New Gemini Images Exemplify the Power of Adaptive Optics
June 2, 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.
 | This composite image shows a small section of the core of the globular cluster M-13 as imaged by the Altair adaptive optics system on Gemini North (upper blue inset; 0.060 arcsecond resolution). Beneath the Altair image is an uncorrected "natural seeing" image (0.26 arcsecond resolution). Wide-field background image courtesy of the Canada-France-Hawai`i Telescope/Coelum/Jean-Charles Cuillandre. |
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.
{mospagebreak title=Adaptive Optics: Straigtening Out Bent Starlight}
Background Information
Learn more about how an adaptive optics system works. Learn more about wavefront optics. 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.
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.
Schematic
Medium-res JPEG (206KB)
To view a fully animated Quick-TimeTM movie of this schematic, click here (13MB).
A schematic of how an adaptive optics systems, like Altair on Gemini North, works to correct distorted starlight. The illustration (1) is an example of a blurry image taken without the help of adaptive optics. When starlight is collected and focused by the telescope, just prior to coming to a focus, the light entering an adaptive optics system is first collimated (2) and is reflected off a deformable mirror (3). After reflecting off the deformable mirror, the light passes through a beam-splitter (4) where the shorter wavelength light (optical) enters the wavefront sensor (5) which takes a "snapshot" of the distortions on the wavefront and sends the information via a computer (6) to the deformable mirror to keep the wavefronts corrected and flat. Finally, the light is focused (7) and imaged on a detector (8) for astronomers to study.
Wavefront Optics
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This image depicts perfectly even and smooth waves, as they would form if someone tossed a stone into a serenely still pond on a calm day. Light, like water, travels in waves, and starlight traveling undisturbed through the universe resembles the perfectly smooth and even waves shown above. |
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Depicted is the effect of a breeze blowing, a form of turbulence, that interferes with the uniform wave pattern creating uneven ripples. |
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When uniform waves of starlight reach Earth's atmosphere, pockets of hot and cold air that produce turbulence, cause the wavefronts of starlight to bend due to the fact that light travels slightly faster in less dense warmer air. |
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A close-up view of starlight bending and rippling once it reaches and passes through the Earth's atmosphere. |
