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Chromatic aberration is an aberration (distortion) in the image of an optical instrument (such as a telescope), such that different wavelengths of EMR are focused differently, for example, a point source (such as a star) may focus like the output of a prism, with the red end of its spectrum not quite at the same point as the blue end of its spectrum. This contributes to a poorer angular resolution. Chromatic aberration is not introduced by mirrors, but lenses always contribute to it because the degree to which they redirect light (refraction) always depends upon the light's wavelength: most of the lens, where light passing through is functioning as a prism, the light in the image slightly dispersed. High quality cameras use a series of lenses (compound lenses) such that they compensate for each other regarding chromatic aberration, reducing it considerably. A reason for astronomy's preference for reflector telescopes is the primary mirror and any subsequent mirrors make no contribution to chromatic aberration.