An eyepiece design which is in effect an achromatic version of the Ramsden eyepiece, and is therefore sometimes known as the achromatic Ramsden. It has a planoconvex field lens and an achromatic eye lens with a flat surface facing the eye. Its field of view is fairly large, 45–50°, and is free from aberrations. For this reason the Kellner eyepiece is often used in binoculars. It is a good all-purpose eyepiece, giving acceptable results even with low f-numbers, although it is prone to ghost images caused by internal reflections. It was invented in 1849 by the German optician Carl Kellner (1826–55).
Subjects: Astronomy and Astrophysics.