Adaptive optics is a technology developed during the Cold War to search for satellites. It is now used by astronomers to image distant stars and galaxies, and could soon allow surgeons to examine the retina of a human eye.
One of the biggest problems faced by astronomers is the atmosphere. Light arriving from a star has to pass through 100km of turbulent air before reaching the Earth's surface. This causes the stars to 'twinkle' and severely distorts astronomical images from telescopes.
Astronomers have tried to minimize the effects of the atmosphere by building observatories on mountain tops, and ultimately launching them into space. However, the new technique of adaptive optics corrects this distortion, allowing astronomers to view clear, sharp images from the Earth's surface.
Adaptive optics works by measuring the shape of the incoming light from a telescope, and correcting it by reflecting the image off a deformable mirror. The shape of the mirror is continuously adjusted to match the measured distortions, so the final beam is straight and gives a clear picture of the sky.
The deformable mirrors are usually made of a number of individual segments, each positioned using 'piezoelectric actuators' (precision pistons). An alternative design uses a continuous membrane, which is deformed using actuators.
The technique was developed by the American military during the Cold War, as a method for trying to image Soviet satellites. The atmosphere makes it impossible to view any detail of satellites orbiting the Earth without correcting for its effects. Following the end of the Cold War, details of military research were published and several astronomical projects started.
In developing Adaptive Optics systems, astronomers have had to overcome numerous technical challenges. The systems require fast computers and electronics, as atmospheric turbulence typically changes 1000 times per second. After a decade of research and experimental projects, adaptive optics is now a feasible technique in astronomy, and will soon be used routinely in observatories around the world.
Laser Guide Stars
An adaptive optics system can only work if it receives a sufficient amount of light to accurately measure the shape of the incoming light. In order to image a dim star or galaxy, astronomers measure the shape of the light from a bright, nearby guide star. Unfortunately, there are not enough bright stars to enable the whole sky to be imaged this way.
The solution to this problem is to create an artificial star by shining a high power laser into the atmosphere. Laser Guide Stars work by focusing a laser beam onto a layer of sodium atoms in the stratosphere. This excites the atoms and makes them emit light, producing, in effect, an artificial star.
Applications of adaptive optics are not limited to defence initiatives and astronomy. The technology could be used in the future to develop more efficient lasers and optical fibres, as well as underwater imaging devices and better microscopes.
Another application of adaptive optics and laser guide stars is in eye surgery. There are experimental projects using this technique to image the human eye.
Although shining high power lasers into the atmosphere and looking into people's eyes may appear to be completely different, the basic principle is the same. If you look at the retina of a human eye, the image is blurred due to distortion caused by the constituents of the eye. Like the atmosphere, these are in continuous motion, as the eye trembles. Shining a low power laser into the eye causes atoms on the retina to fluoresce, creating a 'star' in the eye. By measuring the shape of the distorted light from this 'star', and correcting it with a deformable mirror, we can form a clear image of the retina. This would give surgeons more information about aberrations and could lead to improvements in eye surgery.
These improvements could, in turn, give many adults and children a better view of the night sky.