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An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
s and
microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...
s. It is 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 Focal point may refer to: * Focus (optics) * Focus (geometry) * Conjugate points, also called focal points * Focal point (game theory) * Unicom Focal Point UNICOM Focal Point is a portfolio management and decision analysis tool used by the p ...
of the objective to magnify this image. The amount of magnification depends on the
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
of the eyepiece. An eyepiece consists of several " lens elements" in a housing, with a "barrel" on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be focused by moving the eyepiece nearer and further from the objective. Most instruments have a focusing mechanism to allow movement of the shaft in which the eyepiece is mounted, without needing to manipulate the eyepiece directly. The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. With telescopes and microscopes, however, eyepieces are usually interchangeable. By switching the eyepiece, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying
fields of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...
, and differing degrees of eye relief for the person who looks through them.


Eyepiece properties

Several properties of an eyepiece are likely to be of interest to a user of an optical instrument, when comparing eyepieces and deciding which eyepiece suits their needs.


Design distance to entrance pupil

Eyepieces are optical systems where the
entrance pupil In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front (the object side) of the lens system. The corresponding image of the aperture as seen through the back of the lens system ...
is invariably located outside of the system. They must be designed for optimal performance for a specific distance to this entrance pupil (i.e. with minimum aberrations for this distance). In a refracting astronomical telescope the entrance pupil is identical with the objective. This may be several feet distant from the eyepiece; whereas with a microscope eyepiece the entrance pupil is close to the back focal plane of the objective, mere inches from the eyepiece. Microscope eyepieces may be corrected differently from telescope eyepieces; however, most are also suitable for telescope use.


Elements and groups

''Elements'' are the individual lenses, which may come as simple lenses or "singlets" and cemented doublets or (rarely) triplets. When lenses are cemented together in pairs or triples, the combined elements are called ''groups'' (of lenses). The first eyepieces had only a single lens element, which delivered highly distorted images. Two and three-element designs were invented soon after, and quickly became standard due to the improved image quality. Today, engineers assisted by computer-aided drafting software have designed eyepieces with seven or eight elements that deliver exceptionally large, sharp views.


Internal reflection and scatter

Internal reflections, sometimes called "scatter", cause the light passing through an eyepiece to disperse and reduce the contrast of the image projected by the eyepiece. When the effect is particularly bad, "ghost images" are seen, called "ghosting". For many years, simple eyepiece designs with a minimum number of internal air-to-glass surfaces were preferred to avoid this problem. One solution to scatter is to use thin film coatings over the surface of the element. These thin coatings are only one or two
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s deep, and work to reduce reflections and scattering by changing the
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
of the light passing through the element. Some coatings may also absorb light that is not being passed through the lens in a process called
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflect ...
where the light incident on the film is at a shallow angle.


Chromatic aberration

''Lateral'' or ''transverse''
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
is caused because the
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomen ...
at glass surfaces differs for light of different wavelengths. Blue light, seen through an eyepiece element, will not focus to the same point but along the same axis as red light. The effect can create a ring of false colour around point sources of light and results in a general blurriness to the image. One solution is to reduce the aberration by using multiple elements of different types of glass. Achromats are lens groups that bring two different wavelengths of light to the same focus and exhibit greatly reduced false colour. Low dispersion glass may also be used to reduce chromatic aberration. ''Longitudinal'' chromatic aberration is a pronounced effect of
optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through elect ...
objectives, because the focal lengths are so long. Microscopes, whose focal lengths are generally shorter, do not tend to suffer from this effect.


