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Sophisticated
Star Hunting
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The
Keck observatory is located on a remote outpost on the
summit of Hawaii's dormant Mauna Kea volcano. The telescope
shown here is one of two twin telescopes operating at
this site.
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Of
course with every step forward new complications arise. Once
a mirror is larger than one foot, the improved resolution
is masked by a "twinkling" effect - the distortion of light
by moving air currents. This explains why most big telescopes
are found on mountains surrounded by cloudless, dry, still
air. One technique to counteract this twinkling effect calls
for photographing a star hundreds of times per second. A composite
image effectively cancels the atmospheric jitter by realigning
the individual frames so the star is seen as one image. Another
technique is to bounce the reflected image off a secondary
mirror with a pliable surface whose shape changes thousands
of times a second. This altered surface mimics the motions
of the air currents causing the distortion in the first place.
Modern
telescopes are bigger than ever; for instance, the Subaru
telescope in Hawaii uses a single 27-foot mirror. Once mirrors
get this big, however, they are very hard to control and maintain.
To be workable, mega-telescopes require technological advances
in lightweight, large scale mirror fabrication and quality
control - the tiniest speck or scratch renders the mirror
useless. Optics and computer-control systems, structural support
and telescope enclosures along with computer processing speed
and instrument systems that match the optical quality are
all part of a new generation of telescopes.
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The
Keck mirror is made up of 36 hexagonal mirrors, tiled
together and arranged in a honeycomb "fly's -eye"
pattern.
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The
initial 10-meter Keck telescope, built in the 1980s, was the
first major observatory to get away from trying to polish
a huge single piece of glass for a mirror. Instead, 36 hexagonal
mirrors are tiled together and arranged in a honeycomb "fly's-eye"
pattern. Computers are used to move the mirrors in concert
to maintain a perfect, precise hyperbolic surface -- resembling
one single mirror -- accurate to within a millionth of an
inch.
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