Eyepiece

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Eyepiece , or ocular lens , is a type of lens attached to various optical devices such as telescopes and microscopes. So named because usually the lens is closest to the eye when one looks through the device. The objective lens or mirror collects light and brings it to the focus of creating the image. The eyepiece is placed near the destination focal point to enlarge this image. The number of enlargements depends on the focal length of the eyepiece.

An eyepiece consists of several "lens elements" in the housing, with a "barrel" at one end. The barrel is formed to fit in the special opening of the installed instrument. Images can be focused by moving the lens of the eye closer and further away from the destination. Most instruments have a focus mechanism to allow for the shaft movement in which the eyepiece is fitted, without the need to manipulate the lens of the eye directly.

Binocular eyepieces are usually fixed permanently in binoculars, causing them to have a predetermined glance and field of view. With telescopes and microscopes, however, eyepieces are usually interchangeable. By diverting eyepiece, users can customize what they see. For example, eyepieces will often be exchanged to increase or decrease telescope enlargement. Eyepieces also offer different fields of view, and different eye relief levels for the person who sees them.

Video Eyepiece

Eyepiece properties

Some properties of the eyepiece may be of interest to users of optical instruments, when comparing eyepieces and deciding which eye lenses fit their needs.

Design distance to pupil entered

Eyepieces are optical systems where incoming students are always outside the system. They should be designed for optimal performance for a certain distance to these admittance students (ie with minimum deviations for this distance). In a refracting astronomical telescope, the entrance pupil is identical to its purpose. It may be several feet away from the eyepiece; whereas with a microscope eye lens, the pupil enters close to the rear focal plane of the goal, just a few inches from the eyepiece. Microscope eyepieces can be corrected differently from telescope eyepieces; However, most are also suitable for the use of telescopes.

Elements and groups

Elements are individual lenses, which may come as simple lenses or "singlets" and twin cemented twins or (rarely) triplets. When the lens is cemented together in pairs or triples, the combined element is called the group (lens).

The first eyepieces have only single lens elements, which produce highly distorted images. Two and three design elements were created shortly thereafter, and quickly became the standard due to improved image quality. Currently, engineers assisted by computer-assisted fastening software have designed eyepieces with seven or eight elements that provide a very large and sharp view.

Internal reflections and scatter

Internal reflection, sometimes called a "scatter", causes light passing through the eyepiece to dissolve and reduce the contrast of images projected by the eyepiece. When the effect is very bad, the "ghost image" is seen, called "ghosting". Over the years, a simple eyepiece design with a minimum amount of internal air-to-glass surface is preferred to avoid this problem.

One solution to deploy is to use a thin film layer over the surface of the element. This thin layer is only one or two wavelengths, and works to reduce reflections and scattering by changing the refraction of light passing through the elements. Some coatings can also absorb light that is not passed through the lens in a process called total internal reflection where the incident light in the film is at a shallow angle.

Chromatic aberration

Lateral or transverse chromatic aberrations are caused due to refraction on different glass surfaces for light of different wavelengths. The blue light, seen through the lens element of the eye, will not focus to the same point but along the same axis with the red light. The effect can create a fake color ring around the point of the light source and produce a general blurriness on the image.

One solution is to reduce the deviation by using various elements of various types of glass. Achromats are groups of lenses that carry two different wavelengths of light into the same focus and show greatly reduced false color. Low dispersion glass can also be used to reduce chromatic aberrations.

Longitudinal chromatic aberration is a pronounced effect of the purpose of an optical telescope, because the focal length is very long. Microscopes, whose focal length is generally shorter, do not tend to suffer this effect.

Focal length

The focal length of the eyepiece is the distance from the main field of the eyepiece where the parallel light rays converge with a single point. When used, the focal length of the eyepiece, combined with the telescope's focal length or the purpose of the microscope, is inherent, determining magnification. Usually expressed in millimeters when referring to the lens of the eye only. When exchanging a set of eyepieces on a single instrument, however, some users prefer to refer to identifying each eyepiece by the resulting magnification.

Untuk teleskop, pembesaran sudut MA yang dihasilkan oleh kombinasi lensa mata dan tujuan tertentu dapat dihitung dengan rumus berikut:

${\ displaystyle \ mathrm {MA} = {\ frac {f_ {O}} {f_ {E}}}}  ÂÂ$

dimana:

• ${\ displaystyle f_ {O}}  ÂÂ$ adalah panjang fokus dari tujuan,
• ${\ displaystyle f_ {E}}  ÂÂ$ adalah panjang fokus lensa mata.