Focal length

The
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converge to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in
millimetre 330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter. The millimetre (American and British English spelling differences#-re, -er, ...
s when referring to the eyepiece alone. When interchanging a set of eyepieces on a single instrument, however, some users prefer to refer to identify each eyepiece by the magnification produced. For a telescope, the angular magnification ''MA'' produced by the combination of a particular eyepiece and objective can be calculated with the following formula: :\mathrm= \frac where: * f_O is the focal length of the objective, * f_E is the focal length of the eyepiece. Magnification increases, therefore, when the focal length of the eyepiece is shorter or the focal length of the objective is longer. For example, a 25 mm eyepiece in a telescope with a 1200 mm focal length would magnify objects 48 times. A 4 mm eyepiece in the same telescope would magnify 300 times. Amateur astronomers tend to refer to telescope eyepieces by their focal length in millimetres. These typically range from about 3 mm to 50 mm. Some astronomers, however, prefer to specify the resulting magnification power rather than the focal length. It is often more convenient to express magnification in observation reports, as it gives a more immediate impression of what view the observer actually saw. Due to its dependence on properties of the particular telescope in use, however, magnification power alone is meaningless for describing a telescope eyepiece. For a compound microscope the corresponding formula is :\mathrm= \frac = \frac \times \frac where * D is the distance of closest distinct vision (usually 250 mm) * D_\mathrm is the distance between the back focal plane of the objective and the back focal plane of the eyepiece (called tube length), typically 160 mm for a modern instrument. * f_O is the objective focal length and f_E is the eyepiece focal length. By convention, microscope eyepieces are usually specified by ''power'' instead of focal length. Microscope eyepiece power P_\mathrm and objective power P_\mathrm are defined by : P_\mathrm = \frac, \qquad P_\mathrm = \frac thus from the expression given earlier for the angular magnification of a compound microscope : \mathrm = P_\mathrm \times P_\mathrm The total angular magnification of a microscope image is then simply calculated by multiplying the eyepiece power by the objective power. For example, a 10× eyepiece with a 40× objective will magnify the image 400 times. This definition of lens power relies upon an arbitrary decision to split the angular magnification of the instrument into separate factors for the eyepiece and the objective. Historically, Abbe described microscope eyepieces differently, in terms of angular magnification of the eyepiece and 'initial magnification' of the objective. While convenient for the optical designer, this turned out to be less convenient from the viewpoint of practical microscopy and was thus subsequently abandoned. The generally accepted visual distance of closest focus D is 250 mm, and eyepiece power is normally specified assuming this value. Common eyepiece powers are 8×, 10×, 15×, and 20×. The focal length of the eyepiece (in mm) can thus be determined if required by dividing 250 mm by the eyepiece power. Modern instruments often use objectives optically corrected for an infinite tube length rather than 160 mm, and these require an auxiliary correction lens in the tube.


Location of focal plane

In some eyepiece types, such as Ramsden eyepieces (described in more detail below), the eyepiece behaves as a magnifier, and its focal plane is located outside of the eyepiece in front of the field lens. This plane is therefore accessible as a location for a graticule or micrometer crosswires. In the Huygenian eyepiece, the focal plane is located between the eye and field lenses, inside the eyepiece, and is hence not accessible.


Field of view

The field of view, often abbreviated FOV, describes the area of a target (measured as an angle from the location of viewing) that can be seen when looking through an eyepiece. The field of view seen through an eyepiece varies, depending on the magnification achieved when connected to a particular telescope or microscope, and also on properties of the eyepiece itself. Eyepieces are differentiated by their ''field stop'', which is the narrowest aperture that light entering the eyepiece must pass through to reach the field lens of the eyepiece. Due to the effects of these variables, the term "field of view" nearly always refers to one of two meanings: ;Actual field of view:The angular size of the amount of sky that can be seen through an eyepiece when used with a particular telescope, producing a specific magnification. It ranges typically between 0.1 and 2 degrees. ;Apparent field of view:This is a measure of the angular size of the image viewed through the eyepiece. In other words, it is how large the image appears (as distinct from the magnification). This is constant for any given eyepiece of fixed focal length, and may be used to calculate what the ''actual'' field of view will be when the eyepiece is used with a given telescope. The measurement ranges from 30 to 110 degrees. It is common for users of an eyepiece to want to calculate the actual field of view, because it indicates how much of the sky will be visible when the eyepiece is used with their telescope. The most convenient method of calculating the actual field of view depends on whether the apparent field of view is known. ''If the apparent field of view is known,'' the actual field of view can be calculated from the following approximate formula: :FOV_C= \frac ::or :FOV_C= \frac where: * FOV_C is the actual field of view, calculated in the unit of angular measurement in which FOV_P is provided. * FOV_P is the apparent field of view. * mag is the magnification. * f_T is the focal length of the telescope. * f_E is the focal length of the eyepiece, expressed in the same units of measurement as f_T. The ''focal length'' of the telescope objective is the diameter of the objective times the focal ratio. It represents the distance at which the mirror or objective lens will cause light to converge on a single point. The formula is accurate to 4% or better up to 40° apparent field of view, and has a 10% error for 60°. ''If the apparent field of view is unknown,'' the actual field of view can be approximately found using: :FOV_C= \frac where: * FOV_C is the actual field of view, calculated in degrees. * d is the diameter of the eyepiece field stop in mm. * f_T is the focal length of the telescope, in mm. The second formula is actually more accurate, but field stop size is not usually specified by most manufacturers. The first formula will not be accurate if the field is not flat, or is higher than 60° which is common for most ultra-wide eyepiece design. The above formulae are approximations. The ISO 14132-1:2002 standard determines how the exact apparent angle of view (AAOV) is calculated from the real angle of view (AOV). :\tan\frac= mag \times \tan \frac If a diagonal or Barlow lens is used before the eyepiece, the eyepiece's field of view may be slightly restricted. This occurs when the preceding lens has a narrower field stop than the eyepiece's, causing the obstruction in the front to act as a smaller field stop in front of the eyepiece. The precise relationship is given by :=2 \times \arctan \frac This formula also indicates that, for an eyepiece design with a given apparent field of view, the barrel diameter will determine the maximum focal length possible for that eyepiece, as no field stop can be larger than the barrel itself. For example, a Plössl with 45° apparent field of view in a 1.25 inch barrel would yield a maximum focal length of 35mm. Anything longer requires larger barrel or the view is restricted by the edge, effectively making the field of view less than 45°.


