Astronomical telescopes have standard eyepiece tubes, with which one can use eyepieces bought separately from the telescope. Most astronomical telescopes use eyepieces with a 1 1/4" barrel diameter, although many amateur observers with larger telescopes designed for viewing dim galaxies at relatively low magnification now use eyepieces with a 2" barrel diameter. Some inexpensive telescopes sold on the mass market use interchangeable eyepieces whose barrel diameter is approximately .965".国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材  L( t; s3 y- @# h1 J
In metric, these sizes are:国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材  ]8 e" o% B9 M8 c" r5 J
Inches      Millimetres, [' l) k$ x4 I) d3 ]4 `( [
0.964567"   24.5天文家园欢迎您！  Z( P  b8 b4 t6 W7 k5 Y0 D) D
1 1/4"      31.75
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2 ?  L6 x3 I# k9 O; z6 Owww.astroview.com.cnThe standard size for microscope eyepieces is 23mm, but there are microscopes which use eyepieces with other sizes of barrel.& r' j: @2 V9 r% }) D) f/ u

' h7 R/ u* h- _; D' J- ~( `. owww.astroview.com.cnThe eyepiece contains both the eye lens and the field lens that complete a telescope.
4 a) \& x& R' E3 u国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材
/ Y; \3 S* b! G4 W; Z天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪Some very old telescopes use a threaded barrel 1 1/4" in diameter with 16 threads to the inch; such eyepieces are referred to as RAS thread eyepieces, as the standard which they follow was established by the Royal Astronomical Society in Britain.
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The Questar telescope also uses threaded eyepieces. This has the advantage of making it harder to make a mistake that might cause the eyepiece to fall out and become damaged, but has not proven popular. Since nearly all eyepieces are threaded on the inside for the attachment of filters, one could always make a telescope that used this thread to attach the eyepiece to the telescope!
5 K3 C2 [2 f. Q/ A天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪
- r5 F! p' A; c3 b* a国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材In most eyepieces, though, the field lens is placed just slightly past where the image from the objective comes to a focus. This allows a graticule, a flat glass disk with measuring lines placed on it, to be used with the eyepiece if desired for some forms of observing.www.astroview.com.cn- I. R% k2 p$ L8 ~* u

- e4 w0 c9 f5 [8 M  P& |The magnification of a telescope is determined by dividing the focal length of the telescope by the focal length of the eyepiece. Thus, if a telescope has a focal length of 1200 mm, an eyepiece with a focal length of 40 mm will provide a magnification of 30x; that is, things viewed with the telescope will look 30 times larger in each direction.天文家园欢迎您！) p4 a/ T. A  S/ o3 g3 o7 t
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Often, a telescope is described in terms of its aperture and its focal ratio. Thus, you might see a telescope mentioned as being a 4-inch f/12 telescope. A 4-inch telescope is (often) one with a 100mm aperture: as with the lengths of slide rules, an "inch" is often only 25mm instead of 25.4mm. With telescopes, as with cameras, "f/12" means that the aperture is 1/12 of the focal length. Thus, the focal length of such a telescope would be equal to 100mm times 12, or 1200 mm.
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% `3 E- W  r4 M( j; `& Q/ m, [天文望远镜器材评测和交流论坛[ 本帖最后由 siqingtan 于 2008-6-19 20:27 编辑 ]

When you look at a printed page with a magnifying glass, it will normally look just as bright as it did before; perhaps a little dimmer because of the loss of light in the glass, but that is not usually noticeable (or brighter if the light illuminating the page happens to also be concentrated by the lens, but this isn't applicable to astronomy, since you don't see even the Moon by the light of a lamp situated behind your telescope).
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Stars, however, do look brighter through a telescope. This is because they are so tiny, that they appear as just points of light whatever magnification you use. When a star is made brighter, in effect it is really being enlarged but it retains the same brightness over the now larger surface area, making for a larger total amount of light. But since the "larger area" is still just a point, that point is brighter.
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There is, however, an upper limit to this process.
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3 d, A" ?9 E# N天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪By day, a typical value for the size of the pupil of the human eye is 3mm. At night, a fully dark-adapted eye can have the pupil dilated to about 7mm.天文家园欢迎您！& L3 y8 E- I; a8 U

