New Gemini Images Exemplify the Power of Adaptive Optics
2 Junio 2003
A razor-sharp image was released today revealing new details at the heart of a famous star cluster. The thousands of swarming stars at the cluster's core were made visible by an innovative adaptive optics system called Altair that is currently being commissioned on the Frederick C. Gillett Gemini Telescope on Mauna Kea, Hawai'i.
Among several of the first images from Altair (Altitude Conjugate Adaptive Optics for Infrared), the high-resolution data reveal multitudes of stars with stunning clarity. The dense star cluster known to generations of skywatchers as the Great Hercules Cluster or M-13 is home to hundreds of thousands of stars that, in the center, are often blurred by our atmosphere into a great glowing mass. "The resolution obtained in these images is approximately equivalent to seeing the separation between an automobile's headlights on the Golden Gate Bridge in San Francisco while standing 3,850 kilometers away in Hawai`i," said Observatory Adaptive Optics Scientist Dr. Francois Rigaut.
The close-up images of M-13, with and without Altair, as well as a spectacular reference image of the entire cluster, provided by the Canada-France-Hawaii Telescope, can be viewed and downloaded at: http://www.gemini.edu/media/images_2003-2.html.
The remarkable detail in the Gemini images was made possible by Altair's unique ability to correct starlight that has been blurred by atmospheric turbulence using adaptive optics with altitude conjugation.
Most adaptive optics systems that are currently in use correct for distortions to starlight by assuming that all of the distortions occur where starlight is collected - near the surface of the telescope's primary mirror. In an altitude-conjugated system like Gemini's, the distortions are assumed to be at the dominant turbulence layer of the atmosphere. By conjugating or tuning the system for a specific layer above the telescope, Altair can generate a more accurate model of the starlight's path through our atmosphere.
"Adaptive optics with altitude conjugation is a pioneering new technique that is a powerful way to measure and fix distortions to starlight, which traveled undisturbed for vast distances through space until hitting pockets of warm and cold air in earth's atmosphere," said Glen Herriot, the systems engineer who managed the building of Altair in Victoria, BC at the laboratories of the National Research Council of Canada. Altair is able to precisely correct the distorted starlight up to 1,000 times per second using a sophisticated, deformable mirror about the size of the palm of your hand. "The end result is," says Herriot, "images that rival or even exceed the sharpness of pictures taken from space."
Working with Gemini Observatory personnel, the Canadian team headed by Project Manager Herriot and Project Scientist Dr. Jean-Pierre Véran, have been commissioning Altair on Gemini North from late 2002 through early 2003. The instrument team, comprised of 25 scientists and engineers, guided the Gemini adaptive optics system from design to commissioning over the past six years. "Commissioning a precision instrument on a 7-story, 350-ton, sophisticated telescope is especially challenging because of the extremely intricate coordination required to make all the systems work together seamlessly," said Herriot. Altair's commissioning on Gemini is expected to be complete before the end of 2003.
A key feature of Altair's sophistication is the ability to automatically monitor, adjust and optimize multiple parameters during image exposures. The idea is to make adaptive optics user-friendly for our community. When atmospheric conditions allow, simply point and click and near diffraction-limited images are delivered to a camera or spectrograph. Altair continually measures and reports on the images' level of detail making it one of the most efficient adaptive optics systems in the world. "By routinely delivering infrared images much sharper than is currently possible even from space, Altair gives observers a tremendous advantage in probing deeper in the universe and making more accurate measurements of astronomical objects," Dr. Véran says.
"Altair enormously enhances the quality and power of our imaging and spectroscopy," says Dr. Matt Mountain, Gemini's Director. "Gemini will soon deliver diffraction-limited images in the near-infrared." Gemini's theoretical diffraction limit (maximum resolution) is about 40 milli-arcseconds in the near-infrared H-band (1.6 micrometers wavelength). At this point in commissioning, Altair can deliver 60-milli-arcsecond resolution in the H-band (60 milli-arcseconds is comparable to viewing one grain of sand from about 1.6 kilometers or 1 mile away).
Dr. Mountain pointed out that Altair's commissioning means that one of the most sophisticated adaptive optics system in the world is now built-in to Gemini North as a facility instrument, and will soon be routinely available to all scientists throughout the Gemini partnership.
"This is a major achievement towards our Gemini goal of delivering space-quality images from an 8-meter, ground-based telescope," said Dr. Mountain.
Gemini's Associate Director Dr. Jean-René Roy explains that Altair is a major step forward in Gemini's aggressive plans to maximize the potential of adaptive optics on ground-based astronomical imaging. Dr. Roy elaborates, "Altair, representing the foundation of tomorrow's adaptive optics technology, is important for the success of the next generation of 30- to 100-meter, diffraction-limited, infrared, ground-based telescopes now on the drawing boards."
Future generations of adaptive optics technologies like these will undoubtedly revolutionize ground-based astronomy. For now, Altair is state of the art and provides a powerful new eye on the universe.
Whenever starlight passes through our atmosphere, it is distorted by turbulence that is similar to what we feel when traveling in an airplane. We've all seen the effects of this turbulence on stars. It's called twinkling. Because twinkling blurs images made through a telescope, scientists go to great lengths (and heights) to reduce its effects.
One of the reasons that the Hubble Space Telescope was put high above the Earth's atmosphere was to escape its adverse effects. Since putting Gemini into orbit is not an option, Gemini uses a relatively new technology called adaptive optics. Adaptive optics works simply: the system takes a sample of starlight, determines how the atmosphere bent it, and then uses a deformable mirror to "straighten" the starlight out again. Simple, right? Right, but there's a bit more to it.
Because stars are so far away, starlight passing through our atmosphere consists of parallel wavefronts of light that are bent and distorted by air of different temperatures, and therefore, different densities. Because our atmosphere is constantly changing and mixing together, the effect is very random and quite dynamic.
When starlight enters a telescope like Gemini, if nothing is done, the distortions caused by the atmosphere are magnified. Stars often look more like shimmering blobs than the pinpoints of light they would be if viewed from space. Before starlight passes into many of the instruments or cameras on Gemini, a representative column of starlight is diverted through a beam-splitter into what is called a "wavefront sensor."
The column of light entering the wavefront sensor is a representative sample of the light that is being collected across the entire main mirror of the telescope. In other words, any distortions that are visible to the wavefront sensor correspond directly to distortions somewhere in the atmosphere above the telescope. In order to use this information, the wavefront sensor separates the column of light into many areas or zones, and samples each zone to determine how our atmosphere altered the light.
By taking samples on the order of a thousand times per second, the information from the wavefront sensor is fed back to a "flexible" or deformable mirror, about the size of the palm of your hand, that can be adjusted (like a funhouse mirror) to counteract for the distortions caused by the atmosphere. However, unlike a funhouse mirror, these adjustments are very small and precise, and cannot even be seen while watching the mirror.
Adaptive optics systems work best with longer wavelength light, which means that Gemini will see the most dramatic results with infrared observations. Using this system, it is expected that Gemini will produce the sharpest images yet of the infrared sky and dramatically improve many other types of observations as well.
For more information about how starlight becomes distorted, click here.
For more information about how a generic adaptive optics system works, click here.
Interested parties should contact Peter Michaud at the address below for a video animation of Gemini's adaptive optics system.