Barrel diameter

Eyepieces for telescopes and microscopes are usually interchanged to increase or decrease the magnification, and to enable the user to select a type with certain performance characteristics. To allow this, eyepieces come in standardized "Barrel diameters".


Telescope eyepieces

There are six standard barrel diameters for telescopes. The barrel sizes (usually expressed in
inch Measuring tape with inches The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth ...
es) are: * 0.965 in. (24.5 mm) – This is the smallest standard barrel diameter and is usually found in toy store and
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retail telescopes. Many of these eyepieces that come with such telescopes are plastic, and some even have plastic lenses. High-end telescope eyepieces with this barrel size are no longer manufactured, but you can still purchase Kellner types. * 1.25 in. (31.75 mm) – This is the most popular telescope eyepiece barrel diameter. The practical upper limit on focal lengths for eyepieces with 1.25" barrels is about 32 mm. With longer
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
s, the edges of the barrel itself intrude into the view limiting its size. With
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
s longer than 32 mm, the available field of view falls below 50°, which most amateurs consider to be the minimum acceptable width. These barrel sizes are threaded to take 30 mm
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component that ...
. * 2 in. (50.8 mm) – The larger barrel size in 2" eyepieces helps alleviate the limit on focal lengths. The upper limit of focal length with 2" eyepieces is about 55 mm. The trade-off is that these eyepieces are usually more expensive, will not fit in some telescopes, and may be heavy enough to tip the telescope. These barrel sizes are threaded to take 48 mm
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component that ...
(or rarely 49 mm). * 2.7 in. (68.58 mm) – 2.7" eyepieces are made by a few manufacturers. They allow for slightly larger fields of view. Many high-end focusers now accept these eyepieces. * 3 in. (76.2 mm) – The even larger barrel size in 3" eyepieces allows for extreme focal lengths and over 120° field of view eyepieces. The disadvantages are that these eyepieces are somewhat rare, extremely expensive, up to 5 lbs in weight, and that only a few telescopes have focusers large enough to accept them. Their huge weight causes balancing issues in Schmidt-Cassegrains under 10 inches, refractors under 5 inches, and reflectors under 16 inches. Also, due to their large field stops, without larger secondary mirrors most reflectors and Schmidt-Cassegrains will have severe vignetting with these eyepieces. Makers of these eyepieces include Explore Scientific and Siebert Optics. Telescopes that can accept these eyepieces are made by Explore Scientific and Orion Telescopes and Binoculars. * 4 in. (102 mm) – These eyepieces are rare and only commonly used in observatories. They are made by very few manufacturers, and demand for them is low.


Microscope eyepieces

Eyepieces for microscopes have barrel diameters measured in millimeters such as 23.2 mm and 30 mm.


Eye relief

The eye needs to be held at a certain distance behind the eye lens of an eyepiece to see images properly through it. This distance is called the eye relief. A larger eye relief means that the optimum position is farther from the eyepiece, making it easier to view an image. However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position. The eye pupil should coincide with the exit pupil, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass. Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Until recently, and still quite commonly, eyepieces of a short-focal length have had a short eye relief. Good design guidelines suggest a minimum of 5–6 mm to accommodate the eyelashes of the observer to avoid discomfort. Modern designs with many lens elements, however, can correct for this, and viewing at high power becomes more comfortable. This is especially the case for
spectacle In general, spectacle refers to an event that is memorable for the appearance it creates. Derived in Middle English from c. 1340 as "specially prepared or arranged display" it was borrowed from Old French ''spectacle'', itself a reflection of t ...
wearers, who may need up to 20 mm of eye relief to accommodate their glasses.


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.


Negative lens or "Galilean"

The simple negative lens placed before the focus of the objective has the advantage of presenting an erect image but with limited field of view better suited to low magnification. It is suspected this type of lens was used in some of the first refracting telescopes that appeared in the Netherlands in about 1608. It was also used in
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He ...
'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.