. m' x7 Z. R' P" `2 O! PA pair of 7x50 binoculars (50mm aperture, 7x magnification) is an example of the class of binocular known as a "night glass". When you divide the aperture by the magnification, you get 7 1/7 mm. We can round this off to 7mm. The aperture divided by the magnification yields the size of the exit pupil of the telescope. When the size of the exit pupil matches that of the dark-adapted eye, lowering the magnification of the telescope further won't yield brighter stars, because not all the light from the telescope for each star can enter the eye.
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In the first part of the diagram, the focal length of the objective lens of the telescope, and the effective focal length of the eyepiece, which is the same as the focal length of the eye lens for the design shown here where the field lens coincides with the image plane, are shown.
# g6 t7 \1 p1 Z* p+ D国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材
) B" C3 z- o1 [) [天文家园欢迎您！From the second part of the diagram, you can see why the magnification is proportional to the ratio of the two focal lengths. The red ray passing through the center of the objective lens goes at such an angle as to reach a height h from the optical axis after travelling through a distance equal to the focal length of the objective lens.天文望远镜器材评测和交流论坛, U5 `7 I: C; P5 C% E  Y

6 v' G" `7 E3 Z* \* A3 n国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材Many rays of light go through the objective lens to be focused to form the point defining the height of the image; the other rays are shown in green. Which one is the one bent to be the dark green ray going through the center of the eye lens is not important. But that lens travels at a steeper angle; from the point on the optical axis at the center of the eye lens, it reaches the height h after travelling through a distance equal to the focal length of the eye lens.天文望远镜器材评测和交流论坛' d6 N! x! B& }$ v/ _& l  s  v1 J
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Thus, if we compared the height both rays would reach after travelling the same distance, this ray would reach a height larger than the height the other ray would reach, and the ratio of the two heights would be the same as the ratio of the two focal lengths.
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The third part of the diagram shows why the ratio of the exit pupil to the aperture is also given by the ratio of the two focal lengths; the rays passing through the center of the field lens enter and leave it at the same angles, so the distance between them at either other lens is proportional to the distance of those lenses from it.
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8 _; B: e9 U4 T8 U# X8 K" a2 W, ^天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪The fourth part of the diagram illustrates another important eyepiece characteristic, eye relief. The exit pupil indicates the size of the circular area that can recieve all the light collected by the telescope; but that circular area must be situated a particular distance from the telescope to receive that light, and that distance is known as the eye relief of the eyepiece. Note that the aperture of the objective, in this type of simple telescope, is also the field stop of the system; the rays shown, forming an image of the field stop, belong to what is known as the conjugate system.

The diagram below illustrates several popular types of eyepiece:天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪' a# u0 o/ V- B4 h

The eyepieces are all shown oriented so that the observer's eye is looking into them from the left side of the page.天文家园欢迎您！( P4 }( M$ w  P1 ^

) l: Q3 D  S' e2 D天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪A very interesting history of eyepiece designs is available at the web site of the Brayebrook Observatory, which also has a number of other astronomy-related publications.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪* |! R& F" `) g; u6 o* I$ e& e

