The new laser guide star system mounted on the Keck II infrared telescope atop Mauna Kea is bringing Earthbound astronomers closer to the heavens than ever before

When we look at up the night sky, we see stars as clear and distinct points of light. However, astronomers using high-resolution telescopes struggle to capture clear images of distant stars and celestial objects. Unaltered images of these bodies, which the human eye cannot see, show up smudged and blurry when recorded by highpowered telescopes. The cause of this distortion is atmospheric turbulence. Clouds and winds swirl around the Earth, and the atmosphere contains several dynamic temperature bands. The effect of this turbulence is akin to looking at objects on the bottom of a swimming pool.

For two decades, astronomers have compensated for atmospheric turbulence through the use of adaptive optics. Sophisticated adaptive optics systems measure how the atmosphere distorts images and corrects for those distortions to increase overall image clarity. Astronomers also put a telescope in orbit around the Earth, the Hubble Space Telescope, specifically to minimize image distortions. Two years ago, the Keck II telescope atop Mauna Kea was the first very large telescope in the world to receive a new type of adaptive optics system that is changing the game for stargazers. Called a laser guide star (LGS), the new system allows scientists to see the heavens in far more detail than ever before.

Since the late 1980s, astronomers have been using so-called “natural guide stars” to enhance observations taken from terrestrial telescopes. Using the natural guide star system, an astronomer points a telescope at a nearby bright star in the night sky to capture a relatively clear image. The astronomer then uses sophisticated optical equipment to measure how the atmosphere distorts this image. With this information, the astronomer is able to compensate for the distortion by deforming the mirror of the telescope.

Arrays of very precise motors can reshape the mirror surfaces of modern telescopes hundreds of times per second in order to maintain image clarity. “The light wave you are receiving of an image gets crinkled like a potato chip due to the turbulence. You measure the shape of the wave, how distorted it is, and send that image to the deformable mirror. The mirror shape is changed to shift the crinkled waves into flat waves. Instead of a blurry image, you get a sharp image,” explains Michael Liu, an astronomer at the University of Hawai‘i’s Institute for Astronomy (IFA) who uses adaptive optics for his observations.

Unfortunately, large portions of the sky remain inaccessible to natural guide star systems. Those regions lack sufficiently bright stars. Scientists have long theorized that they could make an artificial guide star using laser beams. The first such system went live at Lick Observatory, near San Jose, Calif., in 1996. The Keck II is the second operating LGS. The Keck II system saw “first light” in 2003 and began full-time scientific operations in late 2004.

Lighting Up The Sky

A laser guide star works in a similar manner to yellowish sodium street lamps. Over time, micrometeorites striking the atmosphere have deposited a thin band of sodium atoms 56 miles above the Earth’s surface. Technicians tune a powerful laser mounted on the telescope to a frequency that excites, or increases the energy level of, sodium atoms. The laser is fired into the sky, exciting a small portion of the sodium band. “In those lamps, the electricity is exciting the sodium and it produces a yellow light. We do the same with the laser guide star and create a fake star. The artificial starlight comes back down into your adaptive optics system, just like real starlight does with the natural guide star system,” says Liu. Scientists can use the system to enhance observations anywhere in the sky.

With the help of the LGS, the Keck II can capture detailed infrared images four times sharper than those captured by the Hubble telescope. The first year of observations using the LGS yielded so many discoveries that the American Astronomical Society dedicated a session at its annual meeting in 2006 to showcase the new science. Numerous publications resulted from the first year of research, with many others now coming out in the second year. The pace of research is a testament to the power of the new tool, which has brought distant objects closer than ever before for astronomers seeking to reveal the secrets of the universe.

Photo: Richard Wainscoat, University of Hawaii, Institute for Astronomy - Schematic Rendering: WM Keck Observatory