirrors to work in unison as though they were a single mirror, the mirrors must be polished and tested to an extreme degree of accuracy. The difficulty is compounded by the fact that the six outer mirrors have an unusual highly asymmetric shape. The testing and measuring of the mirror segments pose some of the greatest manufacturing challenges. The new mirror and tower are a major part of the test arsenal used to achieve the required accuracies.

Below - The furnace for melting the E6 Borosilicate Glass (similar to "Pyrex") to cast the new 3.8 M mirror.