% s1 G* x9 S& O: Y4 U; u2 y' B0 f' oThe eyepieces are grouped by category in the diagram. It begins with designs where the distinction between the eye lens and the field lens is marked beginning with the Huyghenian. Beginning with the Ramsden, it continues by showing designs which are nearly symmetrical. The last two designs consisting of a pair of achromats facing each other, the Brandon and the Kalliscopic, are designs especially aimed at reducing distortion.
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The diagram then continues with designs from the Tolles to the König aimed at low distortion, including particularly the Orthoscopic.
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) j, ^: F2 B9 `天文家园欢迎您！It then concludes with ambitious designs aimed at providing a wide field of view, including the original version of the famed Nagler eyepiece.
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The original version of this chart showed crown elements in a light blue color, and flint elements in a light green; as I have more detailed information on many of these eyepieces available, I have attempted to make the color coding somewhat more detailed. However, there may be inaccuracies, particularly as some of the colorings are purely conjectural, such as those of the Brandon, the Galoc, and the wide-angle Bertele. As well, Plössl, Orthoscopic, and Kellner eyepieces, for example, can be made with many different types of glass. Thus, while the original Kellner eyepiece was designed when there were not many types of glasses available, current Kellner eyepieces are likely to offer improved performance with more modern glasses.天文望远镜器材评测和交流论坛& o9 n- \3 K! P7 L8 D! t
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A wide variety of historical eyepiece types are shown above, many of which are not currently available as production items. A few general comments may be in order to provide the beginning amateur astronomer with some perspective on the merits of these types.天文家园欢迎您！* l/ Y% g( b: x7 s
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The Tolles and Monocentric eyepieces were very specialized eyepieces with a narrow field of view, intended for planetary studies.
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, H3 B+ ]9 Z% R4 C4 m天文家园欢迎您！[ 本帖最后由 siqingtan 于 2008-6-19 20:19 编辑 ]

The Huyghenian and Ramsden eyepieces are very simple eyepiece types, using only one kind of glass. They represent an improvement over a simple concave lens, as used in toy binoculars. They were reasonable types of eyepiece for an amateur to make himself from surplus lenses mounted in, say, a wooden spool of sewing thread. And, back in 1900 or even 1920, they were affordable eyepieces for the amateur astronomer. An Orthoscopic eyepiece, much more expensive, was more than many amateur astronomers could afford.
) i) _! ~7 I, h4 w% x天文望远镜器材评测和交流论坛
  [3 x5 h4 w0 Z2 v国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材Today, these days are past. The simplest type of eyepiece one is likely to find available in a 1 1/4" barrel is the Kellner. The Orthoscopic eyepiece is available, and its freedom from distortion is still valuable for planetary observing. It's still more expensive than a Kellner, but the price difference is not terribly pronounced. For a somewhat wider field of view, the Plössl eyepiece is particularly fashionable these days. A standard Plössl eyepiece offers a 50° field of view, and there are a number of variations on the design offering somewhat wider fields of view.
) A$ p8 ~# `/ @6 X天文望远镜器材评测和交流论坛
+ ?2 N9 L5 F1 D0 {% z9 q" n/ ~9 B天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪Back in the 1950s and the 1960s, the Erfle, particularly in a modified six-element form, was available as a luxury item, offering a 60° field of view, but it was not recommended to the amateur by most serious authorities, because its optical performance was compromised.
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Today, there are many designs that offer a 60° or 68° field of view, and less-expensive eyepieces of this type might cost no more than twice what a Kellner with a major brand name might cost.天文望远镜器材评测和交流论坛) g& l1 z9 u4 B8 p& P

6 }/ S: X7 D0 g5 N* G8 twww.astroview.com.cnAnd then there are the Nagler and the Ethos, from TeleVue, with 82° and 100° fields of view respectively, and excellent optical performance. A few decades ago, that such eyepieces would be available to the amateur would be undreamed-of.

The individual eyepiece types shown, in detail, are:
% @/ I$ i* n- ^& J天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪The Huyghenian uses a large field lens, and a small eye lens. In this design, the field lens, instead of lying just past the focus of the telescope, is placed before it, preventing a graticule from being used.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪% W" r: ^) x) R

$ {1 c8 j. R! U8 x- R7 s2 W. l天文望远镜器材评测和交流论坛The Mittenzwey eyepiece is a small improvement on the Huyghenian, replacing the planoconvex field lens by a meniscus lens.
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The Kellner eyepiece is an inexpensive design which follows more obvious design principles. It has a large, plain, field lens, and an achromatic eye lens.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪  U+ [# W5 E5 K0 C' e
The French is like a Kellner, but with a three-element eye lens; sometimes available as military surplus.天文家园欢迎您！" P# Z2 N5 Z7 H6 Q7 W, c9 w

) [% m3 ^; P# z8 r! g( d天文家园欢迎您！The Ramsden uses two identical planoconvex lenses of crown glass; this design is often used and reccommended for homemade eyepieces.www.astroview.com.cn* P6 R4 b( Z0 M; l
国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材" ~3 r* P+ m( G% W) e7 _1 a' o
The Dialsight or Symmetrical eyepiece is a simple eyepiece, designed especially for use in telescopes with graticules, built around two identical achromatic lenses.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪& `) V/ k# [+ T8 T, ?

