04-05-2012, 04:41 PM
INTRODUCTION TO ADAPTIVE OPTICS AND ITS HISTORY
[attachment=21310]
Why is adaptive optics needed? Turbulence in the Earth's atmosphere limits the performance
of ground-based astronomical telescopes. In addition to making a star twinkle, turbulence spreads
out the light from a star so that it appears as a fuzzy blob when viewed through a telescope. This
blurring effect is so strong that even the largest ground-based telescopes, the two 10-m Keck
Telescopes in Hawaii, have no better spatial resolution than a modest 8-inch backyard telescope!
One of the major motivations for launching telescopes into space is to overcome this blurring due
to the Earth's atmosphere, so that images will have higher spatial resolution than has been possible
to date from the ground. The Figure below illustrates the blurring effect of the atmosphere in a
long-exposure image (left) and a short "snapshot" image (center). When the effects of turbulence
in the Earth's atmosphere are corrected, this distant star would look like the image on the right.
Image credit: Lawrence Livermore National Laboratory and NSF Center for Adaptive Optics.
2. How adaptive optics works. Adaptive optics technology can correct for the blurring caused by
the Earth's atmosphere, and can make Earth-bound telescopes "see" almost as clearly as if they
were in space. The principles behind adaptive optics technology are illustrated in the Figure
below. Assume that you wish to observe a faint galaxy. The first step is to find a relatively bright
star close to the galaxy. a) Light from both this "guide star" and the galaxy passes through the
telescope's optics. The star's light is sent to a special high-speed camera, called a "wavefront
sensor," that can measure hundreds of times a second how the star's light is distorted by the
turbulence. b) This information is sent to a fast computer, which calculates the shape to apply to a
special "deformable mirror" (usually placed behind the main mirror of the telescope). This mirror
cancels out the distortions due to turbulence. c) Light from both the "guide star" and the galaxy is
reflected off the deformable mirror. Both are now sharpened because the distortions due to
turbulence have been removed.
[attachment=21310]
Why is adaptive optics needed? Turbulence in the Earth's atmosphere limits the performance
of ground-based astronomical telescopes. In addition to making a star twinkle, turbulence spreads
out the light from a star so that it appears as a fuzzy blob when viewed through a telescope. This
blurring effect is so strong that even the largest ground-based telescopes, the two 10-m Keck
Telescopes in Hawaii, have no better spatial resolution than a modest 8-inch backyard telescope!
One of the major motivations for launching telescopes into space is to overcome this blurring due
to the Earth's atmosphere, so that images will have higher spatial resolution than has been possible
to date from the ground. The Figure below illustrates the blurring effect of the atmosphere in a
long-exposure image (left) and a short "snapshot" image (center). When the effects of turbulence
in the Earth's atmosphere are corrected, this distant star would look like the image on the right.
Image credit: Lawrence Livermore National Laboratory and NSF Center for Adaptive Optics.
2. How adaptive optics works. Adaptive optics technology can correct for the blurring caused by
the Earth's atmosphere, and can make Earth-bound telescopes "see" almost as clearly as if they
were in space. The principles behind adaptive optics technology are illustrated in the Figure
below. Assume that you wish to observe a faint galaxy. The first step is to find a relatively bright
star close to the galaxy. a) Light from both this "guide star" and the galaxy passes through the
telescope's optics. The star's light is sent to a special high-speed camera, called a "wavefront
sensor," that can measure hundreds of times a second how the star's light is distorted by the
turbulence. b) This information is sent to a fast computer, which calculates the shape to apply to a
special "deformable mirror" (usually placed behind the main mirror of the telescope). This mirror
cancels out the distortions due to turbulence. c) Light from both the "guide star" and the galaxy is
reflected off the deformable mirror. Both are now sharpened because the distortions due to
turbulence have been removed.