Magnification increases, therefore, when the focal length of the lens is shorter or the length of the goal focus is longer. For example, a 25Ã, mm lens in a 1200 mm focal length telescope will magnify the object 48 times. The 4 mm eye lens on the same telescope will enlarge 300 times.

Amateur astronomers tend to refer to the telescope eyepiece by their focal length in millimeters. This usually ranges from about 3 mm to 50 mm. Some astronomers, however, prefer to determine the strength of the resulting magnification rather than the focal length. It is often easier to express an enlargement in the observation report, as it gives a more direct impression of what the real observer sees. Due to its dependence on certain telescope traits used, however, the magnification power alone is meaningless to describe the telescope's eye lens.

Untuk mikroskop majemuk, rumus yang sesuai adalah

${\ displaystyle \ mathrm {MA} = {\ frac {DD _ {\ mathrm {EO}}} {f_ {O} f_ {E}}} = {\ frac { D} {f_ {E}}} \ kali {\ frac {D_ {\ mathrm {EO}}} {f_ {O}}}}  ÂÂ$

dimana

• ${\ displaystyle D}  ÂÂ$ adalah jarak penglihatan yang paling dekat (biasanya 250 mm)
• ${\ displaystyle D _ {\ mathrm {EO}}}  ÂÂ$ adalah jarak antara bidang fokus belakang dari tujuan dan bidang fokus belakang lensa mata (disebut panjang tabung), biasanya 160 mm untuk instrumen modern./li>
• ${\ displaystyle f_ {O}}  ÂÂ$ adalah panjang fokus obyektif dan ${\ displaystyle f_ {E}}  ÂÂ$ adalah jarak fokus eyepiece.

Dengan konvensi, eyepieces mikroskop biasanya ditentukan oleh power daripada focal length. Mikroskop lensa mata ${\ displaystyle P _ {\ mathrm {E}}}  ÂÂ$ dan kekuatan obyektif ${\ displaystyle P _ {\ mathrm {O}}}  ÂÂ$ ditentukan oleh

${\ displaystyle P _ {\ mathrm {E}} = {\ frac {D} {f_ {E}}}, \ qquad P _ {\ mathrm {O}} = { \ frac {D_ {\ mathrm {EO}}} {f_ {O}}}}  ÂÂ$

demikian dari ekspresi yang diberikan sebelumnya untuk pembesaran sudut mikroskop senyawa

${\ displaystyle \ mathrm {MA} = P _ {\ mathrm {E}} \ kali P _ {\ mathrm {O}}}  ÂÂ$

The enlargement of the total angle of the microscope image is then calculated by multiplying the strength of the eyepiece by objective strength. For example, a 10ÃÆ'â € "lens with a goal of 40ÃÆ'â €" will magnify the image as much as 400 times.

The definition of the strength of this lens depends on the arbitrary decision to divide the instrument angle enlargement into separate factors for the lens of the eye and its purpose. Historically, Abbe described different microscope eyepieces, in terms of enlarging the angle of the lens of the eye and the 'initial enlargement' of the goal. While convenient to optical designers, this turns out to be less convenient from a practical microscopy standpoint and then abandoned.

Visually received visual distance from the closest focus ${\ displaystyle D}$ is 250 mm, and eye power is usually determined at the assumption of this value. The power of general eye lenses is 8ÃÆ'â € ", 10ÃÆ'â €", 15ÃÆ'â € ", and 20ÃÆ'â €". The focal length of the eye lens (in mm) can thus be determined if needed by dividing 250 mm by the power of the lens.

Modern instruments often use corrected optical purposes for unlimited tube lengths rather than 160 mm, and this requires additional correction lenses in the tube.

Location of focus field

In some types of eyepieces, such as Ramsden eyepieces (described in more detail below), the eyepiece serves as a magnifying glass, and its focusing plane lies beyond the eyepiece in front of the field lens. Therefore this aircraft can be accessed as a location for graticule or micrometer crosswires. In the Huygenian eye lens, the focus area lies between the eyepiece and the plane, inside the eyepiece, and is therefore inaccessible.

