Eyepiece is used to observe the frontoptical system The visual optics of the resulting image aretelescope、MicroscopeThe main function of the components of visual optical instruments is to magnify the real image magnified by the objective lens again.To cancelaberration, eyepiece usually consists of severallensCombined, with largerfieldAnd viewing angle magnification.
Chinese name
eyepiece
Foreign name
eyepiece
Type
Visual optics
Nature
Main components of microscope
Role
Enlarge the real image magnified by the objective lens again
The eyepiece is also the main part of the microscope. Its main function is to magnify the real image magnified by the objective again, so as toPhotopic distanceForm a clear virtual image;Therefore, its quality will finally affect the quality of the image.In photomicrography, a real image is formed at the ground glass.Some eyepieces (such as compensating eyepiece) can not only magnify, but also reduce the residual produced in the imaging process of the objectiveaberrationCorrect.The construction of eyepiece is much simpler than that of objective lens.Because the beam passing through the eyepiece is nearly parallelSpherical aberrationAnd longitudinal (axial)chromatic aberrationNot serious.Only horizontal color difference (amplified color difference) shall be considered during design.The eyepiece consists of two partslensIt is called eye lens for amplification;The lower lens is called convergence lens or field lens to make the image brightness even.A light bar is arranged in the middle of the upper and lower lenses or at the lower end of the lower lens, where the micrometer, cross glass, pointer and other accessories are installed.EyepieceapertureThe angle is very small, so its resolution is very low, but it is enough to magnify the primary image of the objective lens.Commonly used eyepiece magnifications are 8 ×, 10 ×, 12.5 ×, 16 ×, etc.Installed on the upper end of the lens barrel, 2-3 pieces are usually provided, and the 5 ×, 10 × or 15 × symbols are engraved on them to indicate the magnification. Generally, 10 × eyepieces are installed.[1]
type
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If classified according to the construction form, eyepieces have the following types:
1. Fugen eyepiece: separableOrthotype eyepieceSystem andNegative eyepieceThey belong to two categories.The main focus of the positive eyepiece is outside the field lens. Although it is composed of two or more lensesoptical system Can be regarded as singleConvex lens, so it can be used as a magnifying glass alone under appropriate circumstances.The main focus of the negative eyepiece is within the field lens, that is, between the field lens and the eye lens. Obviously, it cannot be used as a magnifier alone.The focus of the simplest type of eyepiece is between two lenses, which belongs to "negative lens".Fugen eyepiece is the simplest type of negative eyepiece system.It consists of two discrete plano convex lenses without chromatic aberration correction. The one close to the human eye is called eye lens, which plays the role of amplification.The other is called field lens, which plays a role in making the image highly uniform.A light bar is arranged between the two pieces and is located at the front focus of the eye lens.Fugen eyepiece not carried outaberrationCorrection, or only partialSpherical aberrationAfter correction, there is still some aberration and distortion.Its magnification is generally not more than 15 times, suitable for observation or photography with medium and low power objective lens.
2. Resden eyepiece: It is composed of two planoconvex lenses, and its main focus is outside the lower lens (field lens), so it is called positive lens.Resden eyepiece pairImage field curvatureAnd distortion has good correction, spherical aberration is also small, but the magnified chromatic aberration is worse than that of Fogan eyepiece.It can be used not only for observation and photography, but also for magnification.
3. Compensation eyepiece: flat field with vertical axis chromatic aberration of 1.5%~2%Achromatic objective, flat field halfApochromatic objective, flat field apochromatic objective lens, etc. are all objective lenses with insufficient vertical chromatic aberration correction.These eyepieces need to be used together with the eyepiece with excessive vertical chromatic aberration correction, so they are called compensation eyepieces.The compensating eyepiece has the feature of excessively correcting and amplifying chromatic aberration to compensate for apochromatic aberrationSemi apochromatic objectiveResidual chromatic aberration.Since the compensating eyepiece has a certain amount of vertical chromatic aberration and its magnification is high (up to 30 times), it is not suitable for use with ordinary achromatic objective lens, but should be used with apochromatic objective lens or semi apochromatic objective lens to offset the residual chromatic aberration of these objectives.It cannot be used with achromatic objective lens, because "over positive" will cause negative chromatic aberration of the image.
fourMicrometer eyepiece: A piece of calibrated glass sheet is added to the eyepiece for quantitative measurement or for measuring the length of micro indentation.According to the measurement purpose, the scale can be designed on straight lines, cross lines, grids, concentric circles or other geometric figures.
