Rubber Mirrors and Artificial Stars:
The Adaptive Optics/Laser Guide Star Program
at Lick Observatory

Twinkle, Twinkle, Little Star
Because of Earth's turbulent atmosphere, stars seen in large telescopes are never perfectly sharp or steady, looking more like hyperactive amoebas dancing a jig, than well behaved points of light. This is the charming twinkle we see when we look at a star with our naked eyes, but when magnified by a large telescope, it is revealed as a continuously changing blur, severely limiting a telescope's power to resolve detail.

The distorting influence of the atmosphere can be minimized by locating telescopes where the air is at its most steady -- usually the tops of high mountains. Lick Observatory pioneered this approach more than a century ago, becoming the first permanently occupied mountaintop observatory. But even at the best terrestrial locations, the atmosphere still limits a telescope's potential.

Putting telescopes in earth orbit, above the atmosphere, is an excellent solution, as the exquisitely detailed pictures from the Hubble Space Telescope attest. But doing astronomy from space is a very expensive undertaking. So most astronomers, most of the time, must contend with more or less blurry images from Earth's surface.

The blurring is caused by the rapid and incoherent bending of starlight as it passes through atmospheric cells of varying density. One can imagine a light wave from a star as a flat, perfecty smooth sheet of energy, travelling through the vacuum of space. But in the last fraction of a second before reaching our telescope, the wave passes through Earth's atmophere, where the turbulent air bends it this way and that. By the time it reaches us, the smooth sheet of a moment before has been wrinkled like a potato chip. Sheet after sheet gets to the telescope, each a little differently distorted than the one before, resulting in the blurry, jiggling images we see.


Ironing Out the Wrinkles
In recent years however, a new solution has emerged, thanks to an ingenious marriage of advanced optical, electronic, and laser technologies: Suppose we could measure the wrinkles in a light wave as it arrives at our telescope, and that those measurements can be used to deform a special "rubber" mirror into the same shape as the distorted light wave. When the wrinkled starlight encounters the similarly wrinkled mirror, the light wave bounces off, restored to nearly the same flat shape it had in space. The resulting image is nearly as sharp and steady as if it had never passed through the atmosphere.

This is the principle behind a new technique called "Adaptive Optics" (AO), and the key to obtaining images from ground-based telescopes far superior to those thought possible only a few years ago. Of course, it's not as simple as we've made it sound. Because the atmosphere is moving rapidly, the shape of the image changes significantly hundreds of times a second. To keep up with this frenetic dance, the deformable mirror must assume a new shape with equal rapidity. All this requires a host of complicated, delicate optical and electronic components, all working together perfectly.


A Star Is Born
But the technological challenges facing adaptive optics don't stop here. AO, by itself, would likely have had only minor, if interesting, applicaton in astronomy because of an inherent limitation. The limitation arises from the need for lots of starlight to accurately measure the arriving lightwaves' ever-changing wrinkles. Therefore, to lend itself to AO, an object of astronomical interest must itself be sufficiently bright, or a bright star must lie very nearby. This requirement limits AO to the less than 1% of the sky that contains stars of the necessary brightness.

However, rather than accept this limitation, the scientists and engineers developing AO systems devised a scheme by which a bright "star" could always be found near the target, thus opening the whole sky to AO. The idea is, on the face of it, a simple one: if there's no star where you need one, make one. We can't, of course, really make a star, but we can make a bright spot, high in the atmosphere, that to the telescope looks just like a star.

The spot is produced by a powerful laser tuned to the yellow color preferred by atoms of sodium. At about 90 kilometers above the surface of the earth, the laser encounters a layer of sodium gas left in our upper atmosphere by the continual evaporation of small meteors containing traces of that element. The sodium atoms react to the laser light by glowing, thus forming a bright spot that the telescope sees as just another star. Because the laser moves with the telescope, the "star" (called the Laser Guide Star (LGS)) is visible wherever the telescope is pointed, and can be placed next to the true celestial object to be studied.

And It Works!
Despite daunting technical challenges, adaptive optics works. When the "loop is closed" -- as AO engineers call intiaiting the rapid electronic conversation between the device measuring the incoming distortions and the mirror making the corrections -- the wriggling blur of an uncorrected star suddenly becomes a well-behaved dot.
Over the last several years, development of the AO and Laser Guide Star systems has proceeded at the 3-meter telescope on Mt. Hamilton and at Lawrence Livermore National Laboratory. What began as a difficult engineering problem has evolved into a superb astronomical tool. While engineering continues and the system is continually refined, science is now being routinely done both with nautral guide stars (i.e., targets sufficiently bright that no laser is required) and with the more complicated laser guide star mode. AO-corrected images obtained from Mt. Hamilton can achieve resolutions comparable to those made with Hubble Space Telescope.
Technical descriptions of the Lick AO/LGS system, descriptions of AO research, and links to other AO projects can be found at Lawrence Livermore National Laboratory, the Center for Adaptive Optics and on the Lick instrument pages for observers.