secondary spectrum

Definition: Aberrations present in an achromat i.e. an optic with basic correction for chromatic aberrations. * Seen as difference in focal length of one remaining waveband e.g. green where two others are corrected: causes at best lost of definitionl, at worse, colour fringing.* Apochromats or fully corrected optics show no secondary colour. * Do not confuse with secondary colour, one resulting from mixture of primary colours.

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