5. Photographic eyepiece: This eyepiece is specially used for photography or close projection, and cannot be used for microscopic observation or single magnification.hisaberrationThe correction and compensation eyepiece are basically the same, and should be the same as the plane apochromatic objective orSemi apochromatic objectiveUsed to make the image flat enough under the specified magnification.
6. Wide angle eyepiece: the field of view angle of general eyepiece is about 30 °.Wide angle eyepiece refers toField angleA flat field eyepiece with a magnification of 12.5 times or more at 50 ° or more, and a flat field eyepiece with a field angle of 40 ° or more and a magnification of 10 times or less.[1]
features
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1. Marking of eyepiece The eyepiece is engraved with the following marks: eyepiece categoryMagnification。For example, the 10 × flat field eyepiece is engraved with p10 ×;P is the flat field eyepiece, 10 × is the magnification, generalHuygens eyepieceNo marking.
eyepiece
2. Magnification of eyepiece Magnification of eyepiece is specified.The function of eyepiece is toobjective lenseThe magnified real image (middle image) is magnified again, andvisible imageIn the eyes of the observer, the eyepiece is essentially a magnifying glass.The resolution ability of known microscope is determined by the objective lensnumerical aperture And the eyepiece only plays a magnifying role.Therefore, no matter how large the eyepiece is, the structure that cannot be distinguished by the objective lens cannot be distinguished.
Due to different series of eyepiecesoptical design Different, so they cannot be mixed.
The eyepiece used for observation is distinguished according to its focal lengthMagnification, eyepiecefocal lengthThe longer the number, the smaller the magnification, and the larger the field of vision.
Generally speaking, it is above 40mm, which is called low power eyepiece. It is suitable for observing nebulae, star clusters, comets and other dim objects.
25mm - 12mm, called medium power eyepiece, is suitable for observing the lunar surface, planets, binary stars and bright nebulae.
12mm-4mm, called high power eyepiece, is suitable for observing the detailed holesplanetSurface, double star, etc.
For various celestial bodies, eyepieces with different focal lengths should be selected according to different objects. Generally, commercially available telescopes are equipped with two or three eyepieces, which are divided into medium, high, low (such as 12mm, 6mm, 25mm) or medium and high power (such as 18mm, 6mm).It is suggested that if you have the opportunity, you should purchase another 40mm long focal special low-power eyepiece, which is particularly effective in viewing nebulae and star clusters.
It is worth noting that the specification of ordinary eyepiece is 24.5mm, and the specifications of three other "big head" eyepieces are 31.7mm, 36.4mm and 50.8mm. The eyepiece glass becomes larger as the diameter changes, which makes it look like watching a large screen TV.[2]
Basic properties
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Some properties of eyepieces are very important to the function of optical products, and they need to be compared to determine the most suitable eyepiece.
Distance design of incident pupil
EyepieceEntrance pupilAlways designed outside the optical system of the eyepiece, they must be designed to have excellent performance at a specific distance (that is, minimal deformation at this distance).In refractiveAstronomical telescopeThe entrance pupil is usually very close to the objective lens and is usually several feet away from the eyepiece;In microscopes, the entrance pupil is usually close to the rear focal plane of the objective lens, only a few inches away from the eyepiece.Therefore, the eyepiece of a microscope is different from that of a telescope in nature, and it is not interchangeable to obtain proper performance.[2]
Elements and Groups
Each independent lens is called an element. It is usually a simple lens, which can be combined into a single mirror, a glued double mirror or a triple mirror.When these elements are bonded together by two or three, this combination becomes a group.
