Achromatic telescope

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For an achromatic doublet, visible wavelengths have approximately the same focal length.

The achromatic telescope is a refracting telescope that is constructed to correct for chromatic aberration. When an image passes through a lens, the light is diffracted 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. To correct for this, compound lenses are used with material of different indices of refraction. In the achromatic doublet, one of the lenses is normally made of Flint glass, giving it a relatively high index of refraction, while the other if made of Crown glass with a lower index of refraction. The lenses are mounted next to each other and shaped so that the chromatic aberration of one lens is counter-balanced by the chromatic aberration of the other lens. They are designed so as to bring at least two different wavelengths of light to a common focus.

An apochromatic refractor is a variant of the achromatic refracting telescope that uses an arrangement of three or more lenses to correct for chromatic aberration, while also minimizing other types of aberrations that can occur. These lenses can include specialized types of glass, such as flourite or ED glass. They can bring at least three different wavelengths of light to a common focus.

Early refracting telescopes typically used a non-achromatic objective, but their long focal length tended to mask the chromatic aberration. When reflecting telescopes were invented, they had the advantage of having no chromatic aberration. Most modern refracting telescopes use an achromatic or apochromatic design. Catadioptric telescopes use a combination of lenses and mirrors to nearly eliminate aberration.