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achromatic telescope
The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.
==How it works==
When an image passes through a lens, the light is refracted at different angles for different wavelengths. This produces focal lengths that are dependent on the color of the light. So, for example, at the focal plane an image may be focused at the red end of the spectrum, but blurred at the blue end. This effect is particularly noticeable the further an object lies from the central axis of the telescope. The image of a star can appear blue on one side and orange on the other. Early refracting telescopes with non-achromatic objectives were constructed with very long focal lengths to mask the chromatic aberration. An Achromatic telescope uses an achromatic lens to correct for this. An achromatic lens is a compound lenses made with two types of glass with different dispersion. One element, a ''concave'' lens made out of Flint glass, has relatively high dispersion, while the other, a ''convex'' element made of Crown glass, has a lower dispersion. The crown lens is usually placed at the front due to the higher susceptibility of flint glass to atmospheric attack (exception: Steinheil doublet). The lens elements are mounted next to each other and shaped so that the chromatic aberration of one is counterbalanced by the chromatic aberration of the other, while the positive power of the crown lens element is not quite equaled by the negative power of the flint lens element. Together they form a weak positive lens that will bring two different wavelengths of light to a common focus.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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