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Adaptive mirror sharpens starlight

17 Jun 2002

The Very Large Telescope has for the first time used adaptive optics to sharpen images of stars.

Astronomers at the European Southern Observatory (ESO) in the Chilean Andes have witnessed the sharpest images ever captured by the Very Large Telescope (VLT).

Thanks to its newly-installed adaptive optics equipment, the VLT, which has an 8.2 m-diameter mirror, can now capture images that are as sharp as those seen with the Hubble Space Telescope.

In the adaptive system, a 185-actuator deformable mirror corrects for turbulence in the Earth's atmosphere that blurs objects in space.

In the first night of adaptive-optics operation, the VLT was initially pointed at a star in the Milky Way. Next, the correcting system was switched on for the first time and, according to the ESO: "What had seconds before been a jumping, blurry patch of light suddenly became a rock-steady, razor-sharp and brilliant spot of light."

Analysis showed that the diameter of the captured image had been improved by almost an order of magnitude at the first attempt.

The first image was taken in what astronomers call the K-band - corresponding to light at a 2.2 µm wavelength. Operation in the infrared region is a unique capability of the new adaptive system, and it will allow astronomers to view crystal-clear images of highly-embedded infared sources for the first time.

The adaptive system and mirror is the result of a Franco-German collaboration that began in 1997. The French team concentrated on the Nasmyth adaptive optics system (NAOS), while the German team built the CONICA infrared high-angular-resolution camera.

Unlike previous adaptive systems, NAOS-CONICA is equipped with Shack-Hartmann wavefront sensors in both the visible and infrared regions. Once fully tested, this will allow adaptive-optics-assisted imaging, polarimetry and spectroscopy of wavelengths from 0.45 to 5 µm.

For correction, the system currently uses a guide star that is near to the object being imaged. However, in 2003 the VLT is expected to have the added benefit of a laser guide star (LGS). The LGS will provide an artificial guide star at any arbritrary location, and astronomers will be able to do adaptive optics in any part of the sky that they choose.

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