Field of view

Field of view, often abbreviated FOV, describes the target area (measured as the angle of the viewing location) that can be seen when looking through the eyepiece. The field of view seen through the eyepiece varies, depending on the magnification achieved when connected to a particular telescope or microscope, as well as on the lens of the eye itself. Eyepieces are distinguished by their stop fields , which is the smallest gap that light enters the eyepiece must pass through to reach the lens of the eyepiece field.

Because of the effects of these variables, the term "field of view" almost always refers to one of two meanings:

Actual field view

the angular size of the number of sky that can be seen through the eyepiece when used with a particular telescope, generates a certain magnification. It ranges between 0.1 and 2 degrees.

Field of view visible

This is a measure of the angular size of the image viewed through the eyepiece, in other words, how big the image looks (different from the magnification). This is a constant for each specified focal length lens, and can be used to calculate what the actual field of view will be when the eyepiece is used with a given telescope. The measurements range from 30 to 110 degrees.

It is common for the lens user to want to calculate the real field of view, as it shows how much the sky will look when the eyepiece is used with their telescope. The easiest method to calculate the real field of view depends on whether the visible field of view is known.

Jika bidang pandang yang terlihat diketahui, bidang pandang sebenarnya dapat dihitung dari rumus perkiraan berikut:

${\ displaystyle FOV_ {C} = {\ frac {FOV_ {P}} {mag}}}  ÂÂ$

atau

${\ displaystyle FOV_ {C} = {\ frac {FOV_ {P}} {({\ frac {f_ {T}} {f_ {E}}})}} }  ÂÂ$

dimana:

• ${\ displaystyle FOV_ {C}}  ÂÂ$ adalah bidang pandang sebenarnya, dihitung dalam satuan ukuran sudut di mana ${\ displaystyle FOV_ {P}}  ÂÂ$ disediakan.
• ${\ displaystyle FOV_ {P}}  ÂÂ$ adalah bidang tampilan yang terlihat.
• ${\ displaystyle mag}  ÂÂ$ adalah pembesaran.
• ${\ displaystyle f_ {T}}  ÂÂ$ adalah panjang fokus teleskop.
• ${\ displaystyle f_ {E}}  ÂÂ$ adalah panjang fokus lensa mata, dinyatakan dalam satuan pengukuran yang sama dengan ${\ displaystyle f_ {T}}  ÂÂ$ .

Focal length the purpose of the telescope is the diameter of the objective time of the focus ratio. This represents the distance at which the mirror or objective lens will cause the light to converge at one point.

The formula is accurate up to 4% or better up to 40 Â ° field of view is clear, and has a 10% error for 60 Â °.

Jika bidang pandang yang terlihat tidak diketahui, bidang pandang sebenarnya dapat ditemukan dengan menggunakan:

${\ displaystyle FOV_ {C} = {\ frac {57.3d} {f_ {T}}}}  ÂÂ$

dimana:

• ${\ displaystyle FOV_ {C}}  ÂÂ$ adalah bidang pandang sebenarnya, dihitung dalam derajat.
• ${\ displaystyle d}  ÂÂ$ adalah diameter bidang eyepiece berhenti dalam mm.
• ${\ displaystyle f_ {T}}  ÂÂ$ adalah panjang fokus teleskop, dalam mm.

The second formula is actually more accurate, but the size of the field stop is usually not determined by most manufacturers. The first formula will not be accurate if the plane is not flat, or higher than 60 Â ° which is common for the most widespread ultra-wide lens design.

Rumus di atas adalah perkiraan. Standar ISO 14132-1: 2002 menentukan bagaimana sudut pandang jelas yang tepat (AAOV) dihitung dari sudut pandang nyata (AOV).

${\ displaystyle tan {\ frac {AAOV} {2}} = mag \ kali tan {\ frac {AOV} {2}}}  ÂÂ$

If the diagonal or Barlow lens is used before the lens, the lens field of view may be limited. This occurs when the previous lens has a narrower field than the eye lenses, causing the obstruction ahead to act as a smaller field stops in front of the eye lens. The right relationship is given by

$Â\; ÂÂ\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂAÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂAÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂOÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂVÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â=Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â2Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÃ\AE \text{'}\; -Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂaÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂrÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂcÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂtÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂaÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂnÂ\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\frac{Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â0.5Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂdÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â}{f_{Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂEÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â}}Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â}Â\; Â\; Â\; ÂAnnotation\; encoding\; =\; "application/x-tex">\; \{\backslash \; displaystyle\; \{AAOV\}\; =\; 2\; \backslash \; times\; arctan\; \{\backslash \; frac\; \{0.5d\}\; \{f\_\; \{E\}\}\}\}\; Â\; Â$

This formula also shows that, for the design of the eyepiece with a visible field of view, the diameter of the barrel will determine the maximum possible focal length for the lens, since no field can be larger than the barrel itself. For example, PlÃÆ'¶ssl with a field of view 45Ã, Â ° seen in a 1.25-inch barrel will result in a maximum focal length of 35mm. Anything longer requires a larger barrel or a view bordered by edges, effectively making the field of view less than 45 °.

