An eyepiece, or ocular lens, commonly abbreviated EP - is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes and allows a variety of magnifications for viewing.


[top]Overview

An eyepiece, or ocular lens, commonly abbreviated EP - is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

[top]Eyepiece Selection and Usage

Without a good eyepiece the best telescope will not perform well. The simplest of all eyepieces is a single double-convex lens. Different types of eyepieces have different strengths and weaknesses. See Eyepiece Selection for more information.

[top]Eyepiece designs

Technology has developed over time and there are a variety of eyepiece designs for use with telescopes, microscopes, gun-sights, and other devices. Some of these designs are described in more detail below.

[top]Convex lens

Convex lens diagram
A simple convex lens placed after the focus of the objective lens presents the viewer with a magnified inverted image. This early configuration was used in Zaccharias Janssen 1590 compound microscope and proposed as a way to have a much wider field of view and higher magnification in telescopes in Johannes Kepler's 1611 book Dioptrice. Since the lens is placed after the focal plane of the objective it also allowed for use of a micrometer at the focal plane (used for determining the angular size and/or distance between objects observed).

[top]Negative lens or "Galilean"

Negative lens diagram
The simple negative lens placed before the focus of the objective has the advantage of presenting an erect image but with limited magnification. This type of lens was used in the first refracting telescopes that appeared in the Netherlands in about 1608. It was also used in Galileo Galilei's 1609 telescope design which gave this type of eyepiece arrangement the name "Galilean". This type of eyepiece is still used in very cheap telescopes, binoculars and in opera glasses.

[top]Huygens

Huygens eyepiece diagram
A Huygens eyepieces consist of two plano-convex lenses with the plane sides towards the eye separated by an air gap. The lenses are called the eye lens and the field lens. The focal plane is located between the two lenses. It was invented by Christiaan Huygens in the late 1660s and was the first compound (multi-lens) eyepiece. Huygens discovered that two air spaced lenses can be used to make an eyepiece with zero transverse chromatic aberration. If the lenses are made of glass of the same refractive index, to be used with a relaxed eye and a telescope with an infinitely distant objective then the separation is given by:where and are the focal lengths of the component lenses.These eyepieces work well with the very long focal length telescopes (in Huygens day they were used with single element long focal length non-achromatic refracting telescopes, including very long focal length aerial telescopes). This optical design is now considered obsolete since with today's shorter focal length telescopes the eyepiece suffers from short eye relief, high image distortion, chromatic aberration, and a very narrow apparent field of view. Since these eyepieces are cheap to make they can often be found on inexpensive telescopes and microscopes.Because Huygens eyepieces do not contain cement to hold the lens elements, telescope users sometimes use these eyepieces in the role of "solar projection", i.e. projecting an image of the Sun onto a screen. Other cemented eyepieces can be damaged by the intense, concentrated light of the Sun.

[top]Ramsden

Ramsden eyepiece diagram
The Ramsden eyepiece comprises two plano convex lenses with the same focal length and glass, placed less than one focal length apart, a design created by astronomical and scientific instrument maker Jesse Ramsden in 1782. The lens separation varies between different designs, but is typically somewhere between 7/10 and 7/8 of the focal length of the lenses, the choice being a trade off between residual transverse chromatic aberration (at low values) and at high values running the risk of the field lens touching the focal plane when used by an observer who works with a close virtual image such as a myopic observer, or a young person whose accommodation is able to cope with a close virtual image (this is a serious problem when used with a micrometer as it can result in damage to the instrument).A separation of exactly 1 focal length is also inadvisable since it renders the dust on the field lens disturbingly in focus. The two curved surfaces face inwards. The focal plane is thus located outside of the eyepiece and is hence accessible as a location where a graticule, or micrometer crosshairs may be placed. Because a separation of exactly one focal length would be required to correct transverse chromatic aberration, it is not possible to correct the Ramsden design completely for transverse chromatic aberration. The design is slightly better than Huygens but still not up to today’s standards.It remains highly suitable for use with instruments operating using near monochromatic light sources e.g. polarimeters.

[top]Kellner or "Achromat"

Kellner eyepiece diagram
In a Kellner eyepiece an achromatic doublet is used in place of the simple plano convex eye lens in the Ramsden design to correct the residual transverse chromatic aberration. Carl Kellner designed this first modern achromatic eyepiece in 1849, also called an "achromatized Ramsden". Kellner eyepieces are a 3-lens design. They are inexpensive and have fairly good image from low to medium power and are far superior to Huygenian or Ramsden design. The eye relief is better than the Huygenian and worse than the Ramsden eyepieces. The biggest problem of Kellner eyepieces was internal reflections. Today's anti-reflection coatings make these usable, economical choices for small to medium aperture telescopes with focal ratio f/6 or longer. The typical field of view is 40 to 50 degrees.

