Erfle eyepiece
The Erfle eyepiece is the first wide-angle - eyepiece for optical devices. It was developed by Heinrich Erfle in 1917 . Erfle eyepieces are comparatively cheap to manufacture. They are therefore still produced today for amateur astronomy or wide-angle binoculars.
The "Erfle" consists of three groups of lenses. A converging lens is inserted between two opposite achromatic lenses (doublets) . In addition to this 5th lens, it differs from the similar Plössl eyepiece , invented around 1840, by the greater curvature of the front achromatic lens. The construction for the wide-angle eyepiece was patented by Heinrich Erfle for Carl Zeiss in 1921 (1923 in the USA: patent number 1478704).
The "Erfle" typically gives an apparent field of view of up to 68 °. A large field of view is a sought-after property, especially for military applications ( periscope ) and other outdoor use .
Erfle eyepieces tend to astigmatism , a comet-shaped distortion of light sources , at the edge of the field of view . In addition, internal reflections (“ghosting”) can easily occur . This makes them suitable for observing bright objects, e.g. B. of planets , less suitable in observational astronomy . On the other hand, they are ideal for faint, extended objects such as open star clusters and emission or reflection nebulae .