Barrel Diameter

Eyepieces for telescopes and microscopes are usually interchangeable to increase or decrease enlargement, and to allow the user to select a type with certain performance characteristics. To allow this, the eyepieces come in a standard "Barrel diameter".

Telescope eyepieces

There are six standard barrel diameters for the telescope. The size of the barrel (usually expressed in inches) is:

• 0.965 logged in. (24.5 mm) - This is the smallest standard barrel diameter and is usually found in toy stores and retail telescope shopping centers. Many of the 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 produced, but you can still buy Kellner types.
• 1.25 inches. (31.75 mm) - This is the barrel diameter of the most popular telescope lenses. Practical upper limit on the focal length of the eyepieces with 1.25 "barrel is about 32 mm. With a longer focal length, the edge of the barrel itself infiltrated the limit display size. With a focal length of 32 mm, field of view available. Down below 50 Â °, which according to most amateurs is the minimum acceptable width.The size of this barrel is threaded to take 30 mm filters.
• 2Ã, in. (50.8 mm) - Larger barrel size in 2 "eyepieces helps reduce the limit on focal length The upper focal length with 2" eyepieces is about 55 mm. The trade-off is that these eyepieces are usually more expensive, will not fit on some telescopes, and may be quite heavy for the tip of the telescope. The size of this barrel is threaded to take a 48 mm (or rarely 49 mm) filter.
• 2.7Ã, in. (68.58 mm) - 2.7 "eyepieces are made by several manufacturers. They allow for the field of view slightly larger. Many high-end thinkers now accept these eyepieces.
• 3Ã, in. (76.2 mm) - The larger barrel size in 3 "eyepieces allows for extreme focal length and more than 120 Â ° field of view of eyepieces.The disadvantage is that these eyepieces are somewhat rare, very expensive, up to 5 lbs deep weight, and only a few telescopes that have a focus that is large enough to accept it. It weighs a great cause balance problems in Schmidt-Cassegrains under 10 inches, refractor under 5 inches, and a reflector under 16 inches Also, because the field of them quit, without a larger secondary mirror, most reflectors and Schmidt-Cassegrain will have severe vignetting with these eyepieces.The makers of these eyepieces include Explore Scientific and Siebert Optics.The telescopes that can receive these eyepieces are made by Explore Scientific and Orion Telescopes and Binoculars. li>
• 4Ã, in. (102Ã, mm) - these eyepieces are rare and are only commonly used in observatories. They are made by very few manufacturers, and their demand is low.

Microscope eyepieces

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

Lighten the eyes

The eye should be held at a certain distance behind the eyepiece lens to see the image well past it. This distance is called eye relief. Larger eyelids mean optimal position farther from the eyepiece, making it easier to see images. However, if eye relief is too great, it is uncomfortable to hold the eye in the correct position for long periods of time, therefore some eyepieces with the help of long eyes have a cup behind the eyepiece to aid the observer in maintaining the correct observation position. The pupil of the eye must coincide with the pupil out, the incoming student image, which in the case of an astronomical telescope corresponds to a glass object.

Eye reliefs typically range from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long eyepieces of length usually have enough eye relief, but short focal length eyepieces are more problematic. Until recently, and still fairly common, the eyepieces of the short focal length have short eye relief. Good design guidelines suggest a minimum of 5-6 mm to accommodate observer eyelashes to avoid discomfort. The modern design with many lens elements, however, can be true for this, and the look at high power becomes more comfortable. This is especially true for wearers of glasses, which may require more than 20 mm of eye shadow to accommodate their glasses.

Maps Eyepiece

Design eyepiece

Technology has evolved over time and there are various lens designs for use with telescopes, microscopes, rifle sights, and other devices. Some of these designs are described in more detail below.

