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Stars in the night sky appear to twinkle because air moving in the atmosphere disrupts starlight before it reaches our eyes. While this phenomenon might make for a great nursery rhyme, it causes major problems for astronomers trying to study the heavens.
Observing objects with space-based telescopes can avoid the disruptive effects of the atmosphere, and astronomers using Earth-based telescope have turned to a method called ‘adaptive optics’ to compensate for atmospheric light-bending.
Adaptive optics are used with massive reflecting telescopes, the workhorses of modern astronomy. Reflecting telescopes are typically based on two mirrors, a large "primary mirror" and a smaller "secondary mirror". The primary mirror is typically at the 'back' end of the telescope's tube, and the secondary mirror is positioned at a 45-degree angle in the line of sight of the eyepiece which holds a magnifying lens. Based on the intended purpose of the reflecting telescope, each of these mirrors could be concave, convex or flat.
To acquire an image, the telescope is directed at an object to allow light from it to enter the telescope tube. Light from the object strikes the primary mirror and is then directed to the secondary mirror. The secondary mirror directs this light to the eyepiece where it is magnified for greater visibility.
Adaptive optics involve state-of-the-art, deformable mirrors governed by computers that can adjust in real-time for the optical distortion brought on by turbulence in the atmosphere to create images of celestial objects almost as sharp as those captured in space. Adaptive optics enables a reflective telescope to see much finer details than what would otherwise be achievable from ground-based telescopes.
When starlight is gathered by a telescope, just ahead of being focused the light going into an adaptive optics system is aligned and is bounced off a deformable mirror. Coming off this mirror, the light passes through a beam-splitter. The shorter wavelengths of light strike a sensor to gauge distortions from the atmosphere. This information is processed and used to adjust the deformable mirror accordingly. Light from the object to be studied is then focused and imaged.
To be properly calibrated, an adaptive optics system requires a reference star of significant brightness that is relatively near to the object being observed. The reference star is used to determine the disruptive effects of the atmosphere, which are used to adjust the deformable mirror.
Due to the fact that suitable reference stars are not readily available all over the evening sky, scientists have developed a way to generate an artificial reference star by shining a strong laser beam into the upper atmosphere. This method has allowed the entire sky to be imaged using adaptive optics.
Adaptive optics systems function at frequencies too high for a primary mirror of conventional reflective telescopes. Therefore, adaptive optic systems are built to act on the secondary mirror and other optical elements within the telescope.
In addition to requiring a reference star, either real or artificial, adaptive optics systems lose sensitivity for resolution with each addition of an optical element, as these elements scatter light and create a small quantity of heat which affects performance in the infrared part of the spectrum.
Requiring high-speed computer processing and unique deformable mirrors, adaptive optics is considered state-of-the-art technology and it is still very much in the development stage. Some of the greatest leaps forward in the technology came after the end of the Cold War when significant amounts of information became declassified.
In addition to being used in astronomy, adaptive optics is also applied in ophthalmology, vision science, communications, the shaping of laser beams and military intelligence.
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