Rubber Mirrors and Artificial Stars:
The Adaptive Optics/Laser Guide Star
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
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
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.