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Adaptive optics technology has also been used recently to image individual cone photoreceptors in the living, human eye. See here.
When light from a star or another astronomical object enters the Earth's atmosphere, the different temperature layers and different wind speeds distort and move the image in various ways (see astronomical seeing for a proper discussion). The net result is that an 8 meter or 10 meter telescope (like the VLT or Keck), while theoretically capable of milli- arcsecondA second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7. 7E-7 of a circle. It is the angular diameter on object of 1 unit diameter at a distance of 360 resolutionThe word resolution has several meanings, depending on context. Resolution (law), an action taken by a deliberative body, usually a legislature Angular resolution, the capability of an optical sensor to discern small objects Sensor resolution, the precisi, is limited to what the atmosphere permits, which can easily be a factor of 50 or 100 worse.
An adaptive optics system tries to correct these distortions, using a quality-of-image detectorIn telecommunication, the term detector has the following meanings: #A device that is responsive to the presence or absence of a stimulus. In an AM radio receiver, a circuit or device that recovers the signal of interest from the modulated wave. In FM rec, a deformable mirror, and a computer that receives input from the detector and calculates the optimal deformation of the mirror.
This is possible only because the theoretical, perfect image is known in advance: in the case of a point-like star, the image is a circle with certain characteristics dictated by the telescope's apertureAperture an opening. from the Latin apere to open. Photography The aperture defines the size of the opening in the lens, which in advanced cameras can be adjusted to control the amount of light reaching the film or digital sensor ( CCD or CMOS). In combin.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where 'guide stars'In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth. Accurate telescope pointing and tracking is critical for of sufficient luminosityIn physics, luminosity is the density of luminous intensity in a given direction. In astronomy, luminosity is the amount of energy a body radiates in unit time. It is typically expressed in the SI unit watts or in terms of solar luminosities, L; that is, (for current systems, about magnitudeIn science, magnitude refers to the numerical size of something: see orders of magnitude. In mathematics, the magnitude of an object is a non-negative real number, which in simple terms is its length. In astronomy, magnitude refers to the logarithmic meas 11-12) can be found very near to the object of the observation.
An alternative is the use of a laser beam to generate a beacon in the atmosphere, to act as a guide starIn astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth. Accurate telescope pointing and tracking is critical for - (referred to as a laser guide star).
Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include Lucky Imaging and space telescope such as NASA's Hubble Space Telescope