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Telescopes and space missions

Adaptive-optics to cover the sky

07 Jan 2000

When astronomers study the skies, their observations are plagued by the distorting effects of atmospheric turbulence. One way of improving the resolution of astronomical images is to observe the effect of the atmosphere on a bright "guide star" near the area being studied and then to deform the telescope's mirror structure using "adaptive optics" to make corrections in real time. Unfortunately, less than 1% of the sky is close enough to a bright star to benefit from the standard adaptive-optic technique. However, Roberto Ragazzoni of the Astronomical Observatory of Padova, Italy, and colleagues have now found a way of mapping atmospheric fluctuations anywhere in the sky using multiple guide stars. The technique could improve the resolution of an 8-metre telescope by up to one hundred times and make adaptive-optics much more widely used (Nature 403 54).

Adaptive-optic telescopes differ from normal telescopes by using a thin flexible primary mirror that can easily be deformed. A bright guide star near the observing area acts as a “beacon” for the telescope. As the image of the bright star is affected by turbulence, the primary mirror is distorted by computers to correct for the fluctuations.

Ragazzoni and his colleagues propose that a primary mirror made from a collection of smaller deformable mirrors could correct for the effects of atmospheric turbulence in three-dimensions, by compensating for the much wider field of view. In their experiment, they used three stars surrounding a central star in the constellation Aquila. They collected 130 snapshots of the constellation using the Italian 3.6m Galileo telescope and deliberately defocused the images so that the atmospheric distortion was easily highlighted and correctable in the final images. Their technique works both for constellations of real stars and for groups of “artificial” stars – bright beacons generated 90 km up in the atmosphere by lasers. Once a number of outstanding technical challenges have been solved, the new technique could lead to improved resolution images of the entire sky.

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