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National Solar Observatory/Sacramento Peak
Sunspot, New Mexico

Contacts:
Dave Dooling, National Solar Observatory
505-434-7015, dooling{at}nso.edu

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 reach.

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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