GMT’s Second Mirror is Cast at UA’s Steward Mirror Lab
Loading the Ohara E6 glass into the UA's Steward Mirror Lab furnace
(Photo: Ray Bertram/Steward Observatory)
The second of GMT’s seven 8.4 meter (27-foot) diameter primary mirrors was cast on January 14th inside a rotating furnace at the University of Arizona's Steward Observatory Mirror Laboratory. Underneath the football stadium, in the Mirror Lab, the humming furnace (the size of a fairground carousel) spun, while inside 21 tons of glass, heated to 2,140 degrees Fahrenheit, slowly melted and flowed into a complex mold to create the giant honeycomb telescope mirror.
The glass reached its peak temperature around 9PM on Saturday the 14th and was held there for four hours to allow bubbles to clear and the glass to flow into all areas of the mold. During the high temperature phase the furnace spun at 5 RPM to maintain the desired parabolic shape of the glass. Shortly before 1AM the heat was lowered and the glass began cooling. The temperature in the furnace will be closely controlled during a three-month annealing phase. Cameras inside the furnace reveal a clear and apparently flawless front surface to the mirror blank, indicative of a successful casting. The GMT2 mirror will soon join the completed GMT1 primary mirror segment in the processing areas of the mirror lab. Materials for the third segment, GMT3, will begin arriving in Tucson in the coming months.
"Astronomical discovery has always been paced by the power of available telescopes and imaging technology," said Steward Observatory Director Peter Strittmatter. "The GMT allows another major step forward in both sensitivity and image sharpness. In fact the GMT will be able to acquire images 10 times sharper than the Hubble Space Telescope."
The Casting Pilot monitors conditions inside the furnace during the high temperature phase
(Photo: Ray Bertram/Steward Observatory)
Like other mirrors produced by the mirror lab, the GMT mirrors are fabricated in a process called spin casting, achieving the basic front surface in the shape of a paraboloid, the form taken on by water in a basin when spun around its axis.
"This is mirror No. 2, so one could ask, ‘What's the big deal?' Strittmatter said during a news conference on Saturday. " We had to demonstrate that one could actually make such off-axis mirror segments. That concern has been retired. We have succeeded and demonstrated we could fabricate GMT mirror No. 1."
As the furnace spins, technicians in the adjacent control room gather around a computer monitor to watch video taken by a camera looking inside the oven.
First, chunks of glass begin to melt together. Soon, a pool of liquid glass forms over an underlying honeycomb structure pre-assembled from a specialized heat-resistant material called alumina silica fiber board that looks a bit like Styrofoam. And then, rather suddenly, the surface drops, as the molten glass becomes liquid enough to run into the honeycomb scaffold, filling the space between the honeycomb mold structure.
The rotation of the lab’s one-of-a-kind furnace ensures that the glass starts out close to the desired parabolic shape. Two hundred seventy heaters inside the furnace ensure the glass is heated evenly throughout, explained Kirk Kenagy, who helped build the giant rotating oven in 1984.
Later that day, the engineers and technicians initiated the cool-down phase. Once the temperature drops below 1,000 degrees C, the glass starts to solidify. Slow, steady rotation is necessary even after the glass has started to cool to ensure steady air flow inside the oven and uniform cooling throughout the mirror blank, explained Bruce Hille, the mirror lab's facility manager.
GMT Project Director Patrick McCarthy said, “This second GMT casting is going forward now because the primary optics are on the critical path for the project. The polishing of the first off-axis 8.4-meter GMT mirror is essentially complete, with an optical surface accuracy within about 25 nanometers, or about one-thousandth the thickness of a human hair."
The full press release for the GMT2 casting can be found in the “For Press” section of this web site.
A gallery of images from the casting can be found in the “Resources” section of this web site.
GMT Instrument Conceptual Design Reviews
Laura Ferrarese, Simon Morris and Natascha Förster Schreiber pass around a model of the GMT Integral-Field Spectrograph
For the past several years, scientific and technical teams from across the GMT partnership have been developing instrument concepts for the GMT. This phase was completed in October 2011 with a series of conceptual design reviews by experts from around the world. The six reviews included participation by 34 panelists from the US, Europe, Australia, and South Korea. Many of the reviewers are active participants in major projects of their own, including E-ELT, TMT, and LSST.
