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.