Impressive, sharp images
of the Sun can be produced with an advanced adaptive optical
(AO) system that will give new life to existing telescopes and
opens 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.