Negative lens or "Galilean"

A simple negative lens is placed before the goal focus has the advantage of presenting an upright image but with a limited field of view more suitable for low magnification. It is suspected that 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 the Galileo Galilei 1609 telescope that provided this type of lens with the name " Galilea ". This type of eyepiece is still used in very cheap telescopes, binoculars and in opera glasses.

Convex lens

Simple convex lenses are placed after the focus of the objective lens presents the observer with an enlarged enlarged image. This configuration may have been used in the first refracting telescope of the Netherlands and proposed as a way to have a much broader view and higher magnification in a telescope in Johannes Kepler's 1611 Dioptrice book. Since the lens is placed after the focus field of the destination it also allows the use of micrometers in the focus plane (used to determine the angular size and/or distance between the observed objects).

Huygens

Huygens eyepieces consist of two plano-convex lenses with sides of the plane leading to the eye separated by air gaps. These lenses are called eye lenses and field lenses. The focus area is located between two lenses. It was discovered by Christiaan Huygens in the late 1660s and was the first multi-lens lens. Huygens found that two airspace lenses could be used to make the lens of the eye with zero transverse chromatic abnormalities. If the lens is made of glass with the same refractive index, for use with a relaxed eye and a telescope with a distant purpose, then the separation is given by:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂdÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â=Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â\frac{}{}Â\; Â\; Â\; Â\; Â\; Â\; Â\; ÂÂ\; Â\; Â\; Â\; Â\; Â\; Â(Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{           f                        A                                            f               Â                          )               {\ displaystyle d = {\ frac {1} {2}} {f_ {A} f_ {B})}  Â}

di mana ${\ displaystyle f_ {A}}  ÂÂ$ dan ${\ displaystyle f_ {B}}  ÂÂ$ adalah panjang fokus dari lensa komponen.

These eyepieces work well with very long focal length telescopes (in Huygens today they are used with one long focal length non-achromatic refractive telescope element, including a very long focal length air telescope). This optical design is now considered obsolete because with the shorter telescope's focus length this eyepiece suffers from short eye relief, high image distortion, chromatic aberration, and a very narrow field of view. Because these eyepieces are cheap to make them can often be found on cheap telescopes and microscopes.

Because Huygens eyepieces do not contain cement to hold up the lens elements, telescope users sometimes use these eyepieces in the role of "solar projection", ie projecting the Sun image onto the screen. Other cemented eyepieces can be damaged by intense and concentrated sunlight.

Ramsden

The Ramsden eyepiece consists of two plano-convex lenses of the same glass and the same focal length, placing less than a separate eye-lens focal length, a design made by astronomical and scientific instrument maker Jesse Ramsden in 1782. Separation of the lens varies between designs which is different, but usually somewhere between 7/10 and 7/8 of the focal length of the eyepiece, the choice becomes a trade off between the remaining cheromatic aberration (at a low value) and at a high value that runs the risk of field lenses touching the focal plane when used by observers working with near virtual images such as farsighted observers, or young people whose accommodation is able to cope with close virtual images (this is a serious problem when used with a micrometer because it can cause damage to the instrument).

The exact separation of 1 focal length is also discouraged because it makes dust on the field lens distract the focus. Both curved surfaces face inwards. Thus, the focal plane is placed outside the eyepiece and can be accessed as the location where the graticule, or micrometer viewfinder can be placed. Because the exact separation of one focal length is required to correct transverse chromatic aberration, it is not possible to improve the Ramsden design completely for transverse chromatic aberration. The design is slightly better than Huygens but still not up to date.

It remains highly suitable for use with instruments operating using near-monochromatic light sources such as polarimeters.

Kellner or "Achromat"

In Kellner doublet achromatic lenses are used in place of simple plano-convex eyepiece in Ramsden design to fix the remaining transverse chromatic aberration. Carl Kellner designed this first modern acromatic lens eye in 1849, also called "Ramsden who was healed". Kellner eyepieces is a 3-lens design. They are cheap and have a pretty good image from low to medium power and are far superior to Huygenian or Ramsden designs. Eye relief is better than Huygenian and worse than Ramsden eyepieces. The biggest problem of Kellner's eyepieces is the internal reflection. The current anti-reflection coating makes an economical choice that can be used for small to medium-sized aperture telescopes with f/6 or longer focal ratios. The visible field of view is 40-50 Â °.

Source of the article : Wikipedia