Eyepiece - AbsoluteAstronomy.com

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescope

Optical telescope

An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

s and microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

s. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective

Objective (optics)

In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

of the objective to magnify this image. The amount of magnification depends on the focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

of the eyepiece.

An eyepiece consists of several "lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

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

Focus (optics)

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

Field of view

The field of view is the extent of the observable world that is seen at any given moment....

, and differing degrees of eye relief

Eye relief

-Eye Relief and Exit Pupil:The eye relief of a telescope, a microscope, or binoculars is the distance from the last surface of an eyepiece at which the eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained...

for the person who looks through them.

Modern research-grade telescopes do not use eyepieces. Instead, they have high-quality CCD sensors mounted at the focal point, and the images are viewed on a computer screen

Computer display

A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

. Some amateur astronomers

Amateur astronomy

Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

use their telescopes the same way, but direct optical viewing with eyepieces is still very common.

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 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

Objective (optics)

. 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 lens

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

es or "singlets" and cemented doublet

Doublet (lens)

In optics, a doublet is a type of lens made up of two simple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element." With additional degrees of freedom, optical designers have...

s or (rarely) triplet

Triplet lens

A triplet lens is an optical device consisting of three single lenses. The triplet design is the simplest to give the required number of degrees of freedom to allow the lens designer to overcome all Seidel aberrations....

s. 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

Contrast (vision)

Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...

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

Thin film

A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....

coatings over the surface of the element. These thin coatings are only one or two wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s deep, and work to reduce reflections and scattering by changing the refraction

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

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

Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...

where the light incident on the film is at a shallow angle.

Chromatic aberration

Lateral chromatic aberration

Chromatic aberration

In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

is caused because the refraction

Refraction

at glass surfaces differs for light of different wavelengths. Blue light, seen through an eyepiece element, will not focus to the same plane 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

Optical telescope

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

Focal length

of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converges 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

Millimetre

The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

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:

where:

is the focal length of the objective,
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

where

is the distance of closest distinct vision
Least distance of distinct vision
In optometry, the least distance of distinct vision or the reference seeing distance is the closest someone with "normal" vision can comfortably look at something...

(usually 250 mm)
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.
is the objective focal length and is the eyepiece focal length.

By convention, microscope eyepieces are usually specified by power instead of focal length. Microscope eyepiece power

and objective power

are defined by

thus from the expression given earlier for the angular magnification of a compound microscope

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

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

Field lens

In imaging optics, a field lens is a positive-powered lens or group of lenses that comes after the objective lens and before the image plane or the eyepiece, serving to change the size of the image, or to provide image-space telecentricity....

. 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 is typically between one tenth of a degree, and two degrees.
Apparent field of view: this is a measure of the angular size of the image viewed through the eyepiece, in other words, 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 35 to over 80 degree

Degree (angle)

A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s.

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:

where:

is the actual field of view, calculated in the unit of angular measurement in which is provided.
is the apparent field of view.
is the magnification.
is the focal length of the telescope.
is the focal length of the eyepiece, expressed in the same units of measurement as .

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:

where:

is the actual field of view, calculated in degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s.
is the diameter of the eyepiece field stop in mm.
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 aproximations.
The ISO 14132-1:2002 standard determines how the exact apparent angle of view (AAOV) is calculated from the real angle of view (AOV).

Barrel diameter

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

Telescope eyepieces

There are three standard barrel diameters for telescopes. The barrel sizes (usually expressed in inches) are:

0.965 inch (24.5 mm) - This is the smallest standard barrel diameter and is usually found in toy store and shopping mall
Shopping mall
A shopping mall, shopping centre, shopping arcade, shopping precinct or simply mall is one or more buildings forming a complex of shops representing merchandisers, with interconnecting walkways enabling visitors to easily walk from unit to unit, along with a parking area — a modern, indoor version...

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.

1¼ inch (31.75 mm) - 1¼ inch is the most popular telescope eyepiece barrel diameter. The practical upper limit on focal lengths for eyepieces with 1¼ inch barrels is about 32 mm. With longer focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

s, the edges of the barrel itself intrude into the view limiting its size. With focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

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
Photographic filter
In photography and videography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or oblong shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which...

.

2 inch (50.8 mm) - The larger barrel size in 2 inch eyepieces helps alleviate the limit on focal lengths. The upper limit of focal length with 2 inch eyepieces is about 55 mm. The trade-off is that these eyepieces are usually more expensive, won't fit in some telescopes, and may be heavy enough to tip the telescope. These barrel sizes are threaded to take 48 mm filters
Photographic filter
In photography and videography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or oblong shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which...

(or rarely 49 mm).

Microscope eyepieces

Microscopes have standard barrel diameters measured in millimeters: 23.2 mm and 30 mm, slightly smaller than telescope barrels.

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 further 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 Ramsden disc, 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

Glasses

Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

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.

