Adaptive Optics

Adaptive Optics refers to Optical systems that can be modified (such as by adjusting the shape of a mirror) to compensate for distortions. An ideal example is the use of information from a beam of light passing through the atmosphere to compensate for the distortion experienced by another beam of light on its passage through the atmosphere.

Adaptive optics systems are used in observational astronomy to eliminate the “twinkling” of stars and in ballistic missile defense to reduce the dispersive effect of the atmosphere on laser beam weapons. At visible and near-infrared wavelengths, the angular resolution of Earth-based telescopes with apertures greater than 10 to 20 centimeters is limited by turbulence in Earth’s atmosphere rather than by the inherent, diffraction-limited image size of the system. Large telescopes are often equipped with adaptive optics systems to compensate for atmospheric turbulence effects, enabling these systems to achieve imaging on scales that approach the diffraction limit. Adaptive optics systems continuously measure the wave front errors resulting from atmospheric turbulence. Then, using a point-like reference source situated above the distorting layers of Earth’s atmosphere, compensation is achieved by rapidly adjusting a deformable optical element located in or near a pupil plane of the optical system.

For example, in the adaptive optics system built for the 2.54-meter (100 inch) telescope at the Mount Wilson Observatory, the incoming light reflected from the telescope mirror is divided into several hundred smaller beams or regions. Observing the beam of light from a star, the system sees hundreds of separate beams that are going in different directions because of the effects of Earth’s atmosphere. The electron circuits in the system compute the bent shape of a deformable mirror surface that would straighten out the separate beams so that they are all going in the same direction. Then a signal is beamed to the deformable mirror to change its shape in accordance with these electronic signals. Simply stated, in an adaptive optics system a crooked beam of light hits a crooked mirror and a straight beam of light is reflected.

Questions to Ponder:

• What is the purpose of Adaptive Optics?
• How does Adaptive Optics correct the problems arising due to atmospheric turbulence?