AN EARTH-BASED telescope is taking pictures that are just as clear and sharp
as images from the Hubble Space Telescope. A flexible mirror that can compensate
for the Earth鈥檚 distorting atmosphere allows the Yepun telescope in Chile to
take pictures even better than those taken in space.
Ground-based telescopes don鈥檛 have the size constraints of space-based ones,
which have to be launched on a rocket. So they can have larger mirrors, giving
sharper pictures. They鈥檙e also much cheaper to build. But Earth-bound telescopes
must view the heavens through the turbulent atmosphere, which distorts light,
making images blurry.
The European Southern Observatory鈥檚 Yepun telescope鈥攐ne of four that
make up the Very Large Telescope in Chile鈥檚 Atacama Desert鈥攃an now cut
through the fog. Its newly installed optics system, called the Nasmyth Adaptive
Optics System (NAOS), works by monitoring distortions in the pattern of light
streaming from a reference star near the target celestial body. NAOS鈥檚 computers
continually tweak Yepun鈥檚 8.2-metre mirror using 185 tiny motors, adjusting its
curvature to compensate for the atmospheric distortion. A new infrared camera,
called CONICA, snaps the pictures.
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Both Yepun and Hubble can image objects using near infrared and visible
light. But Yepun鈥檚 pictures are three times as sharp as Hubble鈥檚, because its
mirror has three times the diameter. 鈥淥ur ground-based telescope is limited only
by the diffraction of light, which is determined by the size of the telescope,鈥�
says ESO astronomer Alan Moorwood.
Adaptive optics has been increasingly employed over the past 10 years, but
NAOS allows Yepun to use much fainter reference stars than other telescopes. The
new-found resolution could help scientists search for planets outside the Solar
System or hunt for the earliest stirrings of galaxy formation in the Universe.
ESO researchers hope to eliminate the need for a reference star by using a
powerful laser to create an 鈥渁rtificial star鈥� about 90 kilometres above the
Earth.
But Yepun鈥檚 success doesn鈥檛 spell the end of space telescopes. Some types of
telescope have to be based in space because they sample X-rays and gamma rays,
which are blocked by the Earth鈥檚 atmosphere.