Convex lens

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

Johannes Kepler

Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

'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).

Negative lens or "Galilean"

The simple negative lens placed before the focus of the objective has the advantage of presenting an erect image

Erect image

An erect image, in optics, is one that appears right-side up.Some telescopes and other devices, such as the camera obscura present an inverted image on the viewing surface. Various means are used to achieve an erect image. A common example of an erect image is the image of a person in a standard...

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

Galileo Galilei

Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

'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

Opera glasses

Opera glasses, also known as theater binoculars or Galilean binoculars, are compact, low-power optical magnification devices, usually used at performance events, whose name is derived from traditional use at opera performances. Magnification power below 5x is usually desired in these circumstances...

Huygens

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

Non-achromatic objective

A non-achromatic objective is an objective lens which is not corrected for chromatic aberration. In telescopes they can a be pre-18th century simple single element objective lenses which were used before the invention of doublet achromatic lenses...

refracting telescope

Refracting telescope

A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

s, including very long focal length aerial telescope

Aerial telescope

An aerial telescope is a type of very-long-focal-length refracting telescope built in the second half of the 17th century that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held...

s). 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

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

onto a screen. Other cemented eyepieces can be damaged by the intense, concentrated light of the Sun.

Ramsden

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

Jesse Ramsden

Jesse Ramsden FRSE was an English astronomical and scientific instrument maker.Ramsden was born at Salterhebble, Halifax, West Riding of Yorkshire, England. After serving his apprenticeship with a cloth-worker in Halifax, he went in 1755 to London, where in 1758 he was apprenticed to a...

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.

Kellner or "Achromat"

In a Kellner eyepiece an achromatic doublet

Achromatic lens

An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths into focus in the same plane....

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

Achromatic lens

eyepiece in 1849, also called an "achromat

Achromatic lens

ized 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

Anti-reflective coating

An antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...

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.

Plössl or "Symmetrical"

The Plössl is an eyepiece usually consisting of two sets of doublets

Doublet (lens)

, designed by Georg Simon Plössl

Georg Simon Plössl

Simon Plössl was an Austrian optical instrument maker. Initially trained at the Voigtländer company, he set up his own workshop in 1823. His major achievement at the time was the improvement of the achromatic microscope objective...

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+ degree apparent field of view along with relatively large FOV. This makes this eyepiece ideal for a variety of observational purposes including deep sky

Deep sky

Deep-sky objects are astronomical objects other than individual stars and solar system objects . The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies.-Origins and...

and planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

ary viewing. The chief disadvantage of the Plössl optical design is short eye relief

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

Anti-reflective coating

and well made ones.

Orthoscopic or "Abbe"

The 4-element orthographic eyepiece consists of a plano convex singlet

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

eye lens and a cemented convex-convex triplet field lens

Triplet lens

achromatic field lens. This gives the eyepiece a nearly perfect image quality and good eye relief

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 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

Anti-reflective coating

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

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

in between. They were invented during the first world war for military purposes, described in US patent by Heinrich Erfle

Heinrich Erfle

-References:...

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

Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

and ghost images. However, with lens coatings

Anti-reflective coating

at low powers (focal length

Focal length

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

Doublet (lens)

and a convex~flat positive singlet

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

. 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

Eye relief

— the highest eye relief

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 doublet

Doublet (lens)

s and singlets. The most typical adaptation is to add a positive, concave-convex simple lens

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

before the doublet

Doublet (lens)

, 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

Edmund Scientific Corporation

Edmund Scientific Corporation was a company based in Barrington, New Jersey that specialized in supplying surplus optics and other items via its mail order catalog and Factory Store. During four decades from the 1940s to the 1970s Edmund Scientific was virtually unique in its offerings to...

, 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 some ambiguity about what RKE stands for. According to Edmund Scientific Corporation

Edmund Scientific Corporation

, RKE stands for Rank Kellner Eyepiece. Others speculate it stands for Rank Kellner Edmund or Reversed Kellner Eyepiece; the latter because it is in effect a reversed version of the Kellner design on which it is based.

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. Nagler's latest design, the Ethos claims 100°. 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 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

Doublet (lens)

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

Barlow lens

The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal ratio of an optical system as perceived by all components after it in the system...

combined with a long focal length

Focal length

eyepiece. This design has been widely copied in other wide field

Field of view

The field of view is the extent of the observable world that is seen at any given moment....

or long eye relief

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.

External links

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Eyepiece properties

Design distance to entrance pupil

Elements and groups

Internal reflection and scatter

Chromatic aberration

Focal length

Location of focal plane

Field of view

Barrel diameter

Telescope eyepieces

Microscope eyepieces

Eye relief

Eyepiece designs

Convex lens

Negative lens or "Galilean"

Huygens

Ramsden

Kellner or "Achromat"

Plössl or "Symmetrical"

Orthoscopic or "Abbe"

Monocentric

Erfle

König

RKE

Nagler

See also

External links