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Smoothing Out The Wrinkles In Our View Of The Sun

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Old June 22nd 03, 07:28 AM
Ron Baalke
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Default Smoothing Out The Wrinkles In Our View Of The Sun

National Solar Observatory/Sacramento Peak
Sunspot, New Mexico

Dave Dooling, National Solar Observatory

Embargoed until 12 EDT, June 18, 2003

Smoothing out the wrinkles in our view of the Sun

Impressive, sharp images of the Sun can be produced with an advanced adaptive
optical system that will give new life to existing telescopes and open the way
for a generation of large-aperture solar telescopes. This AO system removes
blurring introduced by Earth's turbulent atmosphere and thus provides a clear
vision of the smallest structure on the Sun.

Solar scientists face the same challenge as night-time astronomers when
observing from the ground: Earth's atmosphere blurs the view. Astronomers speak
of being "seeing limited," or restricted to what atmospheric turbulence allows.
The turbulence acts as a flexible lens, constantly reshaping what we are
studying, and putting many of the answers about solar activity just beyond our

Bigger telescopes can see fainter objects but with no more detail than mid-size
telescopes. The closeness and brightness of the Sun make no difference: sunlight
passes through the same atmosphere (usually more disturbed because the Sun heats
the ground and air during the day). Solar observations from Earth have the same
limit of about 1 arc-second as nighttime astronomy (1 arc-second = about
1/1920th the apparent size of the Sun or Moon; 1/1,296,000th of a circle).

An innovative solution, evolving since the 1990s, is to measure how much the air
distorts the light and then adjust mirrors or lenses to cancel much of the
problem. This is adaptive optics (AO), a sophisticated blend of computers and
optics. For more than a decade night astronomers have used AO to let a larger
number of telescopes operate closer to their difraction limit, the theoretical
best set by the size of a telescope and how light forms images.

Applying AO to solar astronomy is a bigger challenge, though. Where night
astronomers have high-contrast pinpoints -- stars against a black sky -- to
measure how the light is distorted, solar astronomers have large, low-contrast
targets -- such as sunspots and granules -- comprising an infinite number of
point sources. This has required a different approach.

Since the late 1990s the National Solar Observatory has been advancing the
Shack-Hartmann technique. We divide the solar image into subapertures then
deform a flexible mirror so each subaperture matches one reference subaperture.
In 1998 we applied a low-order AO system to the Dunn Solar Telescope, thus
allowing it to operate near its diffraction limit under moderately good
atmospheric conditions. This technology now is applied at several solar
telescopes around the world.

NSO continues this important research and in late 2002 demonstrated a high-order
AO system that will allow the Dunn to operate at its diffraction limit under a
wider range of atmospheric conditions. Our goal is to expand this capability to
support a system that is 100 times as complex and capable to support the planned
4-meter Advanced Technology Solar Telescope (ATST). This will let us grasp many
of the details that are beyond our reach now and that we need to start answering
vital questions about solar activities.

The current High-Order Adaptive Optics (AO) development project is a partnership
between NSO and the New Jersey Institute of Technology, supported by the NSF's
Major Research Instrumentation division.

[NOTE: Images supporting this release are available at
http://www.nso.edu/press/AO/ .

Additional information is available at
http://www.nso.edu/press/AO/AO76.html ]


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