+ N; o3 H! o$ a1 y) G8 b- [7 W天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪The Plössl eyepiece is one of the most popular eyepieces today. It is often used for deep-sky observing, because it offers a comparatively wide field of view of 50 degrees.
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The Brandon eyepiece, manufactured by Vernonscope, although superficially similar to the Plössl, is specifically designed to have optical properties similar to those of the Orthoscopic. It was designed using four different types of glass
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The Kalliscopic eyepiece is another design whose properties are similar to the Orthoscopic.天文家园欢迎您！4 u0 q1 A5 l1 F
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The Tolles eyepiece was made from a single thick piece of glass, so thick that each surface should be considered as a lens in itself. Thus, the strongly curved surface near the eye is the eye lens, and the less strongly curved surface is the field lens. A groove around the lens serves as a field stop, and thus this lens resembles the Huyghenian in principle.天文望远镜器材评测和交流论坛: V5 X  V/ F3 F1 g7 [6 w* @# e: c
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The Monocentric eyepiece, consisting of a thick cemented three-element lens whose surfaces, as the name indicates, are all parts of spheres with a common center, was designed by Stenheil, and was an early design aimed, like the Orthoscopic, at providing the highest-quality images. It had a narrow field of view and lacked eye relief, but because the images were of excellent quality, it was popular among early astronomers who studied the planets despite the inconvenience.
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: z& Z, L8 q& E% G0 ~. {The Orthoscopic eyepiece is a design due to the noted optician Ernst Abbé and the famed mathematician Karl Friedrich Gauss. Unlike many other eyepieces, the elements of crown glass, shown in blue, are on the outside of the eyepiece. Crown glass is harder than flint glass (shown in green) and is basically the same as the ordinary glass used for most purposes, although optical glass is made more carefully to be of a higher quality; flint glass has a higher index of refraction, but it also has dispersion that is not only larger than that of crown glass, but is larger proportionately than its index of refraction. The simplest forms of flint glass are similar in composition to lead crystal, (but usually have an even higher proportion of lead oxide) the "sparkle" of which is a consequence of the additional dispersion. Thus, flint glass elements normally are of the type opposite to the function performed by the lens, as they serve to correct chromatic aberration.

An unusual thing about the Orthoscopic is that the eye lens has only one element, but the field lens has three. Since, like most eyepieces, the field lens is actually located somewhat past (to the left of, in this diagram) the image plane, it is still possible for corrections made in the field lens to have an effect on the image of the eyepiece.国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材2 M7 M" s* h3 o/ U- h
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Orthoscopic eyepieces tend to be made in high powers, as they are chiefly used for observing the planets at high magnification. The special feature of this design is that it causes almost no distortion of shapes, such as barrel or pincushion distortion.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪) t- b' b3 W8 V" b

0 B4 c" K0 X0 _9 R6 u! I$ C' x: e天文望远镜器材评测和交流论坛Because the crown elements are on the outside, the Orthoscopic eyepiece has proven to be more popular than the other designs with this property.
; q: E/ w  z/ |; }国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材
" A: v7 Z; b/ v! q- Z+ P9 fThe Reversed Kellner eyepiece also illustrates the principle of placing additional elements in the field lens instead of the eye lens to correct aberrations. The illustration here attempts to approximate the RKE eyepiece sold by Edmund Scientific.天文家园欢迎您！7 d- A) [: l  l* M0 f  @% D