The first eyepiece is only a single lens element, and the image obtained is highly deformed.After the design of two or three elements was invented, it soon became the standard design due to the improved image quality.The engineer's design with the help of computer planning provides an excellent image with seven or eight elements.[2]
Internal reflection and scattering
The internal reflection is sometimes called scattering, which causes the light passing through the eyepiece not only to disperse but also to reduce the contrast of the image produced by the eyepiece.When the effect of the image is poor, "ghost" will appear, which is called illusion.For many years, making a small air gap between glass and glass during design can effectively improve this problem.
yesThin lensThis problem can be solved by coating the element surface.This layer is only one or two wavelengths thick, which can change the refraction of light through elements to reduce reflection and scattering.Some coatings can betotal reflectionThe process of absorbing these rays at a low angle so that they do not pass through the lens.[2]
Lateral chromatic aberration
chromatic aberrationBecause different colors (wavelengths) change from one medium to anotherRefractive index。For eyepiece, color difference comes from crossing the interface between air and glass.Blue light and red light cannot be focused at the same time after passing through the element of eye diameterfocusOn the contrary, this phenomenonPoint sourceAs a result, it is possible to produce a fuzzy color ring around the focus, and the usual result is that the image is blurred.
There are several ways to alleviate this problem. One is to use the film to correct the elements of the eyepiece.The more traditional method is to use a number of different glass and curvature elements to reduce deformation.
Longitudinal color differenceOptical telescopeMedium, becausefocal lengthLong and significant effect;Microscopes are not affected by this effect because the general focal length is very short.
Usually, when eyepiece is used to improve color difference, both of them need to be corrected.[2]
Focal length
The focal length is the distance between the point of convergence of parallel light passing through the eyepiece and the main plane of the eyepiece.When in use, the combination of eyepiece focal length and objective lens focal length determines the auxiliaryMagnification。When eyepiece is mentioned separately, its unit is usually mm;When used on an instrument that can replace the eyepiece, some users prefer to use the magnification obtained after passing the eyepiece as the unit.
eyepiece
For telescopes, some special eyepieces can produce different angular magnification, and the combined magnification of telescopes and microscopes can be calculated using the following formula: MA is the angular magnification to be calculated, fO is the focal length of the telescope objective lens, fE is the focal length of the eyepiece, and the same formula should be usedUnit of measurementTo represent.The common formula of fT for a compound microscope is the closest distance (usually 250mm). DEO is the distance between the back focus surface of the objective lens and the back focus surface of the eyepiece (called the tube length). In modern instruments, the overall length of this distance is 160mm. O is the focal length of the objective lens, and FE is the focal length of the eyepiece.Therefore, improveMagnificationThe focal length of the eyepiece can be shortened, or the focal length of the instrument itself can be lengthened.For example, an eyepiece with a focal length of 25mm is used on a telescope with a focal length of 1200mm, and the magnification is 48 times;The eyepiece with focal length of 4mm is used on the same telescope, and the magnification is 300 times.
The eyepiece of the telescope used by amateur astronomers tends toFocal lengthMark it.In astronomy, focal length is usually expressed in millimetres (mm), ranging from 3 to 50 mm.ActualMagnificationIt depends on the focal length of the telescope used.
However, when describing the observation phenomenon, astronomers often use the magnification instead of the focal length to mark the eyepiece.It is more convenient to use magnification on the observation report, because it gives a more direct hint of what the observer actually sees.Since the magnification depends on the telescope used, it is meaningless to mention the magnification alone for the eyepiece of the telescope.
