Eyepiece Totally Explained

Everything about Eyepiece totally explained

ť For the device for looking through a camera, see viewfinder.

An eyepiece, or ocular lens, is a type of lens that's attached to a variety of optical devices such as telescopes and microscopes. It is so named because it's usually the lens that's 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 at the focal point of the objective to magnify this image. The amount of magnification depends on the focal length 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's 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, and differing degrees of eye relief for the person who looks through them.
   Modern research-grade telescopes don't use eyepieces. Instead, they've high-quality CCD sensors mounted at the focal point, and the images are viewed on a computer screen. Some amateur astronomers 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 (for example 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 wavelengths deep, and work to reduce reflections and scattering by changing the refraction of the light passing through the element. Some coatings may also absorb light that isn't being passed through the lens in a process called total internal reflection where the light incident on the film is at a shallow angle.

Chromatic aberration

Lateral chromatic aberration is caused because the refraction at glass surfaces differs for light of different wavelengths. Blue light, seen through an eyepiece element, won't 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 objectives, because the focal lengths are so long. Microscopes, whose focal lengths are generally shorter, don't tend to suffer from this effect.

Focal length

The 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's attached, determines the magnification. It is usually expressed in millimetres 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 produced by the combination of a particular eyepiece and objective can be calculated with the following formula: ť

mathrm (f_A + f_B)

where

f_A

and

f_B

are the focal lengths of the component lenses.

Ramsden

The Ramsden eyepiece, created by astronomical and scientific instrument maker Jesse Ramsden in the 18th century, comprises two plano convex lenses with the same focal length and glass, placed less than one focal length apart. The 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 isn't 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 for example polarimeters.

Kellner or "Achromat"

Carl Kellner designed this first modern achromatic eyepiece in 1850, also called an "achromatized Ramsden". Kellner eyepieces are a 3-lens design. An achromatic doublet is used in place of the eye lens in the Ramsden design to correct the residual transverse chromatic aberration. They are inexpensive and have fairly good image from low to medium power and are far superior to Huygenian or Ramsden design. 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.

Abbe or "Ortho"

The 4-element Abbe eyepiece was invented by Ernst Abbe in 1880, and is called "orthoscopic" or "orthographic" because of its low degree of distortion; usually the eyepiece is simply called an "ortho". The Abbe design uses a convex-convex triplet field lens and a convex-flat singlet eye lens. Orthos have nearly perfect image quality and good eye relief, but a little bit narrow apparent field of view — about 40°–45°.
Until the advent of multicoatings and the popularity of the Plössl, orthos were the most popular design for telescope eyepieces. Even today these eyepieces are superior to most others for planetary and lunar viewing.

Erfle

Erfles were invented during the first world war for military purposes, described in US patent by Heinrich Erfle number 1,478,704 of Aug 1921. They are a 5-element design which is a logical extension to wider fields of the four lens military eyepiece design. In effect, they're Plössls with extra lenses.
Erfle eyepieces are designed to have wide field of view (about 60 degrees), but they're 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're acceptable, and at 40 mm they can be excellent. Erfles are very popular because they've large eye lenses, good eye relief and can be very comfortable to use.

König

The König eyepiece was designed in 1915 by German optician Albert König (1871−1946). The original design is a simplified Abbe, with a leading doublet instead of a triplet. The original 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.
König's original 1915 form is the simplest, and is composed of two lens groups: 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.
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°.

Plössl

Originally designed by Georg Simon Plössl in 1860, several versions can be found on the amateur astronomy market. By far the Plössl eyepiece is currently the most widely used design. The name Plössl eyepiece covers a range of eyepieces with at least four optical elements. Usually consisting of two sets of doublets, a and element sandwiched together, the lens provides a large apparent field of view along with relatively large FOV. This makes this lens 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, which is restricted to about 70-80% of focal length. The short eye relief is more critical in short focal lengths, when viewing can become uncomfortable.
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.

RKE

An RKE eyepiece is an adaptation of a Kellner eyepiece 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.
There is some ambiguity about what RKE stands for. According to an e-mail from Edmund, RKE stands for Rank Kellner Eyepiece. Others speculate it stands for Rank Kellner Edmund or Reversed Kellner Eyepiece; the latter because the elements within the eyepiece in effect have been reversed from the Kellner design on which it's based. This arrangement makes the design similar to a widely spaced version of the König design.

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. This is achieved using exotic high-index glass and up to eight optical elements in 4 or 5 groups; there are 5 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 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, which is enough to unbalance many telescopes. Amateurs fondly refer to Naglers as "paperweights", because of their heft, or "hand grenades", because of their size and shape. 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 Link

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