A powerful new laser that shoots 90 kilometres into the atmosphere to produce an "artificial star" has been tested for the first time by the Very Large Telescope in Chile.
Astronomers will use the observations of the laser to correct the blurring in telescope images caused by turbulence in the Earth's atmosphere. This will allow them to image celestial objects with the VLT at a resolution 10 times finer than currently possible.
Atmospheric turbulence is the one of the key factors limiting the resolution and light-collecting ability of Earth-based telescopes. So astronomers have been developing adaptive optics systems that measure the turbulence using lasers, then correct for it by adjusting the shape of a telescope's various mirrors about 100 times per second.
"Laser guide star systems" vastly improve the sensitivity and resolution of observations at infrared wavelengths. But, improvements at the shorter optical wavelengths would require deforming a mirror 1000 times per second.
The laser systems are already installed on four large telescopes in the northern hemisphere, including the Keck Observatory in Hawaii. Now, astronomers have tested the first such system in the southern hemisphere. It is installed on Yepun, one of the four 8.2-metre telescopes that make up the VLT in Cerro Paranal, Chile.
The laser is beamed into the sky, where it causes a layer of sodium atoms at an altitude of about 90 kilometres to glow. The sodium layer originates from meteorite impacts. The glow produces an artificial star whose change in appearance allows astronomers to measure atmospheric turbulence.
The VLT system comprises a powerful orange laser that sits inside a special clean-room next to the telescope and an optical fibre that sends the laser light to a small "launch" telescope at the top of Yepun. The launch telescope sends a 50 centimetre-wide beam of light into the sky to produce the artificial star.
Work on the laser began in 2005, but was not easy, says Richard Davies, project manager for the laser's development at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany. The orange light it uses is at a wavelength not used commercially and the laser also requires 1000 times more power than typical instruments – it could not be bought off-the-shelf.
The laser was tested by itself for the first time on 28 January, then used again with two adaptive optics instruments on 9 and 10 February. These tests showed the artificial star was about twice as large on the sky as it was designed to be, Davies told New Scientist: "It's not so big we can't use it, but it's not as small as would be optimal."
The problem is probably due to a small earthquake that shook the site and bent the launch telescope's mirror. Telescope managers plan to build a new mount for the telescope that will straighten out the mirror. The laser guide star system is expected to begin science operations later in 2006.
Such systems will lay the groundwork for more advanced adaptive optics systems planned for future telescopes, which may boast mirrors as wide as 60 metres. "To really make use of these big mirrors, they will have to have laser guide stars as standards," Davies says.
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