Adaptive optics systems are, by definition, closed loop. This means, among other things, that any change in the optical beam performed by the AO system is sensed by the system. Consider the optical system pictured in Fig. 2.1. This system took eons to develop and probably could not have been developed so elegantly without divine intervention. Most of us possess this basic slow AO system. The eye works as a detector, the brain as a computer, and the arm as an actuator. While we all see what is being done in this system, and some of us experience it often, it is not simply reflex, an open loop operation. Our eye senses an image and transmits the information to the brain. The brain, through years of training, knows pretty well that an image is not focused correctly. We often can't define blurry, but we know it when we see it. If the image is blurry, our nervous system instructs the muscles in our hand to change the focus setting on the optical device. The focus control, acting like a deformable mirror, adjusts the focus of the beam and therefore the appearance of the image to our eyes. We sense the image getting better or worse, i.e., closing the loop. If the image gets worse, we stop our adjustment and reverse its direction. We eventually zero in on an image in its best focus.
For those who wear eyeglasses, the analogy continues. If we focus the binoculars while not wearing the eyeglasses, the binoculars compensate for the aberrations in our eye that are not corrected. Thus, the AO system compensates for all the aberrations in the system. It really doesn't know, and doesn't particularly care, where they come from. If the object moves away, like that of a rapidly moving car, we must constantly readjust the binoculars to keep the image clear. If, however, we happen to have binoculars with a large depth of focus, the adjustment doesn't have to be made often, nor very rapidly. The integrated adaptive optics system depends upon all the optics around it, the type and magnitude of the aberrations, the speed of the sensor, and the quick-wittedness of the control computer. Although large telescopes require hundreds of channels of compensation with a few hundred hertz bandwidths, our human AO system gets by with about 3 channels and 2 Hz - a good start, but still requiring a little help from technology.
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