Astronomers have successfully tested a new method to remove atmospheric blurring from large, ground-based telescopes.
The experiments were made in November 2002 and January 2003 at the 6.5-meter (21-foot) telescope at the MMT Observatory on Mount Hopkins, Ariz. The project is a collaboration of the University of Arizona's Steward Observatory and Italy's Osservatorio Astrofisico di Arcetri in Florence. It uses revolutionary new technology developed with support from the U.S. Air Force.
Large ground based telescopes can make images as sharp as or sharper than the Hubble Space Telescope, but only if atmospheric blurring is corrected. Previously, the deformable mirrors available to do this were small, flat, and relatively inflexible. They could be used only as part of complex instruments attached to conventional telescopes.
But in this new work, one of the two mirrors that make up the telescope optics is used to make the correction directly. The new secondary mirror makes the entire correction with no other optics required, making for a more efficient and cleaner system.
Like other secondary mirrors, this one is made of glass over 2 feet in diameter and is a steeply curved dome shape. But under the surface, it is like no other. The glass is less than 2 millimeters thick (less than eight-hundredths of an inch). It literally floats in a magnetic field and changes shape in milliseconds, virtually real-time. Electro-magnetically gripped by 336 computer-controlled "actuators" that tweak it into place, nanometer by nanometer, the adaptive secondary mirror focuses star light as steadily as if Earth had no atmosphere. Astronomers can study precisely sharpened objects rather than blurry blobs of twinkling light.
"The reason no one has done this before is because it turns out to be enormously technically challenging," said Michael Lloyd-Hart of the UA Center for Astronomical Adaptive Optics (CAAO), project scientist for the MMT adaptive optics system. Lloyd-Hart and chief system engineer Francois Wildi led the design and engineering effort. "We found out the hard way just how difficult it is. It's taken us a number of years to build a mirror with a shape that deforms in real time," Lloyd-Hart said.
One of the system's unique features is its ability to make corrections according to the detail of the distortion measurements. If very faint stars provide no useful data, the shape can be fixed to mimic a conventional mirror of solid glass. The mirror can also be used to rapidly "chop" the viewpoint of the telescope, as required for infrared imaging.
"It's been 25 years or so since anyone's tried a radically new way of building a deformable mirror, and this technology really is different, " said Laird Close, UA assistant professor of astronomy and CAAO scientist. "But the ability to build large, curved, deformable mirrors for adaptive optics is a boon to astronomers using the giant new ground-based telescopes."
"The adaptive secondary mirror made for the UA and Smithsonian Institution's 6.5-meter telescope at the MMT Observatory is a tremendous advance over conventional adaptive optics, which are systems that involve extra relay optics and mirrors in a box separate from the telescope, Lloyd-Hart said.
"One key feature is that we correct for blurring effects of the atmosphere at a mirror which is an integral part of the telescope," Lloyd-Hart said. "The conventional approach has been to build the telescope, then build a box of optical tricks to improve the resolving power of the telescope."
The new system solves a big problem astronomers face when they try to observe at longer infrared wavelengths, where special targets like Jupiter-like planets and circumstellar disks are brightest.
"Everything in the world glows with thermal energy, or heat. When you try to do astronomy at those wavelengths, even the optics that you are looking through glow. So the more optical surfaces and telescope parts you can eliminate -- the simpler the system is optically, the better it is for doing infrared astronomy," Lloyd-Hart said.
The Steward Observatory Mirror Lab made the MMT's large deformable convex, aspheric secondary mirror. Learning how to make glass 2mm thick so that it's "infinitely floppy" was a major challenge to building the system, Lloyd-Hart said.
The biggest equivalent flexible mirror available commercially at this time is 15 inches (38 cm) across, Close said.
But once Steward Observatory researchers realized how to do it, they also realized they could make big deformable mirrors for use in space. Steward Observatory has been developing new space-based optics as a spin-off of this ground-based technology.
The Italian partners designed the magnetic flotation system and the very powerful computer electronics that drive the MMT's adaptive secondary mirror with a cluster of 168 microprocessors, which are all packed in an electronics crate that is mounted behind the secondary mirror. The computer cluster is essentially a supercomputer, more powerful than any computer available during the Apollo space era. It senses the positions and drives the actuators.
