Above: The first of seven mirrors for the Giant Magellan Telescope after removal from the furnace. The back surface of the mirror is shown here during an inspection of the holes used to ventilate the mirror during operation in the telescope. (Photo: Ray Bertram/UA)
The Giant 8.4-meter advanced Mirror has been polished at the Steward Observatory Mirror Lab, below the Arizona Wildcats football stadium.
Scientists use a great oven to spin-cast molten glass, in order to produce the most perfect telescope mirrors in the world.
The first of seven mirrors for the Giant Magellan Telescope being polished at the UA Steward Observatory Mirror Laboratory. The polishing head, shown center, changes shape to match the curvature of the mirror as it moves across the surface. (Photo: Ray Bertram/UA)
Ceramic molds placed on the GMT mirror during construction
Giant Magellan Telescope atop a mountain in Chile’s Atacama Desert.
From the University of Arizona:
For the past several years, a group of optical scientists and engineers working at the UA Steward Observatory Mirror Laboratory underneath the UA’s football stadium have been polishing an 8.4-meter (27 ½ feet) diameter mirror with an unusual, highly asymmetric shape.
By the standards used by optical scientists, the “degree of difficulty” for this mirror is 10 times that of any previous large telescope mirror. The mirror surface matches the desired prescription to a precision of 19 nanometers – so smooth that if it were the size of the continental U.S., the highest mountains would be little more than a half-inch high.
This mirror, and six more like it, will form the heart of the 25-meter Giant Magellan Telescope (GMT), providing more than 380 square meters, or 4,000 square feet, of light-collecting area. The Giant Magellan Telescope will lead a next generation of giant telescopes that will explore planets around other stars and the formation of stars, galaxies and black holes in the early universe.
Image credit: Giant Magellan Telescope atop a mountain in Chile’s Atacama Desert.
Buell Jannuzi, director of the UA Steward Observatory and professor of astronomy, said, “Making this first GMT mirror required all the expertise and experience that the University has built up over 25 years of making telescope mirrors and a great deal of innovation to push beyond previous limits in optical fabrication and testing. In achieving this remarkable milestone, the team built and demonstrated all the equipment and techniques that will lead to efficient production of the remaining mirrors for the GMT.”
The mirror was cast at the mirror lab from 20 tons of glass, melted in a rotating furnace until it flowed into a honeycomb mold. Once the glass had cooled and the mold material was removed, scientists at the lab used a series of fine abrasives to polish the mirror, checking its figure regularly using a number of precision optical tests.
The mirror has an unconventional shape because it is part of what ultimately will be a single 25-meter (82 feet) optical surface composed of seven circular segments, each 8.4 meters (27 ½ feet) in diameter.
“We need to be certain the off-axis shape of this mirror, as well as the other six that will be made for GMT, is precisely right, to an accuracy of 1/20 of a wavelength of light,” said Buddy Martin, polishing scientist at the Mirror Lab. “Only then will the seven large mirrors form a single, exquisitely sharp image when they all come together in the telescope in Chile. We have now demonstrated that we can fabricate the mirrors to the required accuracy for the telescope to work as designed.”
source University of Arizona