"When we started with the first mirror," McCarthy said, "there was a lot of learning going on, and it was a time when the industry was moving away from optical polishing as an art and more of a predictive science using computer-controlled polishing." That's right, robots took our glass polishing jobs. And the world is a better place for it.
"Computer-controlled polishing is now at the state of the art for small optics but less so for these very large mirrors," McCarthy continued. "Because there isn't as much of a large industrial throughput."
Through trial and error, the GMT team developed a sophisticated model for the material removal rate as a function of time (that is, how much you can grind away in x seconds), and the testing equipment needed to ensure it was accurate. Of course the team still independently measures the mirror's shape. Once the polishing run has been programmed, McCarthy said, the team typically scales it back by 30 percent "Because as the director of the mirror lab likes to remind us, 'It's easy to take the glass off, it's harder to put it back on.'"
The polishing tools themselves are computer controlled as well, changing shape several times per second. This is so "it always has the shape that we want while we're rubbing the glass," McCarthy explained. "These off-axis mirrors they have a different shape on every part of the glass, there is no symmetry." Hence the need for automation.
"We plan for this telescope to be revealing new discoveries of the universe for the next 50 years," Shelton exclaimed. "But with advances in computing and advances in electronics, there will be regular upgrades with new instrumentation. We've got a couple of generations of workhorse expectations out of the GMT."
The GMT is designed to observe light in the optical and near-infrared range. For older telescopes operating on that part of the spectrum, atmospheric distortion -- aka the phenomenon that makes stars twinkle -- would be a problem. But thanks to the GMT's built-in adaptive optics, that won't be an issue.
"The resolving power of your telescope, once you get above about a half meter in size, [due to atmospheric distortion effects] they're kind of all the same," McCarthy explained. "You're not realizing the full potential of the telescope." Adaptive optics, however, use lasers to sense the distortion and counter it in much the same way as noise-cancelling headphones block external sounds.
But the GMT won't be staring into the sky on its own; the observatory will collaborate with other telescopes both on the ground and in space. Take the Hubble's existing work with ground-based observatories, for example. "The Hubble can look at parts of the [light] spectrum that are difficult to observe from the ground, like the ultraviolet," Shelton explained. "The Hubble has unparalleled imaging quality but can't take spectra that are particularly interesting." However, by augmenting its abilities with facilities on the ground, astronomers have managed to make huge advancements in the cosmological field.
Once it gets up and running, the GMT may also turn its eye toward closer targets, potentially supplementing the observations of spacecraft orbiting the planets and moons of our solar system. "The interesting thing for solar system targets is how you connect the in situ measurements from spacecraft," McCarthy said. "With ground-based measurements with big apertures where we can take spectra and look for polarization."
The GMT team is also looking forward to working with the The Large Synoptic Survey Telescope, which is currently being constructed a few hundred miles away on the El Peñón peak of Cerro Pachón. The LSST will survey the entire sky every few days looking for things that move or change brightness, then the GMT will go back and study those objects in greater detail. "We think there's a great synergy between us and the SST," Shelton said.
"After that, the question is whether you can make something bigger," McCarthy interjected. "And never say never, only 15, maybe 20 years ago the Europeans were going to make a 100-meter telescope. They eventually scaled it back but that doesn't mean it isn't going to happen."
He expects that humans will eventually build 100-meter-plus telescopes, "Whenever people build the biggest telescope of their generation, they say 'this will never be outdone, this is as big as it can get.'"
Indeed, just two years after it begins operations, the GMT will lose its title of world's largest observatory to the European Southern Observatory's Extremely Large Telescope, when it and its 978-square-meter collection area, come online in 2024.
But even though it won't be the biggest for very long, the GMT will continue to upgrade its systems throughout its operational lifespan. McCarthy foresees further growth in aperture size as well as advances in adaptive optics. "We've got one adaptive mirror on the telescope, but you could put in multiple adaptive mirrors to push the adaptive optics to shorter wavelengths."
"There's also the emerging field of photonics, which are miniaturized self-contained optical systems that effective use every photon," Shelton said. "They work at the quantum level and have people thinking about photonic spectrographs, telescopes that work in the aperture plane rather than the focal plane. There's a whole interesting frontier of photonic applications, we just don't know which of those will bear fruit and which of those will be great ideas that are just not practical."