According to the protocol, the eyepiece of the microscope usually marks the specific magnification to replace the focal length.The magnification PE of the microscope and the magnification PO of the objective lens thus represent the angular magnification of the front end of a compound microscope as:
The definition of magnification is based on the instrument's ability to magnify any easy separation angle between the eyepiece and the objective lens.Different from the analysis of microscope eyepiece in history, it is based on the angle of eyepieceMagnification, and the original magnification of the objective lens.This is very convenient for optical designers, but it lacks convenience from the perspective of practical microscopy, so it was abandoned.The magnification of general eyepiece is 8X, 10X, 15X, and 20X.These multiples are obtained by comparing with the shortest visual distance that normal people can see clearly, D250mm, so the focal length of the eyepiece can be calculated by dividing 250mm by the magnification.Although the accepted standard distance is 250mm, the microscope will be designed with a focal length of only 160mm, making the instrument very compact.It may also be designed that the tube is actually infinitely long (using an auxiliary lens in the lens barrel).The overall angular magnification of the microscope image is the product of the eyepiece magnification and the objective magnification.For example, the combination of 10X eyepiece and 40X objective lens will result in 400X magnification.[2]
Position of focal plane
There are some eyepieces, likeRansden eyepieceThe focal plane is located in front of the field lens outside the eyepiece, so it is suitable to be used as the location of the reticle such as the marking line or the micrometer.In Huygens eyepiece, the focal plane is located between the eye and the field lens in the eyepiece, which is not easily accessible.[2]
view
Field of vision, often using abbreviationsFOV, describes the target that can be seen through the eyepiece (angle measured from the observer's location).The field of vision of the eyepiece will vary according to the magnification of the telescope or microscope they are combined with, and it is also related to the nature of the eyepiece itself.Eyepieces are distinguished by their perimeters, which are the narrowest apertures through which light entering the eyepiece passes before reaching the field lens.
Due to these variable factors, the term "vision" usually has two meanings, and always refers to only one of them.
The real field of view is the angle of the real sky that can be seen through the eyepiece due to the specific amplification effect when using a telescope. Its range is usually between 0.1 and 2 degrees.The visual field is a constant value of all the eyepieces being measured, ranging from 35 degrees to more than 80 degrees.It itself is obviously an abstractnumerical valueHowever, the real field of view can be measured through the magnification obtained by combining the telescope with the eyepiece.The visual field of the eyepiece is usually marked as the characteristics of the eyepiece, providing a convenient method for users to calculate the real field of view when using their own telescope.The user of eyepiece usually needs to calculate the real field of view, because this indicates the size of the sky that can actually be seen when the eyepiece is combined with the telescope.[2]
Barrel diameter
eyepiece
The smallest standard barrel diameter of eyepiece is 0.965 inches (24.5mm), but it has almost been abandoned.Telescopes still using this diameter are either goods in toy stores or poor quality telescopes that are usually only available in shopping malls (hypermarkets).Many eyepieces on such telescopes are made of plastic, and some even lenses are made of plastic.High quality telescopes are no longer the size of eyepiece.Most eyepieces have a tube diameter of 1.25 inches (31.75 mm). The upper limit of the focus of this tube diameter eyepiece in practice is about 32 mm.For the eyepiece with a longer focal length and the eyepiece with a longer focal length than 32mm, the edge of the cylinder diameter limits the size of the visual field to no more than 50 °, and most amateurs believe that this is the minimum acceptable field of vision.This cylinder diameterscrewA 30mm filter can be inserted.
The eyepiece with a diameter of 2 inches (50.8 mm) is often used.The focal length limit of a 2-inch eyepiece is about 50 mm, and the cylinder diameter larger than 2 inches (50.8 mm) is mainly used to help extend the focal length limit of the eyepiece.This eyepiece is usually expensive and may be heavy enough to overturn the telescope.The screw of this eyepiece is suitable for 48mm filter (or 49mm filter).The eyepiece of the microscope uses mm as the unit, and the standard cylinder diameter is 23.5mm and 30mm, which are smaller than the cylinder diameter of the telescope.[2]
Appropriate eye distance
Principle of eyepiece
The eye needs to view the image formed by the eyepiece within a certain distance behind the eyepiece, which is called the appropriate distance.A large eye distance means that the better the quality of the eyepiece, the easier it is to see the image.However, if the eye distance is too large, the eyes should be in the correct position for a long time, which will cause discomfort to the eyes.For this reason, some eyepieces with long eye distance are designed with an eye cup behind the eyepiece lens, which can help the observer observe the target at the correct distance for a long time.The size of the exit pupil should match the size of Ramsden disk.In the case of astronomical telescopes, the image of the incident pupil corresponds to the size of the objective lens.
