Third GMT Primary Mirror Segment Cast
In late August, the third GMT primary mirror segment was successfully cast in the rotating furnace beneath Wildcat Stadium at the University of Arizona. Nearly 20 tons of low-expansion glass was heated to the melting point, some 2100 F˚. While the furnace and honeycomb mold spun at 4.5 rpm, the molten glass flowed into the complex ceramic mold and settled into the approximate shape of the final mirror.
GMTO and the University of Arizona hosted three days of events around the mirror casting, culminating in a gala dinner at the La Paloma resort in the Tucson foothills. At the reception GMTO Board Member Edward “Rocky” W. Kolb described the construction of the world’s first nuclear reactor under University of Chicago’s original football stadium – Stagg Field.Kolb, the University of Chicago’s Dean of Physical Sciences, compared the 1942 technological advancement to the impressive work being done in the Steward Observatory Mirror Lab, which resides under the east wing of the University of Arizona football stadium.
Kolb’s message: Important scientific breakthroughs take place under, rather than on, football fields.
In the beautiful banquet hall, complete with a dazzling ice sculpture modeled after the mirror, the comparison between the nuclear reactor and the mirror lab was partly poetic, but GMT’s primary mirrors are indeed technological marvels.Among the largest in the world, each of the telescope’s seven mirrors will span 8.4 meters (or 27.5 feet) in diameter. They will be lightweight, rigid, and able to quickly adapt to their surrounding temperatures, thanks to a honeycomb-shaped inner core design that includes hollow spaces throughout. Each mirror will have such a smooth surface that the typical imperfection will be no taller than 20 nanometers (less than one millionth of an inch) high.
In a video entitled “A Perfect Mirror,” Wendy Freedman, Chair of the GMTO Board of Directors, describes the high level of precision:
“The surface of this mirror is so smooth that if we took this 27-foot mirror and then spread it out from coast to coast—in the United States, east to west coast—the height of the tallest mountain on that mirror would be about half an inch.”
Reaching this level of near-perfection requires cutting edge technology to cast, to polish, and to test the mirror. (For more on the process, click here.)
During a presentation at breakfast on the casting day, Project Director Patrick McCarthy noted that casting the third mirror keeps the project on schedule for first light in 2020 and that three of the seven mirrors alone would make GMT the largest telescope in history.
Guests at the event had the privilege of touring the Steward Observatory Mirror Lab while staff was on hand to explain each step in the process. They watched as 18 tons of hand-inspected glass melted and filled a ceramic mold inside the enormous spinning furnace that reached 1148 C˚ (2100 F˚).
They saw the complex polishing equipment suspended over another mirror, with its highly sensitive contact surface capable of making a thousand adjustments per second to ensure that the tiniest of imperfections are smoothed away without harming the rest of the mirror.
They saw the tower of optics, holograms, and laser ranging devices specifically constructed to test GMT’s highly off-axis mirrors before they are ready to leave the lab.In attendance were distinguished guests, including top leaders from the GMTO consortium institutions: Astronomy Australia Ltd., Australian National University, Carnegie Institution for Science, Harvard University, the Korean Astronomy and Space Science Institute, the Smithsonian Institution, Texas A&M University, the University of Arizona, the University of Chicago, and the University of Texas at Austin.
The event was also attended by Tristan Bullard, this year’s official Eagle Scout Astronomer selected by the National Eagle Scout Association, and an anonymous supporter—possibly a member of the hotel staff—who left an unsigned note on one of the reception tables:
I didn’t get a chance to say this in person, but on behalf of all humanity I thank you for your contribution to this project.The third mirror, dubbed GMT3, is currently cooling and will be removed from the furnace hearth in December of this year. The mirror will then go through the polishing and testing phases, which will take approximately four years to complete, before making its journey down to the Las Campanas Observatory in Chile.
The weekend event included a tribute to George Mitchell (1919 – 2013), a long-time supporter of the Giant Magellan Telescope. To honor George and his wife Cynthia Woods Mitchell for their support for the project, two of Giant Magellan’s seven primary mirrors will be named after them. (See the tribute here.)
A Distinguished Scientist Returns to the GMT Project
GMTO recently announced the appointment of Rebecca A. Bernstein as GMT Project Scientist. She joined the organization on November 1st to provide technical and scientific leadership for the design and construction of the Giant Magellan Telescope.
For someone who only recently moved into her office, Bernstein already knows a lot about the Giant Magellan project. While at the University of Michigan she was a member of the core design team that developed the original concept for the GMT. Indeed, Bernstein’s appointment as Project Scientist – and her move to Pasadena – is more of a homecoming than a new arrival.
After earning a B.S. in physics from Princeton University and a Ph.D. in astrophysics from the California Institute of Technology, Bernstein took a prestigious NASA Hubble Postdoctoral Fellowship to the Carnegie Institution for Science. “I lived in Pasadena longer than I’ve lived anywhere since I graduated from high school. It definitely still feels like home.”