1 f  o" |; |7 D1 Y' w. S! d- U. C+ j天文望远镜器材评测和交流论坛The König eyepiece shown here (many other eyepiece designs are also due to Albert König) is based on the Orthoscopic design, but splits up the three-element field lens with one extra air gap, so that it consists of a single eye lens, a two-element lens in the middle, and a single field lens. This creates an additional degree of freedom for correcting abberrations, allowing the design to be modified to provide a wider field.
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# r$ R, q0 r/ V% X- Q天文家园欢迎您！The Wide-Angle eyepiece shown here is a design illustrated in N. Howard's Standard Handbook of Telescope Making as a wide-angle eyepiece that can be homemade by an amateur telescope maker.天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪* K$ o$ Z& V* n4 `9 F

/ j- H# _- @5 _- Y' b' k) p; I天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪The Erfle eyepiece offers an even wider field of view than the Plössl (about 60 degrees), and again is popular for deep-sky observing. This design originally had five elements, but many were later offered with six; both designs are shown. It is somewhat more expensive, but now there are available some considerably more expensive and complicated eyepieces for deep-sky observers, offering fields of view as wide as 82 degrees or more. The six-element version of this design is a modification due to Kaspereit, an associate of Heinrich Erfle.
6 ~% e4 G0 i% a: u, l7 Vwww.astroview.com.cn天文望远镜器材评测和交流论坛7 |& ]" {# j; ]* [  ?
The Galoc eyepiece is an early wide-field eyepiece, often used in military equipment. Many modified versions of this design have been made, using special glasses or even an aspheric surface.www.astroview.com.cn; X3 D; w- |/ {: h

( U% I' n7 M0 M# \6 \& @天文望远镜器材评测和交流论坛The Bertele eyepiece is one that is designed, like the Orthoscopic eyepiece, to minimize distortion, but it also serves as the basis for several wide-angle eyepiece types. A version currently available has a field of view of 56 degrees. Ludwig Bertele designed several other types of eyepiece, and it likely one of the more complex designs that is the one noted by some sources as providing an 80 degree field of view. Ludwig Bertele was also the designer of the Ernostar lens used on the Ermanox camera, which was used to great effect by the photographer Erich Salomon.国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材1 x5 _" ?& c2 B! }3 g8 a

- `( e/ ~1 [* k8 u5 T1 y1 |天文望远镜器材评测和交流论坛The Wild eyepiece provides a 70 degree field. This eyepiece resembles, but is not identical to, one of several designs described in a 1951 patent by Ludwig Bertele. Note the series of progressively bent elements; this is normally a way of minimizing spherical abberration.
2 b  g2 s1 h$ u) _6 R# dwww.astroview.com.cn
. Z, ~. B( d) f) j& T& u: S天文望远镜器材评测和交流论坛The Scidmore eyepiece, designed by Wright H. Scidmore for the U.S. military at the Frankford Arsenal in Philadelphia, is a design that existed in the early 1960s and offered an incredible 90 degree field of view. Like the original Nagler, it used only two kinds of glass, however, it didn't include an integral Barlow (although the last doublet in the lens is slightly negative in power, the telescope forms its image before the light enters the eyepiece), and it may not have been quite as well corrected towards the edge of the field, since it was not until the Nagler that eyepieces with such a wide angle of view gained acceptance.天文望远镜器材评测和交流论坛; O* t. W0 o8 H1 }$ _