Guido Brusa, CAAO/Large Binocular Telescope adaptive optics scientist, said that he is "personally very happy" with the results of effort, achieved during the past 7 to 8 years through "patient and persevering work of many people in both Arizona and Italy. It is great to see that the adaptive secondary mirror performs beautifully, even in the presence of relatively strong wind (20 to 30 mph) and in environmental conditions very different from those in the lab."
Brusa called the success "an important step" in integrating adaptive optics into an astronomical telescope. An adaptive primary telescope mirror is "a foreseeable future development," he said.
In the MMT's new adaptive optics, a wavefront sensor camera mounted at the base of the telescope senses atmospheric turbulence and sends that information to the MMT adaptive secondary mirror. The powerful computer cluster behind the secondary mirror sends electronic current through coils so that each of 336 actuator magnets spaced across the mirror is instantly moved to the desired position. The result is a "flat," non-wavy wavefront seen by the astronomer's science camera.
The unique adaptive optics system also includes its own plumbing. "Its half-water, half-methanol liquid cooling system can dissipate up to a kilowatt of heat," chief system engineer Francois Wildi said.
Detwinkling and Blocking Starlight
UA astronomer Phil Hinz' observing run Jan. 22 illustrates why the new adaptive optics system is ideal if you're looking for planetary disks or planets around bright, nearby stars.
On Jan. 22, Hinz and UA astronomer emeritus Bill Hoffmann took Hinz' nulling interferometer called "BLINC" and Hoffman's infrared "MIRAC" camera to the MMT, while UA, Italian and Smithsonian Institution MMTO staff ran the new adaptive optics.
"Without adaptive optics, nulling interferometry is able to suppress the star to only 5 to 10 percent of its original brightness," Hinz said. "In addition, the intensity of the star rapidly changes because of atmospheric turbulence, so the star appears to blink on and off."
Nulling interferometry works by creating two "sub-telescopes," both looking at the same bright star, but positioned so starlight from each sub-telescope travels in slightly different paths before hitting the detector. When properly aligned, crests of lightwaves from one sub-telescope will line up with the troughs of the lightwaves from the other, in effect canceling the light of the bright star.
Hinz, who used the 6.5-meter MMT as two 3-meter sub-telescopes, said the initial Jan.22 observations were successful in showing the power of adaptive optics to stabilize the star and suppress all but two percent of its light.
"Once we've refined this technique, we should be able to stabilize and suppress all but one-tenth of a percent, down to three-hundredths of a percent of the starlight and see faint, planetary dust disks much like our own solar system around nearby stars," Hinz said.
"Our own dust disk is about one-hundredth of one percent of the brightness of the sun, which sets the ultimate goal of this technique. This is the level of suppression we're aiming for with the Large Binocular Telescope Interferometer," he added.
Adaptive Optics for the LBT
University of Arizona scientists are developing two adaptive secondary mirrors for the Large Binocular Telescope (LBT) on Mount Graham, said UA astronomer John Hill, who directs the LBT project. The LBT won't have conventional secondary mirrors, Hill said. Each of the LBT's 8.4-primary mirrors will have an adaptive concave (rather than convex) secondary mirror 91 cm (36 inches) across, held by 672 actuators that will bend it moment by moment to the required shape.
In principle, even a 6.5-meter ground-based telescope could be used to image a Jupiter-like planet in a solar system like our own within the 8 parsec neighborhood, Mirror Lab director and CAAO director Roger Angel has noted in research papers. (Eight parsecs is about 26 light-years, or more than 153 trillion miles.)
As for the really giant telescopes of the future -- telescopes with 20-meter-or-more diameter primary mirrors -- ground based telescopes with adaptive secondary mirrors should be able to directly detect and study nearby Earth-like planets, Angel predicts.
Success in making deformable adaptive secondary mirrors for large telescopes is "a natural stepping stone to so-called 'multiple-conjugate adaptive optics,'" Lloyd-Hart said. This system would use several deformable mirrors in series and correct for atmospheric turbulence in 3 dimensions.
"You could cancel the atmospheric error anywhere you look. You'd have a very large field of view with high resolution all at once. And when you can capture huge fields of view and see them with extreme clarity, then you're talking real scientific progress," Lloyd-Hart said.