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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’s distorting atmosphere allows the Yepun telescope in Chile to
take pictures even better than those taken in space.

Ground-based telescopes don’t 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’re 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’s Yepun telescope—one of four that
make up the Very Large Telescope in Chile’s Atacama Desert—can 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’s computers
continually tweak Yepun’s 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.

Both Yepun and Hubble can image objects using near infrared and visible
light. But Yepun’s pictures are three times as sharp as Hubble’s, because its
mirror has three times the diameter. “Our ground-based telescope is limited only
by the diffraction of light, which is determined by the size of the telescope,”
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 “artificial star” about 90 kilometres above the
Earth.

But Yepun’s success doesn’t 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’s atmosphere.