Astronomers are celebrating 'first light' at the newly adapted Keck II 10-metre telescope in Hawaii. Keck II now has a laser guide-star system — the first on such a large telescope. By creating an artificial star in the sky, it will greatly improve the telescope's ability to image distant galaxies.

The system is based on adaptive optics (AO), an established technique by which images taken at Earth-bound telescopes can be corrected for the blurring caused by the planet's atmosphere. Using AO produces much sharper images. But the scheme usually relies on there being a bright guide star in the vicinity of the object under observation, against which the correction is calibrated.

At Keck II, a 15-watt laser beam is fired into the sky, creating a glowing patch in a natural layer of sodium atoms, 90 km above the Earth's surface. Using this glow as an artificial guide star, the images from the telescope become sharper still — as shown in the picture here, of the Strehl ratio (a measure of image 'perfection') for a star without AO, with AO and with AO plus the laser guide star (left to right).

But it's not just a question of resolution. The laser system can also be directed at any region of sky: astronomers need no longer rely on good fortune to find a guiding star.