Solar Adaptive Optics Project
Smoothing out the wrinkles in our view of 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.
Sample images with AO off (left) and on (center). In the original, each frame covers an area 45x45 arc-sec and was taken at 550 nm wavelength (10 nm interference filter) using the Baja Technology camera in April 2003. The relative size of Earth and a 1 arc-sec square are superimposed in the last frame.
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.
Left: The mirror at center right doesn't look like an ironing board, but that's its basic role in a new high-order adaptive optics system that cancels most of the atmosphere's blurring.
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.