AN OBSERVATORY in Chile will be the first to try out the latest weapon in the
battle for clearer views of the night sky. An imposing array of laser beams and
computer-controlled mirrors will cut through the turbulence caused by the
Earth鈥檚 atmosphere, which makes images from ground-based telescopes fuzzy.
Telescopes in space can produce sharper images, but their mirrors are smaller
and don鈥檛 provide as much detail. Astronomers already boast that they can cancel
out the turbulence that affects ground-based telescopes with adaptive optics, so
the view is as clear as you would get from space
(快猫短视频, 15 December 2001, p 12).
But with current systems you need a guide star near the target object, as
only by knowing what the star is meant to look like can a computer work out how
the image is being altered by turbulence. It then sends signals to motors that
adjust the shape of the mirror and correct the image. Astronomers also tend not
to mention that the correction only works for a narrow region within the overall
field of view, and that moments of clarity are fleeting.
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Astronomers at Gemini South in Cerro Pachon are now aiming to be the first to
overcome these problems and cancel out turbulence over a much greater proportion
of the image, using a new technique known as multi-conjugate adaptive optics
(MCAO). They will solve the problem of needing conveniently placed guide stars
by creating artificial ones鈥攗sing lasers to make sodium atoms high in the
atmosphere fluoresce.
A prototype MCAO system is being built at the University of Durham, and tests
of a sodium laser guide star system have just begun at the Keck Observatory in
Hawaii. The overall system will use an array of five laser guide stars, to give
a clear picture over a much wider field of view
(see Graphic). It will also have
three mirrors working together, each cancelling out turbulence at a different
height from the ground, to make the corrected images more stable.
Patrick Roche, project scientist at Gemini South, says the technique should
produce a tenfold increase in the region of sky that is freed of turbulence:
鈥淭hat will allow us to see much more detail in complex, extended objects like
colliding galaxies or clusters of stars.鈥� But he admits that getting the
technique to work reliably will be tough. 鈥淢ost challenging are the lasers,
which will have to stay tuned to the sodium wavelength in the relatively harsh
conditions of mountain-top observatories.鈥� If all goes well, he hopes the system
could be operating at Gemini South within three years.