Adaptive optics
Adaptive optics ( short AO) is a technique that the quality of optical systems improved in that they present wavefront disturbances such as those caused by air turbulence, best reduced or compensated.
The technique of adaptive optics was developed in the 1970s in the military sphere, the first installation scarce twenty years later, in the ground-based observational astronomy in the civilian sector.
Application in astronomy and microscopy
With an AO can be for example the wavefront noise generated by turbulent layers of the atmosphere during the passage of starlight (exact phase faults) compensate. Measuring it is a beacon or generated by a laser artificial guide star. Without AO all ground-based astronomical telescopes of the optical section work well below their theoretical possibilities. This means, for example, for a reflecting telescope with a 10 m opening that his resolution or sharpness of the image by a factor of 10-50 (depending on wavelength) is worse than dictated by the telescope optics. The limitation of the image quality is therefore not on the telescopic, but the thermal- optical turbulent air layers.
To correct these atmospheric disturbances ( seeing ) large telescopes with AO are equipped. Also in solar telescopes, this technique is being used. But AO plays an important role in the laser - communication or the laser beam guidance through the atmosphere or in military reconnaissance.
In recent years, the use of these methods for microscopy and in ophthalmology is increasingly being explored in order to compensate for the aberrations of the human eye and either provide diagnostic procedures better resolution, or improve human visual performance.
On the technique of adaptive optics
An AO is usually made up of three components. ( 1) A wavefront sensor - for example, a Hartmann- Shack sensor - measures the optical interference, and ( 2) a control computer calculates correction signals with which ( 3) correction elements can so drive that corrected the result wave fronts are generated. The three components form a closed loop that is traversed in astronomical applications typically several hundred times per second.
In the simplest case, as the correction element is a 2 -axis tilt mirror are used with its help the atmospherically induced image motion can be compensated. The image motion can be measured in this case for example with a Position Sensitive Device.
The compensation of optical aberrations of higher order, such as defocus, astigmatism, coma, etc., (see also Zernike polynomials ) falls within the field of active optics and requires, as well as the "right" adaptive optics, mirrors with a deformable surface or liquid - mirror.
Another technique - the active optics - is used to compensate the mirror curvatures, for example, arises when swiveling the telescope. In astronomy, the two correction methods differ in the speed of implementation: at active optics 1 time per second is controlled in the order in adaptive optics, however, much faster in the order of 100 times per second
Another application of adaptive optics laser cutting systems with carbon dioxide lasers. Since the beam is guided by movable mirror and not via optical fibers, the length of the optical path between laser and workpiece changes depending on the position of the cutting head. To stay in focus, adaptive optics are used. Examples of these are hollow copper mirror. By applying a water pressure, the surface can be curved, whereby the focal position is shifted.