A team of University of Arizona astronomers has produced the clearest visible- light pictures of stars and an exoplanet ever taken, using an adaptive-optics system to take out the blurring effects of Earth’s atmosphere.
They did it with a broken and repaired secondary mirror that is programmed to reshape itself to compensate for turbulence caused by the atmosphere.
The team, led by astronomer Laird Close of the UA’s Steward Observatory, used a “repurposed” mirror that had been broken before it could installed at the UA’s Large Binocular Telescope.
They took it to the high, dry Las Campanas Observatory in the southern Atacama Desert in Chile and installed it above the 6.5-meter (21-foot) primary mirror of the Clay Magellan Telescope, one of twin telescopes at the site whose mirrors were cast at the UA’s Steward Observatory Mirror Lab.
The combination of a prime site, a superbly functioning telescope and an adap tive optics system that worked as well as their models predicted, allowed them to take “the highest-resolution images ever taken on a telescope,” Close said.
The system resolved images down to 0.02 of an arcsecond, the diameter of a dime seen from 100 miles away, Close said. The apparent size of the moon, for comparison, is about 2,000 arcseconds across.
The team published its first science results Wednesday in Astrophysical Journal, based on data from an observing run in late 2012 that focused on the Orion Nebula (M42).
“To be honest, I was very skeptical that we would be able to do this,” said Close. “The National Science Foundation rejected it two times. They said it’s not going to work.” Close eventually won $3 million in awards from the NSF to complete the work.
Their first target was the brightest star in the system, Theta 1 Ori C, actually known to be two stars but never before seen in a visible image.
When they “closed the loop,” using the adaptive secondary system to take out the aberrations caused by the Earth’s atmosphere, it appeared clearly as two stars.
“It was exciting,” said Close, although photos posted on the team’s website show a serious-looking bunch observing the results.
Jared Males, a NASA Carl Sagan Fellow who was in charge of doing computer simulations of the system and creating the software to make it work, said he did not have time to celebrate.
“It was like ‘OK, it works.’ I was just frantically trying to make sure the data was storing.”
Males stayed up long past the others once the system was functioning, Close said. “I finally get this thing to look at the stars I wanted to look at for years. I wasn’t going to sleep,” Males said.
Males, who earned his doctorate on the project, was recently selected as a Sagan Fellow for his work on VisAO. He plans to use his three-year stipend and research budget to perfect the VisAO technology and use it to look for Jupiter-sized planets in the habitable zones of their stars. “I have about 20 good candidates,” he said.
Males has already taken photographs of one of them with the visible-light system. The scientific paper on it won’t be published until later this year, but preliminary results have already been presented at astronomical conferences.
The unpublished exoplanet work is “probably the most exciting work we’re doing,” Close said.
The results are a culmination of work done over the past 20 years by astronomers at Steward, working with partners at Arcetri Observatory in Florence, Italy, Close said.
Adaptive optics systems they developed are available on the MMT Telescope on Mount Hopkins and on the Large Binocular Telescope on Mount Graham.
The earlier systems work in longer wavelengths in the near infrared. Close said his VisAO is the first time UA astronomers have used adaptive optics to look “in the blue” — the shorter wavelengths visible to the human eye.
“The big step forward is the shorter wavelengths. It allows us to make even sharper images. We can get some of highest resolution images possible,” said Phil Hinz, director of Steward’s Center for Astronomical Adaptive Optics.
Hinz, who recently combined the two giant 8.4-meter eyes of the LBT and its adaptive mirrors, said Close’s results are “sharper resolution than anything the LBT has accomplished.”
“One of the things it has shown us how to do is to really fine-tune a system to work at shorter wavelengths.” He said he plans to use those new techniques on the LBT.
“This is a great example of the long-term investment we and the Italian group have made in this technology,” Close said.
Close and his graduate students moved to Florence for a year to test the system, pairing it with a wave-front sensor developed by their Italian partners. The wave-front sensor sites a nearby guide star and measures the atmospheric turbulence, then sends messages to the deformable secondary mirror to adapt its shape.
In Florence, the team lived together in a big house with Close, his wife and two children to save money.
“The project was severely underfunded,” said Close. The big savings came when the group found it could repurpose the “shell,” a thin high-tech mirror built for the Large Binocular Telescope on Mount Graham.
Originally, it was a 36-inch mirror, only 1.6 mm (about six-thousandths of an inch) thick, with 672 computer-controlled acutators capable of changing the shape of the mirror 1,000 times a second to compensate for atmospheric turbulence.
It was broken only on the edge, and technicians at Steward Observatory cut it to produce a mirror about 33.5 inches in diameter with 585 actuators.
Charles Beichman, director of the NASA Exoplanet Science Institute at Caltech, called the breakthrough “a fabulous direction for adaptive optics.”
“We started out doing it in the infrared, and the race now is to push down to shorter wavelengths. A combination of bigger telescope primary mirrors and shorter wavelengths is needed to image planets orbiting stars, he said. “You really start to buy improvements in your ability to resolve two stars together or get rid of the light of a star and be able to see a planet.”
A second paper published Wednesday, authored by UA graduate student Ya-Lin Wu, used the VisAO to record light from ionized hydrogen gas being blown away from two smaller stars by the more massive Theta 1 Ori C.
A third paper, whose principal author is UA doctoral student Kate Follette, examined the silhouette or dust disk around another star in Orion.
“The result that’s exciting to us is to find out how the mass of this disk changes as you get farther out. This is actually the holy grail for people who study dust around objects,” said Close.