The typical range of eye distance is between 2mm and 20mm, depending on the structure of eyepiece.The eyepiece with long focal length usually has a wide suitable eye distance, but the suitable eye distance of the eyepiece with short focal length is problematic.Until recently, this is still quite common and common. The proper eye distance of short focus eyepieces is relatively short.Good design guidelines suggest that the eye distance should be at least 5-6mm to avoid discomfort caused by eyelashes.Modern design can add many lens elements, which can not only improve this aspect, but also become more comfortable in high magnification observation.Especially for observers with glasses, they need at least 20mm distance to fit their glasses.[2]
Category structure
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Huygens eyepiece
Huygens, a Dutch scientist, designed it in 1703. It consists of two flat convex lenses. The front is a field mirror and the back is an eyepiece. Their convex faces face one end of the objective lens. The focal length of the field mirror is generally 2-3 times that of the eyepiece, and the lens spacing is half of the sum of their focal lengths.Huygens eyepiecefieldIt is about 25-40 degrees.In the past, Huygens eyepiece was the first choice for small refractors, but with the increase of the telescope's light power, its obvious shortcomings of small field of view, low contrast, chromatic aberration and spherical aberration field curve gradually exposed, so this structure is generally used for microscope eyepieces.[2]
Ransden eyepiece
It was successfully designed in 1783. It is also a two piece two group structure, consisting of two plano convex lenses with opposite convex faces and the same focal length.The spacing is 2/3-3/4 of the sum of their focal lengths, and the color difference is slightly larger, and the field curve is significantly reduced,fieldIt is about 30-45 degrees, which is rarely used.[2]
Kelner eyepiece
eyepiece
It was developed on the basis of Ransden eyepiece and appeared in 1849. The main improvement was to change the single piece eyepiece to double glued eyepieceAchromatic lens, greatly improving the color difference and edge image quality,fieldWhen it reaches 40-50 degrees, it is comfortablePupillary exit distanceTherefore, it is widely used in some low and medium power telescopes, but it does not perform well at high power.In addition,KyleField mirror approach of nano eyepiecefocal planeIn this way, the dust on the field mirror will be easy to image and affect the observation, so special attention should be paid to cleaning.An American companyKelner eyepieceOn the basis of further improvement, RKE eyepiece is developed, and its edge image quality is better than that of the classic structure.[2]
Abbe distortionless eyepiece
In 1880AbbeIt is designed as a four piece two group structure, in which the field lens is a three glued lens, and the eyepiece is a plane convex lens. The eyepiece successfully controls the chromatic aberration and spherical aberration, and reduces ghost images and field curvature to a level that is difficult to detect. It also has a flat field of view of 40-50 degrees and sufficient pupil exit distance. It has good performance at all magnification, and has been widely used.[2]
Ellever wide-angle eyepiece
It was successfully developed in 1917, specifically for the needs of large field of viewMilitary telescopeThe design is the ancestor of all subsequent wide-angle eyepieces, with a structure of five pieces and three groups, and a field of view up to 60-75 degrees.Very suitable for observationdeep space , because the edge existsAstigmatism, so it is not suitable for high magnification design, and its performance at low magnification is very excellent.[2]
PROSOL eyepiece
Also called symmetrical eyepiece.It is composed of two sets of identical double glued achromatic lenses. Its parameter performance is similar to that of OL eyepiece, but it has larger pupil exit distance and field of view, lower cost, and is applicable to allMagnificationIt is the most widely used eyepiece and has derived many improved types.[2]
Nagler eyepiece
A high-end eyepiece designed by Americans in 1979, it has an amazing 82 degree field of view, high-quality edge image quality and comfortable pupil exit distance, as well as complex structure and high price, and weighs more than one kilogram.[2]
