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Wavelength - The distance between two peaks in a wave of light.  Light includes all electromagnetic radiation from gamma rays to radio waves, including visible light.  Gamma rays have very short wavelengths, while radio waves have long wavelengths.  Within the visible spectrum, blue light has the shortest wavelengths, while red light has the longest.  Visible light wavelengths are usually measured in nanometers (nm), or billionths of a meter.  Sometimes, a unit called Angstroms (Å), a tenth of a nanometer, is used.  The visible spectrum covers the range of wavelengths from about 400-700nm.  For comparison, gamma rays have a wavelength measured in picometers (1/1000th of a nanometer), while FM radio waves have wavelengths of 1-10 meters.  Optical errors in a telescope are often measured in fractions of a wavelength of visible light.  The inverse of wavelength is frequency; the shorter the wavelength, the higher the frequency.

Wedge - An equatorial wedge is used to convert an alt-azimuth mount into an equatorial mount.  This is necessary for long-exposure astrophotography to eliminate field rotation.

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