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Adaptive optics sees more clearly than ever

19 Apr 2007

Astronomers at the European Southern Observatory have achieved a startlingly clear view of Omega Centauri from the ground.

They used MAD, a new "multi-conjugate adaptive optics demonstrator" installed at the Paranal Obervatory, Chile. Paranal already hosts seven conventional single-conjugated adaptive optics systems, but MAD is something different.

"MAD is a technological and conceptual step forward in compensating for atmospheric turbulence, using not one but two deformable mirrors and multiple wavefront sensors," Enrico Marchetti, MAD project manager, told optics.org.

"The two mirrors, one 100 mm and one 150 mm in diameter, are optically conjugated at 0 and 8.5 km above the telescope. Sixty actuators behind the mirrors cause rapid and deliberate deformations, based on the measured turbulence from reference stars in MAD's field of view (FoV).The fastest correction frequency we achieved was 400 Hz," said Marchetti.

The MAD project team had to develop new software to handle all the subsystems and data, but the general opto-mechanics and wavefront sensors are similar to other existing AO systems. "This is one of the keys to MAD: keep the system simple, using as much existing technology and ESO instrumentation as possible," commented Marchetti. "There are no items which are particularly unusual, but the concept and the integration of the components are new."

MAD can look at an FoV of 2 arcminutes, the central portion of the full FoV of the Melipal telescope to which it's attached. Larger fields can be built up as mosaics by moving the telescope, as long as suitable reference stars are available to provide the baseline turbulence information. The images of Omega Centauri, the brightest globular cluster in the sky, were built up in this way to cover a full 4 arcminutes, using three magnitude 11 stars as reference points. Magnitude measures the brightness of an astronomical object, with larger numbers indicating fainter objects.

The view from Paranal is among the best in the world and exceptionally free of turbulence. The team estimate that the limiting magnitude attainable with MAD was magnitude 12, achieved at the peak correction frequency of 400 Hz, and Marchetti concedes that this result is actually one or two magnitudes brighter than other AO systems can achieve.

"MAD is a demonstrator, and pushing the wavefront sensor detection to its limits was not one of the main goals," said Marchetti. "But it's a pathfinder for what's to come. It's a robust test bench for the implementation of similar instruments elsewhere in the ESO's Very Large Telescope, of which Melipal is one unit, as well as in the proposed European Extremely Large Telescope. MAD has opened the way, and future instruments will build on it."

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