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Glossary | Telescopes | Optics | Lens design | Doublet

A refractor with two lenses. Compared to an achromat, it displays significantly less chromatic aberration and more contrast.

Telescopes classed as apochromats with a doublet lens have a two-lens objective design. The classification no longer makes a distinction between semi-apos and two-lens apochromats.

The second element is usually made of ED glass (ED stands for extra-low dispersion). However, minor chromatic aberrations are still visible, since only a triplet apochromat with a three-lens construction can completely correct them.