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Adaptive optics is a method for removing the blurring of images caused by the changing distortions within optical systems. Turbulence in the Earth's atmosphere causes blurring of astronomical images. In an analogous manner internal imperfections and fluids in the eye cause blurring of images striking the retina. The use of adaptive optics allows ground based telescopes to see clearly as if they were in space, and these techniques, when used to look at the retina of the human eye, dramatically sharpen images of the retina. -The Center for Adaptive Optics
Step 1: The Mirror
| Waves of light entering an adaptive optics system first reach a deformable mirror. Like the wavy mirrors in a funhouse, which distort your body shape from inhumanly wide or skinny to amusingly tall or short, the deformable mirror distorts the information carried by the light waves. It can do this because the deformable mirror is actually a network of many tiny mirrors, each of which can be adjusted by minute angles. By adjusting these angles, a computer controls the distortions caused in the reflected light. Imagine that by using a computer, someone could control how tall or thin the funhouse mirror made you appear; this is the purpose of the deformable mirror. When light is reflected of off this mirror, it is split in half. Half of the light is imaged by a camera, and the other half is used to recalibrate the system. |
Step 2: The Detector (see demonstration)
The light sent to recalibrates the system next enters a detector. The detector consists of an array of lenslets (small lenses) and a CCD camera. Each of the lenses in the array creates a distinct image of the source (a guide star) on the camera. If you have ever looked through a window with divided windowpanes and seen your reflection in each pane, or looked at a shelf of television screens and seen your reflection in all of them simultaneously, you have experienced this effect.
Step 3: Analysis
Each distinct image on the CCD camera should line up with the center of its corresponding lenslet. In other words, light entering the array of lenses should travel straight though the lens and hit the CCD camera like a bulls eye on a target. The Earth’s atmosphere, however, does not allow this to happen. If you shoot an arrow at a target, as long as the arrow flies in an exactly straight line, it should hit the center every time (assuming it is properly aimed). However, if the arrow enters a windstorm, the wind will alter its course and the arrow will likely miss the center. Likewise, light travels in a straight line until the Earth’s atmosphere, like the windstorm, shifts different parts of the image different amounts from their original position. These shifts cause fuzziness and blurriness in images taken of objects in space. |
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Step 4: Correction
A computer analyzes the images from the lenses to determine how to “undistort” the incoming light. It does this by calculating the deviation of each image from its ideal center on the target, and this information is used to reconstruct the distorted wave of incoming light. Once the computer has this reconstruction, it calculates the shape the mirror needs take to cancel out the distortions. The computer then adjusts the mirror to cancel the distortions and restore the image. This means that as the distorted light wave hits the distorted mirror, it bounces off straight. If you stand in front of a circus mirror that makes you look wide, and then look at the reflection your “wide” self in a mirror that makes you look skinny, you will appear normal. This is essentially how an adaptive optics system works.
Adaptive optics is a circular process. Incoming light reflects off of a mirror, and half of the light is imaged while the other half travels through the adaptive optics system to recalibrate the mirror. As the mirror is being adjusted, new light is continuously entering the system to be imaged and to recalibrate the mirror again. The recalibration process must be extremely fast since the Earth’s atmosphere is constantly changing. In order for the adaptive optics process to be effective, light must come in, reflect off the deformable mirror, travel through the detector, be analyzed, and the mirror be readjusted at least 1,000 times every second.
Adaptive optics has been an invaluable tool in expanding our knowledge of the universe and ourselves. This system has been used to obtain amazing results, and astronomers and vision scientists can only dream of what the future may hold for this type of imaging.