Turbulence in the atmosphere causes point sources of light, such as stars, to appear to shift rapidly and in random directions. In effect the atmosphere acts as a chaotic lens distorting the image of the stars in our skies. It was once thought that this formed a fundamental limit on ground based astronomy, and led directly to the development of space based telescopes such as the Hubble Space Telescope Mission. An alternative was to correct for these atmospheric distortions, in effect refocusing this image, by rapidly deforming one or more of the telescope mirrors. This idea was suggested as far back as 1953, but the technology of the time was not up to the task.
A modern reflecting telescope uses two or more mirrors to focus light into a single image onto the eyepiece, or more commonly onto an electronic detector (see Cornell University telescope tutorial here). A telescope equipped with adaptive optics focuses on a scientific target such as a faint distant galaxy. The distortion of this image is assumed to be approximately the same as that of a nearby reference star. The distortion of this object is then measured and a feedback system corrects for this distortion. You can see an example of this remarkable technology in the image of the binary star IW Tau.
A modality is a tool or set of methods of investigation. As with the case of Adaptive Optics the tools used to image distant stars are now being turned to fields that the inventors could not have imagined. In the field of anatomy, underlying layers of the retina are obscured by other layers. Nanometer sized fluorescent beads can be positioned within the layers of the retina and can act as artificial guide stars. A similar method can be used to peer into the interior of cells undergoing cell division.
********* To complete this discussion activity:
Do your own internet research, and then write an initial response to the following three discussion prompts. Post your response to the discussion board.
Describe the operation and limitations of adaptive optics system as used in astronomy. In your response be sure to use the terms, guide star, artificial guide star, field of view, feedback, and atmospheric distortion.
Describe the source of distortion and the means of correction in terms of the wave model of light and wave optics. Be sure to use the terms ray, wave, reflection, refraction and focal point in your response.
Discuss an alternative application of this technology to other areas of scientific research and industry. See if you can come up with your own example of a process limited by optical distortion.