[top]Plössl or "Symmetrical"

Plössl eyepiece diagram
The Plössl is an eyepiece usually consisting of two sets of doublets, designed by Georg Simon Plössl in 1860. Since the two doublets can be identical this design is sometimes called a symmetrical eyepiece. The compound Plössl lens provides a large 50+ degreeapparent field of view along with relatively large FOV. This makes this eyepiece ideal for a variety of observational purposes including deep sky and planetary viewing. The chief disadvantage of the Plössl optical design is short eye relief compared to an orthoscopic since the Plössl eye relief is restricted to about 70–80% of focal length. The short eye relief is more critical in short focal lengths below about 10 mm, when viewing can become uncomfortable especially for people wearing glasses.The Plössl eyepiece was an obscure design until the 1980s when astronomical equipment manufactures started selling redesigned versions of it. Today it is a very popular design on the amateur astronomical market, where the name Plössl covers a range of eyepieces with at least four optical elements.This eyepiece is one of the more expensive to manufacture because of the quality of glass, and the need for well matched convex and concave lenses to prevent internal reflections. Due to this fact, the quality of different Plössl eyepieces varies. There are notable differences between cheap Plössls with simplest anti-reflection coatings and well made ones.

[top]Orthoscopic or "Abbe"

Orthoscopic eyepiece diagram
The 4-element orthographic 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 multicoatings and the popularity of the Plössl, orthoscopics were the most popular design for telescope eyepieces. Even today these eyepieces are considered good eyepieces for planetary and lunar viewing. Due to their low degree of distortion and the corresponding globe effect, they are less suitable for applications which require an excessive panning of the instrument.

[top]Monocentric

Monocentric eyepiece diagram
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 Adolf Steinheil around 1883. This design, like the solid eyepiece designs of Robert Tolles, Charles S. Hastings, and E. Wilfred 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 field of view of around 25° and is a favorite amongst planetary observers.

[top]Erfle

Erfle eyepiece diagram
An erfle is a 5-element eyepiece consisting of two achromatic lenses with extra lenses in between. They were invented during the first world war for military purposes, described in US patent by Heinrich Erfle number 1,478,704 of Aug 1921 and are a logical extension to wider fields of four element eyepieces such as Plössls.Erfle eyepieces are designed to have wide field of view (about 60 degrees), but they are unusable at high powers because they suffer from astigmatism and ghost images. However, with lens coatings at low powers (focal lengths of 20 mm and up) they are acceptable, and at 40 mm they can be excellent. Erfles are very popular because they have large eye lenses, good eye relief and can be very comfortable to use.

[top]König

König eyepiece diagram
The König eyepiece has a concave-convex positive doublet and a convex~flat positive 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) as a simplified Abbe. The design allows for high magnification with remarkably high eye relief — the highest eye relief proportional to focal length of any design before the Nagler, in 1979. The field of view of about 55° makes its performance similar to the Plössl, with the advantage of requiring one less lens.Modern versions of Königs can use improved glass, or add more lenses, grouped into various combinations doublets and singlets. The most typical adaptation is to add a positive, concave-convex simple lens before the doublet, with the concave face towards the light source and the convex surface facing the doublet. Modern improvements typically have fields of view of 60°−70°.

[top]RKE

RKE eyepiece diagram
An RKE eyepiece has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner eyepiece. 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 it design similar to a widely spaced version of the König.There is much ambiguity about what RKE stands for. According to Edmund Scientific Corporation, RKE stands for Rank Kellner Eyepiece. It is in effect a reversed version of the Kellner design on which it is based. The trademark RKE was first used in commerce on October 14, 1977 by Edmunds, and in an amendment to their trademark application on Jan 16, 1979 they indicated that RKE stands for Rank, Kaspereit, Erfle, the three designs from which the eyepiece was derived!

[top]Nagler

Nagler type 2 eyepiece diagram
Nagler type eyepiece diagrams
Invented by Albert Nagler and patented in 1979, the Nagler eyepiece is a design optimized for astronomical telescopes to give an ultra-wide field of view (82°) that has good correction for astigmatism and other aberrations. Introduced in 2007, the Ethos is an enhanced ultra-wide field design developed principally by Paul Dellechiaie under Albert Nagler's guidance at Tele Vue Optics and claims a 100°-110° AFOV. This is achieved using exotic high-index glass and up to eight optical elements in four or five groups; there are five similar designs called the Nagler, Nagler type 2, Nagler type 4, Nagler type 5, Nagler type 6. The newer Delos design is a modified Ethos design with a FOV of 'only' 72 degrees but with a long 20mm eye relief.The number of elements in a Nagler makes them seem complex, but the idea of the design is fairly simple: every Nagler has a negative doublet field lens, which increases magnification, followed by several positive groups. The positive groups, considered separate from the first negative group, combine to have long focal length, and form a positive lens. That allows the design to take advantage of the many good qualities of low power lenses. In effect, a Nagler is a superior version of a Barlow lens combined with a long focal length eyepiece. This design has been widely copied in other wide field or long eye relief eyepieces.The main disadvantage to Naglers is in their weight. Long focal length versions exceed 0.5 kg (1 lb), which is enough to unbalance small telescopes. Another disadvantage is a high purchase cost, with large Naglers' prices comparable to the cost of a small telescope. Hence these eyepieces are regarded by many amateur astronomers as a luxury.

[top]Eyepiece Discussion Forums

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[top]Eyepiece Terms


AFOV - Apparent Field of View
TFOV

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