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One important example of an adaptive system occurs in optics.
Adaptive optics was developed in the 1990s and refers to the process
of rapidly and automatically correcting distortions that can occur in
optical systems. For instance, telescopes on the surface of Earth (unlike
the orbiting Hubble Space Telescope) must view astronomical objects
through the atmosphere. But air affects the path of light, as is common-
ly seen in mirages such as the “water” that appears in the distance as a
driver travels across a long stretch of road on a sunny day. (The “water”
is actually the blue sky because the warm air rising from the pavement
bends light rays coming from the sky.) Distortion can ruin a telescopic
image and is a source of much frustration for astronomers, which is one
of the reasons most ground-based telescopes are located high on moun-
tains—the great height means that light does not have to travel through
as much atmosphere as it would if the telescope were at sea level.
But even at the thin atmosphere at high altitudes, atmospheric dis-
tortion limits the ability of telescopes to discern faint objects. Adap-
tive optics helps by sensing this distortion and instantly adjusting the
optical properties of the instrument to counteract the harmful effects.
The process must operate continually since air moves around, and the
extent and nature of atmospheric distortion changes rapidly.
The W. M. Keck Observatory, located on the summit of 13,796-foot
(4,206-m) Mauna Kea in Hawaii, has employed adaptive optics since
1999. Sensing the distortion requires examining the light of a bright star
in the patch of sky under interest or by shining a laser beam of a thin
layer of atoms high in the atmosphere (thereby creating a “virtual” star
that can be examined even in regions of the sky with no bright stars).
After sensitive optical instruments determine the amount of distortion,
a computer makes corrections by adjusting the path that light takes
through the telescope. These adjustments occur when actuators bend
a six-inch (15-cm) deformable (“rubber”) mirror, which is one of the
mirrors that route the light through the telescope and into the imaging
device. (The main mirror of each of the two Keck telescopes is much big-
ger—394 inches [10 m].) The small deformable mirror is made of thin
glass, and 349 actuators alter its shape slightly, hundreds of times every
second. Only small adjustments are necessary in optics—the Hubble
Space Telescope required a repair in 1993 due to a flaw in the shape the
size of 1/50th the diameter of a human hair—and the actuators bend
the deformable mirror by similar distances. But the small adjustments
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