  E4 A% k" U+ K国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材This wide angle Bertele eyepiece described in U.S. patent 2,549,158 from 1951, also has a 90 degree field of view, and with only six elements. The eyepiece is designed so that the image formed by the telescope should be formed on the flat surface of the field lens, which does have the problem that any dust on that lens will be clearly visible rather than out of focus when using the telescope.
( n; A) @( y% ?7 u4 x& c/ [天文望远镜器材评测和交流论坛+ T" m8 C  f4 Y: f
The wide-angle Bertele pictured next is also described in U.S. patent 2,549,158. The image is formed within the thick field lens in the design (the groove around that lens serve as a field stop); thus, it passes through a concave surface, and is expanded. This patent was, not surprisingly, cited as prior art in the original Nagler patent.
6 s% W! Q4 r# y3 Q8 z$ R6 A2 E天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材2 s2 H) F- c  s$ `9 u; ?
The Dilworth eyepiece, by Don Dilworth, an optical designer and ATM, is a very recent design with excellent corrections. It is a bit on the bulky side, resembling the Speers-WALER in that respect, but it is an amazing design. The image formed within the lens seems to be very close to the air-glass surface of the fourth element from the objective; a ray trace shows that it is sometimes inside the glass, and sometimes outside, as becomes more visible when the exit pupil is increased. I had thought that quite strange, but that is something which is also true of some versions of the Kellner eyepiece.
# ?, X4 m+ U& _3 l+ K+ s: gwww.astroview.com.cn天文家园欢迎您！, e" _6 m5 _. s8 S& T7 j
The Nagler eyepiece, a patented design whose name is also a trademark, available from TeleVue, is shown here in its original form (current Nagler eyepieces use more exotic glasses, have a more advanced design, and are, as a result, somewhat more compact for any given focal length): this eyepiece, with its wide (82 degree) field of view and high optical quality, even on telescopes with fast focal ratios (partly explained by the fact that the first two elements of the eyepiece, shown here on the right, act like a Barlow lens) is generally felt to have sparked a revolution in deep-sky observing.
! ?2 ?5 k3 B) [6 x4 Q! w+ m* {/ p. e天文望远镜器材评测和交流论坛
% A; D2 \; I4 a6 q! V天文望远镜器材评测和交流论坛The Koehler eyepiece shown here was designed at Zeiss in 1960, and is an example of an eyepice with a 120 degree apparent field.

Of course, many other eyepiece designs are not included here.天文家园欢迎您！1 F; ~- {. D; Q" G

" U; Z7 \4 r) M1 R, S天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪As one of the important and interesting attributes of an eyepiece is field of view, this diagram shows the fields of view of several illustrative eyepiece types (some common, some exemplary) for comparison:
+ N$ v" z; y  C+ ?! ^" pwww.astroview.com.cn
: d& W" }; b! t% m$ T( Q0 v天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪

The radius of the circle representing a particular field of view is proportional to the tangent of half the angle giving the field of view. The bottom part of the diagram illustrates this, and the vertical line in that part of the diagram shows the distance your eye would need to be from the diagram to see the various fields of view correctly.
" F* U( l( w/ Z6 p天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材* @$ @' [8 g7 M+ g. ]
Only a few eyepiece types are shown in the diagram, some of the most common traditional eyepiece types, and some modern eyepiece types with very wide fields of view:, D) r9 s3 c! X2 s. z# I
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40 degrees: the field of view typically provided by the basic Kellner eyepiece;国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材' ?: \, h2 Y* A( T" a9 v: `

, M8 ~, l9 h9 z45 degrees: the field of view typically provided by classic Orthoscopic eyepieces;www.astroview.com.cn) R; q4 x, A8 \+ O5 I3 c- v' m* j

7 O# d2 Y. s5 |# H& J, _50 degrees: the field of view provided by a Plössl eyepiece, a type of eyepiece that has become quite popular in recent years; note that some eyepieces of this type have been made to provide an even larger field of view, whether just slightly larger at 52 degrees, or wider still;
7 [% M: k. C+ u! m9 L( K国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材
( |( c" @1 Q1 Y: i% {, }" r- U2 l天文望远镜器材评测和交流论坛60 degrees: a typical field of view provided by an Erfle eyepiece; this type of eyepiece was the type of eyepiece used by amateur astronomers looking for a very wide field of view in the 1960s and previously, as noted above, there were five and six element designs called by this name, and it involved some compromises of optical quality; other eyepieces with this same field of view, and higher optical quality, include the form of the König eyepiece currently available, and the Radian eyepiece by TeleVue;
" X+ k8 [- b5 T6 B! y. b! S% ?  i天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪天文望远镜器材评测和交流论坛8 v* }. d" G9 t7 F
68 degrees: this value for field of view is provided by several popular types of eyepiece, such as the Panoptic from TeleVue, the Hyperion eyepiece from Baader Planetarium, the Ultima eyepiece from Celestron, the Superview eyepiece from GSO, the Stratus eyepiece from Orion and some focal lengths of the König eyepiece; and the current Super Wide Angle from Meade (older versions having a 67 degree field of view); the popularity of this particular value of field of view stems from the fact that it corresponds well to the normal human field of acute vision;
* S- C9 d2 B  O9 B( J( W8 J国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材4 q, h' K: i2 L
82 degrees: this is the field of view of the original Nagler eyepiece; this is, of course, the eyepiece devised by Al Nagler, and sold through TeleVue Optics, that sparked the current revolution in wide-field viewing (there are now a number of other brands of eyepieces with similar fields of view, such as 80 or 84 degrees);
0 b) L) F) s4 v; I0 k天文望远镜器材评测和交流论坛天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪' z; {' W# Q6 L9 b. n/ E& g
100 degrees: this is the field of view of the forthcoming (at this time of writing) Ethos eyepiece, also from TeleVue Optics;www.astroview.com.cn1 g5 b9 J9 O( v5 E  t1 {9 Q0 f
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120 degrees: this, the field of view provided by the Köhler eyepiece, designed for use with military binoculars, perhaps illustrates the ultimate practical limit to which field of view may someday be increased for the amateur astronomer.

