4.7 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Eyepiece | 6/7 | https://en.wikipedia.org/wiki/Eyepiece | reference | science, encyclopedia | 2026-05-05T09:47:24.682125+00:00 | kb-cron |
=== Orthoscopic or "Abbe" ===
The 4-element orthoscopic eyepiece consists of a plano-convex singlet eye lens and a cemented convex-convex triplet field lens achromatic field lens. This gives the eyepiece a nearly perfect image quality and good eye relief, but a narrow apparent field of view — about 40°–45°. It was invented by Ernst Abbe in 1880. It is called "orthoscopic" or "orthographic" because of its low degree of distortion and is also sometimes called an "ortho" or "Abbe". Until the advent of multicoating and the popularity of the Plössl, orthoscopic eyepieces were the most popular design for telescope eyepieces. Even today these eyepieces are considered good eyepieces for planetary and lunar viewing. They are preferred for reticle eyepieces, since they are one of the wide-field, long eye-relief designs with an external focal plane; slowly being supplanted by the König. Due to their low degree of distortion and the corresponding globe effect, they are less suitable for applications which require an extensive panning of the instrument.
=== Monocentric ===
A Monocentric is an achromatic triplet lens with two pieces of crown glass cemented on both sides of a flint glass element. The elements are thick, strongly curved, and their surfaces have a common center giving it the name "monocentric". It was invented by H.A. Steinheil around 1883. This design, like the solid eyepiece designs of Tolles, Hastings, and Taylor, is free from ghost reflections and gives a bright contrasty image, a desirable feature when it was invented (before anti-reflective coatings). It has a narrow apparent field of view around 25° but was favored by planetary observers.
=== Erfle ===
An Erfle is a 5 element eyepiece consisting of 2 achromatic doublets with an extra simple lens between them. They were invented by Heinrich Erfle during World War I for military use. The design is an elementary extension of 4 element eyepieces such as Plössls, enhanced for wider fields. Erfle eyepieces are designed to have wide field of view (about 60°), but are unusable at high powers because they suffer from astigmatism and ghost images. However, with lens coatings at low powers (focal lengths of 20~30 mm and up) they are acceptable, and at 40 mm they can be excellent. Erfles are very popular for wide-field views, because they have large eye lenses, and can be very comfortable to use because of their good eye relief in longer focal lengths.
=== König ===
The König eyepiece has a concave-convex positive doublet and a plano-convex singlet. The strongly convex surfaces of the doublet and singlet face and (nearly) touch each other. The doublet has its concave surface facing the light source and the singlet has its almost flat (slightly convex) surface facing the eye. It was designed in 1915 by German optician Albert König (1871−1946) and is effectively a simplified Abbe. The design allows for high magnification with remarkably high eye relief – the longest eye relief proportional to focal length of any design before the Nagler, in 1979. The field of view of about 55° is slightly superior to the Plössl, with the further advantages of better eye relief and requiring one less lens element. Modern improvements typically have fields of view of 60°−70°. König design revisions use exotic glass and / or add more lens groups; the most typical adaptation is to add a simple positive, concave-convex lens before the doublet, with the concave face towards the light source and the convex surface facing the doublet.
=== RKE ===
An RKE eyepiece has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner eyepiece, with its lens layout similar to the König. It was designed by Dr. David Rank for the Edmund Scientific Corporation, who marketed it throughout the late 1960s and early 1970s. This design provides slightly wider field of view than classic Kellner design and makes its design similar to a widely spaced version of the König. According to Edmund Scientific Corporation, RKE stands for "Rank Kellner Eyepiece'". In an amendment to their trademark application on 16 January 1979 it was given as "Rank-Kaspereit-Erfle", the three designs from which the eyepiece was derived. Edmund Astronomy News (March 1978) called the eyepiece the "Rank-Kaspereit-Erfle" (RKE) a "redesign[ed] ... type II Kellner". However, the RKE deign does not resemble a Kellner, and is closer to a modified König. There is some speculation that at some point the "K" was mistakenly interpreted as the name of the more common Kellner, instead of the fairly rarely seen König.
=== Nagler ===