Convex lens

A simple convex lens placed after the focus of the objective lens presents the viewer with a magnified inverted image. This configuration may have been used in the first refracting telescopes from the Netherlands and was proposed as a way to have a much wider field of view and higher magnification in telescopes in
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
'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).


Huygens

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 Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists o ...
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 Abbe number, to be used with a relaxed eye and a telescope with an infinitely distant objective then the separation is given by: : d= \frac (f_A + f_B) where f_A and f_B 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 telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses an ...
s, 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 for prolonged periods of time. Cemented eyepieces are traditionally regarded as potentially vulnerable to heat damage by the intense concentrations of light involved.


Ramsden

The Ramsden eyepiece comprises two plano-convex lenses of the same glass and similar focal lengths, placed less than one eye-lens focal length apart, a design created by astronomical and scientific instrument maker
Jesse Ramsden Jesse Ramsden FRS FRSE (6 October 1735 – 5 November 1800) was a British mathematician, astronomical and scientific instrument maker. His reputation was built on the engraving and design of dividing engines which allowed high accuracy measurem ...
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 eye-lens, 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.


Kellner or "Achromat"

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 apparent field of view is 40–50°.


Plössl or "Symmetrical"

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° or more ''apparent'' field of view, along with relatively large FOV. This makes this eyepiece ideal for a variety of observational purposes including deep-sky and
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
ary 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 manufacturers 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.


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 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.


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 Hugo 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 coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the ef ...
s). It has a narrow field of view of around 25° and is a favorite amongst planetary observers.


Erfle

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 August 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 length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
s 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.


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) 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 of 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°.


RKE

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 its design similar to a widely spaced version of the
König König (; ) is the German word for "king". In German and other languages applying the umlaut, the transliterations ''Koenig'' and ''Kœnig'', when referring to a surname, also occur. As a surname in English, the use of ''Koenig'' is usual, and som ...
. According to Edmund Scientific Corporation, ''RKE'' stands for "Rank Kellner Eyepiece'". In an amendment to their trademark application on January 16, 1979 it was given as "Rank-Kaspereit-Erfle", the three designs from which the eyepiece was derived. A March 1978 Edmund Astronomy News (Vol 16 No 2) ran the headline "''New Eyepiece Design Developed By Edmund''" and said "The new 28mm and 15mm Rank-Kaspereit-Erfle (RKE) eyepieces are American redesigns of the famous Type II Kellner eyepiece."


Nagler

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 several similar designs called the ''Nagler'', ''Nagler type 2'', ''Nagler type 4'', ''Nagler type 5'', and ''Nagler type 6''. The newer Delos design is a modified Ethos design with a FOV of 'only' 72 degrees but with a long 20 mm 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 The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...
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; they are often ruefully referred to as ‘
hand grenade A grenade is an explosive weapon typically thrown by hand (also called hand grenade), but can also refer to a shell (explosive projectile) shot from the muzzle of a rifle (as a rifle grenade) or a grenade launcher. A modern hand grenade ...
s’ because of their heft and large size. Long focal length versions exceed , which is enough to unbalance small to medium-sized 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.


See also

* Barlow lens *
List of telescope parts and construction Hardware Accessories *Finderscope *Iron sight * Reflector (reflex) sight * Cheshire collimator: A simple tool to collimate a telescope Control *Clock drive * GoTo Mechanical construction *Mirror support cell * Serrurier truss * Silvering Mounts ...
*
Optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of micro ...
* Monocle *
Optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through elect ...
* Pocket comparator


References

* A. E. Conrady, ''Applied Optics and Optical Design, Volume I''. Oxford 1929. * R. Kingslake, ''Lens Design Fundamentals''. Academic Press 1978. * H. Rutten and M. van Venrooij, ''Telescope Optics''. Willmann-Bell 1988, 1989. . * P. S. Harrington, ''Star Ware: An Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories: Fourth Edition''. John Wiley & Sons, Inc.


External links

*
EYEPIECE EVOLUTIONA. Nagler - United States Patent US4286844A. Nagler - United States Patent US4747675A. Nagler - United States Patent US4525035A. Nagler - Finder scope for use with astronomical telescopesThe evolution of the astronomical eyepiece, in-depth discussion of various design and theoretical background
* ttp://www.optics.arizona.edu/detlab/Classes/Opti340/OPTI340_Spring09/Patents/4286844.pdf United States Patent Office: Ultra wide ocular NAGLER. {{Authority control Lenses Microscope components Telescopes