| adaptive optics | A system for modifying the shape of the mirror of a telescope to compensate for atmospheric seeing and to produce sharp images.
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| charge coupled device (CCD) | An array of photosensitive electronic elements that can be used to record an image falling on it.
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| detector | A device used to measure light once it has been brought into focus by a telescope.
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| dispersion | The separation of white light according to wavelength.
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| Doppler effect | The change in the frequency of a wave (such as electromagnetic radiation) caused by the motion of the source and observer toward or away from each other.
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| electromagnetic wave | A periodic electrical and magnetic disturbance that propagates through space and transparent materials at the speed of light. Light is an example of an electromagnetic wave.
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| energy flux | The rate at which a wave carries energy through a given area.
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| focal length | The distance between a mirror or lens and the point at which the lens or mirror brings light to a focus.
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| focal plane | The surface where the objective lens or mirror of a telescope forms the image of an extended object.
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| focal point | The spot where parallel beams of light striking a lens or mirror are brought to a focus.
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| frequency | The number of oscillations per second of a wave.
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| index of refraction | The ratio of the speed of light in a vacuum to the speed of light in a particular substance. The index of refraction, which always has a value greater than 1.0, describes how much a beam of light is bent on entering or emerging from the substance.
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| interferometry | The use of two or more telescopes connected together to operate as a single instrument. Interferometers can achieve high angular resolution if the individual telescopes of which they are made are widely separated.
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| light-gathering power | A number, proportional to the area of the principal lens or mirror of a telescope, that describes the amount of light that is collected and focused by the telescope.
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| objective | The main lens or mirror of a telescope.
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| photon | A massless particle of electromagnetic energy.
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| pixel | A "picture element," consisting of an individual detector in an array of detectors used to capture an image.
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| reflection | The bouncing of a wave from a surface.
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| reflectivity | The ability of a surface to reflect electromagnetic waves. The reflectivity of a surface ranges from 0% for a surface that reflects no light to 100% for a surface that reflects all the light falling on it.
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| reflector | A telescope in which the objective is a mirror.
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| refraction | The bending of light when it passes from a material having one index of refraction to another material having a different index of refraction.
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| refractor | A telescope in which the objective is a lens.
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| resolution | The ability of a telescope to distinguish fine details of an image.
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| seeing | A measure of the blurring of the image of an astronomical object caused by turbulence in the Earth’s atmosphere.
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| spectrograph | A device used to produce and record a spectrum.
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| spectroscopy | The recording and analysis of spectra.
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| wave | A regular series of disturbances that moves through a material medium or through empty space.
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| wavelength | The distance between crests of a wave. For visible light, wavelength determines color.
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