Dan Jaffe (UT Austin) leading a discussion
on gratings during the GMTNIRS review
Each review lasted two days and examined the instrument’s scientific potential, the technical approach adopted by the team and program management, including cost and risks. The review panels presented their findings to the teams and the GMT Project Office. The reports highlight the many scientific opportunities offered by these instruments and their innovative designs.
The panel reports, along with a response from the teams, are being considered by a high-level panel that will recommend a roadmap for instrument development during the construction and early operations phases of the project. In addition to the panel reports they will consider scientific priorities identified in the GMT science case and operational issues and commissioning plans as they consider how to best maximize the scientific return from the facility.
The University of Texas at Austin: A Unique Astronomy Program Grounded in Tradition
The University of Texas McDonald Observatory. The two large domes in the foreground house the Otto Struve Telescope (left) and the Harlan J. Smith Telescope (right). The Hobby-Eberly Telescope (HET) can be seen between the two, atop Mt. Fowlkes
It’s been more than eight decades since Paris, Texas banker William J. McDonald left most of his estate to an unsuspecting University of Texas at Austin to establish an astronomical observatory. Today McDonald Observatory hosts the consortium-run 9.2-meter Hobby-Eberly Telescope, which is currently being upgraded to begin the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). The observatory’s other research telescopes include the 2.7-meter Harlan J. Smith Telescope, the 2.1-meter Otto Struve Telescope, and several smaller instruments. Program scientists have access to one of the world’s best supercomputing facilities: the university’s Texas Advanced Computer Center in Austin.
In 2004, the university joined the Giant Magellan Telescope (GMT) consortium. Funding for consortium participation has come from various offices within the university, and a gift of $1 million dollars from George P. Mitchell of Houston. The University of Texas intends to have a ten percent share of the GMT and is working to raise the $70 million dollars needed to do so.
The University of Texas astronomy program was founded in the 1960’s and has grown to be one of the leading programs in the country. The program is made up of both the Department of Astronomy and McDonald Observatory. The department includes twenty faculty members, sixteen postdoctoral fellows, dozens of graduate and undergraduate students, and a small number of support personnel. The observatory is comprised of twenty research scientists and around 100 support personnel in Austin and West Texas.
The McDonald Observatory and department work together to do research across topics ranging from dark energy (the field of the 2011 Nobel Prize in physics), to Earth-like planets around other stars and to the nature of comets in our own solar system. This work takes place at the Observatory, on space facilities like the Hubble space telescope and other satellites, at national and international observatories, as well as with supercomputers and even pencil and paper.
In addition to educating college students, producing significant research, and building and maintaining telescopes and instruments, the UT astronomy program runs a substantial public outreach program. This includes StarDate Radio, the longest running nationally syndicated science program in the United States, StarDate Magazine, the Frank N. Bash Visitors Center at McDonald Observatory which welcomes 50,000 visitors annually to daily tours and thrice-weekly star parties, as well as continuing education programs for elementary and secondary science teachers.
GMT director Patrick McCarthy says the university brings valuable skills to the project. “UT is one of the unique, or nearly unique astronomy departments left in the United States in which you have an outstanding strong scientific group –they do theory, they do observation – and is a group that still knows how to build scientific instruments, how to build and operate telescopes. UT has classic, old-school, experimental astronomers who build their own instruments. They go out and do their own observations, and they and their postdocs and graduate students write great scientific papers. That’s the kind of tradition we see as vital to a successful project like the GMT.”
The university is involved in instrument development for GMT. UT Astronomy Department Chairman, Dan Jaffe leads a team of scientists studying a powerful new kind of spectrograph for the GMT, GMTNIRS or the GMT Near Infrared Spectrograph. This is one of six candidate instruments, and is being developed under a half-million dollar design contract funded by the GMTO. This study is being jointly carried out by The University of Texas at Austin, The Korea Astronomy and Space Science Institute and Kyung Hee University.
Visit UT Austin’s Website
Preparations are underway to begin work on the site at Las Campanas. The U.S. Ambassador to Chile and other dignitaries will be visiting Las Campanas in the coming weeks to view the GMT site. See the GMT Spring Newsletter for details.
The Instrument Development Advisory Panel will meet in Pasadena in early February.
The GMT Project is looking for highly-motivated and qualified individuals to work on this world-class facility. Find out about current open positions.