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adaptive optics  A system for modifying the shape of the mirror of a telescope to compensate for atmospheric seeing and to produce sharp images.
charge coupled device (CCD)  An array of photosensitive electronic elements that can be used to record an image falling on it.
detector  A device used to measure light once it has been brought into focus by a telescope.
dispersion  The separation of white light according to wavelength.
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.
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.
energy flux  The rate at which a wave carries energy through a given area.
focal length  The distance between a mirror or lens and the point at which the lens or mirror brings light to a focus.
focal plane  The surface where the objective lens or mirror of a telescope forms the image of an extended object.
focal point  The spot where parallel beams of light striking a lens or mirror are brought to a focus.
frequency  The number of oscillations per second of a wave.
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.
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.
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.
objective  The main lens or mirror of a telescope.
photon  A massless particle of electromagnetic energy.
pixel  A "picture element," consisting of an individual detector in an array of detectors used to capture an image.
reflection  The bouncing of a wave from a surface.
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.
reflector  A telescope in which the objective is a mirror.
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.
refractor  A telescope in which the objective is a lens.
resolution  The ability of a telescope to distinguish fine details of an image.
seeing  A measure of the blurring of the image of an astronomical object caused by turbulence in the Earth’s atmosphere.
spectrograph  A device used to produce and record a spectrum.
spectroscopy  The recording and analysis of spectra.
wave  A regular series of disturbances that moves through a material medium or through empty space.
wavelength  The distance between crests of a wave. For visible light, wavelength determines color.

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