At Carnegie, Bernstein led the development of a high-resolution spectrograph for the 6.5m Clay (Magellan II) Telescope at Carnegie’s Las Campanas Observatory in Chile—the same observatory where the Giant Magellan Telescope is being constructed. Talking about that period, she recalls, “That time at Carnegie was really a high point for me. I was learning a lot really fast working with Steve Shectman on MIKE, and it was an exciting time to be at Carnegie with the Magellan telescopes starting operations.”
After finishing her Hubble fellowship, Bernstein became a faculty member at the University of Michigan. Her research was focused on developing new methods for measuring chemical abundances of stars in other galaxies, and she stayed heavily involved with the Magellan telescopes. From 2004 to 2007, she was also part of the Project Scientists Working Group that started the Giant Magellan Telescope project.
“The GMT partnership is a group that I have a lot of experience with and that I’ve enjoyed working with in the past. I’m looking forward to being in that partnership again!”
Talking about her new role as Project Scientist, Bernstein doesn’t rush into the details of a fixed job description. “Project scientist means different things in different organizations. The roles were very different within Gemini, Keck, and Magellan. I’m excited to see how the role will evolve in GMT.” But, she says, her background in instrumentation and optical design will enable her to have more technical input than project scientists typically have. That will extend to the instruments too.
Dr. Wendy Freedman, Chair of the GMTO Board said “Dr. Bernstein brings a unique combination of technical excellence and scientific breadth to a critical leadership position within the project.”
Ten universities and research institutes make up the international consortium working to complete the Giant Magellan Telescope. The partners were also a draw for Bernstein.
“There’s a sense of community in this telescope project that I value very much. And that’s really the culture that started at Carnegie and with the Magellan project, and I think it has grown with GMT.”
In October Bernstein finished her role as the Principal Investigator for the wide-field, optical spectrograph for the Thirty Meter Telescope project at the University of California, Santa Cruz. As part of her new Project Scientist appointment, Bernstein also joins the research faculty at the Observatories of the Carnegie Institution for Science.
GMT’s Adaptive Optics System Passes Major Milestone
Giant Magellan’s Adaptive Optics system is one of the telescope’s most complex components. The system passed its Preliminary Design Review in July when an external panel of international experts gave the team a strong endorsement. Keith Raybould, GMT Project Manager, reports that the panel pronounced the AO system ready to proceed to the next level of development, and that they felt that all major technical risks have been addressed.
The effect of the Earth’s atmosphere on ground-based observation of distant objects is witnessed in the twinkling of stars. Light rays traveling to the Earth’s surface from a distant object are distorted by the temperature and density inhomogeneities in the atmosphere. It’s comparable to a large crowd of people (the incoming light rays) all trying to cross a busy street (the atmosphere). Even if they all start crossing at the same time, they won’t all reach the other side together, as some slow down and others speed up to avoid the passing traffic.
As a consequence, images taken through the atmosphere are typically blurred by about 1 arcsecond, the size of a quarter seen from 3 miles (5 km) away. Finer details in astronomical objects cannot be distinguished without correcting for these atmospheric effects.
To overcome this obstacle, the Giant Magellan Telescope will make use of a technology known as Adaptive Optics.
The Adaptive Optics system of GMT includes all of the equipment dedicated to correcting the image quality degradation caused by the atmosphere.
At the heart of the system are seven secondary mirrors, each equipped with 674 electromagnetic actuators that can deform the mirror faces. This shape-changing is the basis for the adaptive ability of the system: the mirrors can adjust themselves to correct for the timing errors of the incoming light.
Revisiting the busy street analogy, the Adaptive Optics system serves to alter the distance each person has to travel by changing the width of the street at different points. The flow of traffic is unchanged, but each person is able to cross the street in the same amount of time.
The challenge, as GMT Adaptive Optics Scientist Antonin Bouchez notes, is that the adjustments in distance must be accurate to nanometers – a few billionths of a meter – and updated over 1000 times per second.
The commands to the adaptive secondary mirror are computed using high-speed sensors that analyze the light of bright stars near the scientific target. If no bright stars are available, lasers are used to generate artificial stars in the Earth’s upper atmosphere. The sensors and laser projection systems were also included in the Preliminary Design Review.
As part of their design work for the Adaptive Optics System, the team carried out studies that included long-term characterization of atmospheric conditions at Las Campanas Observatory, wind tunnel testing of a scaled model of the telescope, and extensive computer modeling. The team included the GMT AO Group, scientists and engineers at the Australian National University, Harvard-Smithsonian Center for Astrophysics, Arcetri Observatory, and the University of Arizona, along with consultants and industrial contractors.
GMT Project Director Patrick McCarthy said, “The successful review by a distinguished panel of experts verifies what we have known for some time – we have an outstanding AO team and a great design.”