Volume 2, Issue 13
Seeing Space
Most of us are enchanted by the twinkle of the stars in the night sky. For astronomers though, that twinkle represents a problem that has plagued stargazers since the days of Galileo. The twinkle of stars is caused by atmospheric distortion, air turbulence that warps the light waves as they travel to the Earth's surface. UC Berkeley astronomer James Graham is focused on fighting that distortion. Someday, his work could enable scientists to directly photograph planets orbiting distant stars.
A simulated image from the Gemini Extreme Adaptive Optics Coronagraph depicting a Jupiter-mass extrasolar planet in orbit around a solar-type star. The star is located behind an occulting spot. In hour-long exposures, the system will be 13 times more sensitive.
"For hundreds of years, terrestrial telescopes have been completely limited by the Earth's atmosphere," Graham says. "We're trying to overcome those limitations using modern technology to measure and correct the distortion of the atmosphere. That would allow the most grandiose telescopes to achieve their true potential."
For nearly a decade, Graham has aided several of the world's largest telescopes in adopting a technology called adaptive optics to boost their performance and limit atmospheric interference. The method works by measuring the distortion caused by the atmosphere and rapidly correcting for it by physically changing the shape of a deformable, or "rubber," mirror hundreds of times each second via an array of actuators. When used to its fullest potential, adaptive optics can dramatically improve the angular resolution, the minimum distance between distinguishable objects in the sky before they completely blur together. For example, the 10-meter Keck Observatory in Hawaii, fitted with adaptive optics in 1999, achieves angular resolution exceeding that of the Hubble Space Telescope at infrared wavelengths, Graham says.
At left, a star cluster in a nearby galaxy observed with the Hubble Space Telescope. At right is the same field imaged with adaptive optics at the Keck Observatory. The Keck provides far superior angular resolution. (Keck Observatory/James Graham & and Nate McCrady)
Already, Graham and his colleagues have used the tools they helped pioneer to do valuable science. Last year, Graham and graduate student Marshall D. Perrin employed the adaptive optics system of UC Berkeley's Lick Observatory and a camera Graham built to collect sharp images of faint stars thousands of miles away in the Milky Way. The adaptive optics system was augmented with an "artificial star" generated by a laser mounted on the telescope. The light from the artificial star aids the adaptive optics system in measuring the atmospheric turbulence so the rubber mirror can adjust accordingly. Those results were published in the journal Science.
There's always room for improvement though. According to Graham, today's adaptive optics systems make "modestly good" measurements of atmospheric distortion but still, something gets lost in the translation when the shape of the mirror is shifted in response.
"The corrections are somewhat crude," he says.
A sodium dye laser beam pierces the sky over Mt. Hamilton's Lick Observatory on July 22, 2003. The laser is the final piece of the laser guide star adaptive optics system that allows twinkle-free viewing of the entire nighttime sky. The beam, which reaches 60 miles into the upper atmosphere, is visible in scattered light for several kilometers. The yellowish cast of the dome is due to the street lights of nearby San Jose, Calif. (Marshall Perrin/UC Berkeley)
With increased speed and better resolution though, the telescopes should offer higher contrast. High contrast is key for astronomers to discern objects that may be near one another — planets orbiting stars, for example. Indeed, a star is often a billion times brighter and millions of times larger than the planet orbiting it. As a result, exosolar planets are far too small and faint to be seen against the star's glare. That's why planet hunters, like UC Berkeley's Geoff Marcy, make their discoveries indirectly, by detecting the wobble of the star caused by the planet's gravitational pull and then measuring the change in the wavelength of light coming from the star as the planet completes an orbit.
While a Jupiter-like planet might take a dozen years to orbit its parent star, a Neptune "clone" would have an orbital period of 160 years. According to Graham, advanced adaptive optics could enable astronomers to image and study extrasolar planets directly without waiting for the completion of an orbit.
To that end, Graham and Lawrence Livermore National Laboratory (LLNL) astrophysicist Bruce Macintosh are leading a large effort to build the Gemini Extreme Adaptive Optics Coronagraph, an adaptive optics system that would directly image distant planetary systems and help scientists understand the formation of stars and planets. The project is under the umbrella of the National Science Foundation-supported Center for Adaptive Optics, with members from UC Berkeley, LLNL, UC Santa Cruz, and the Jet Propulsion Laboratory, in collaboration with the Herzberg Institute of Astrophysics, the American Museum of Natural History, Université de Montréal, and UCLA. The planet-finder adaptive optics system is designed for the international Gemini Observatory consisting of two eight-meter telescopes, one on Mauna Kea in Hawaii and the other on Cerro Pachon, Chile.
An image of the star LkHalpha 198 taken at Lick Observatory in 2004 by Graham and his colleagues with the aid of a laser guide star system. At left, the star is seen through the adaptive optics system alone, and at right, through adaptive optics plus a polarimeter the researchers developed to separate unpolarized starlight from polarized scattered light cause by dust around the star. By viewing only the polarized component of the light, the polarimeter makes the dust envelope around the star more easily visible. (Marshall Perrin, James Graham/UC Berkeley)
The new system involves a novel method to control the deformable mirror. Current deformable mirrors are handmade, with each costing around $1,000 each. The Lick Observatory has just 127 actuators while Keck II is outfitted with 349. It takes several thousand actuators to achieve the fine-grain control necessary to see planets, Graham says. The Extreme Adaptive Optics system makes use of tiny micro-electromechanical systems (MEMS) actuatos. Fabricated inexpensively in bulk using processes similar to the way integrated circuits are manufactured, more than 4,000 of the MEMS actuators could be built right on the back of a deformable mirror. The technology is not unlike that found in modern desktop video projectors.
"Rather than starting off with a multi-million dollar price tag to build a telescope, MEMS enable you to do adaptive optics orders of magnitude more cheaply," Graham says.
Along with the MEMS actuators, the Extreme Adaptive Optics system will feature a state-of-the-art optical metrology system to measure the atmospheric interference and wave-front of the light with unprecedented accuracy. Meanwhile, a complex system of masks will limit the diffraction errors caused by reflecting the starlight.
The researchers have conducted multiple design studies and are expecting funding approval to begin building the device. The goal, Graham says, is to complete construction within four years and image the first planets shortly after the system is first switched on.
"The option is either to go into space and avoid the atmospheric distortion problem completely, or develop advanced adaptive optics like this so you can observe more from Earth," he says.