This diagram, because of its small scale, does not really do justice to how spectacular the view through a Nagler eyepiece is, and not just because the radius of the circle representing its field of view ended up being rounded down by 0.48 of a pixel.' i$ _. m9 E' H- g' h- w

% u& W$ A) |' d3 {8 t2 j+ @# E  b天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪As noted, the vertical line in the diagram at the bottom, showing the angles involved in the various fields of view shows how close one's eye would need to be to the diagram to see them properly.
$ ~; I" y: T3 y天文望远镜器材评测和交流论坛天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪) V6 U$ d) h" E; \3 k) F$ P

$ i! Q% }7 R1 o6 _天文家园欢迎您！Having begun by showing crown glass as blue, and flint glass as green, I decided to go further, and use color-coding to show more detail about the type of glass used in some eyepieces. This site, by Peter Smith, an amateur astronomer in Australia, shows complete construction details for some eyepieces that he has designed, for people with a more intense interest in the subject.
6 s9 v8 l! ^$ m) o$ W+ K! Y天文望远镜器材评测和交流论坛
6 h3 X# j; d. n9 r# G* D国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材The diagram has been divided up into areas representing common glass types. The diagram basically follows the convention used by Schott and others; the lines between Extra Dense Flint and Double Extra Dense Flint, Hard Crown and Soft Crown, and Light Barium Crown and Medium Barium Crown, as used by Chance-Pilkington, are added, and the line between Lanthanum Crown and Dense Lanthanum Crown, as used by Ohara, is added and therefore their line, rather than that used by Schott, is used to separate Lanthanum Flint and Dense Lanthanum Flint. The lines between Dense Barium Crown and Extra Dense Barium Crown, between Phosphate Crown and Borosilicate Crown, and between Extra Light Flint and Crown Flint, on the other hand, are ones specific to Schott. Also, the titanium flints, which are increasing in importance, are addtionally subdivided by dividing lines based on one used by Ohara for their Eco Glass product line (which is divided into categories very different from the ones on this chart). This is done so that the colors better indicate the function a titanium flint element performs in a lens system. Note, though, that glasses using titanium are not confined to this low index region.
. H! v7 Q$ L' M, T# ^6 H9 o
+ g* G2 T4 ^8 g7 `& M9 q国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材Except for the line separating crown glasses from flint glasses, the lines are not part of a standard, but are illustrative, and some glass types with designations reflecting their chemical composition may be found in one or more of the compartments along with the type whose name is given to the compartment.

Key to the glass types:天文望远镜器材评测和交流论坛( C- o1 I0 o4 Y  \7 U6 \
www.astroview.com.cn" K: b- \" h5 l
    names                                       examples国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材( O8 T/ t% ^; ~! e# _* u0 p

1 ]4 `& P! @; t7 Q$ G- p天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪A)  Phosphate Crown
5 }; r, e" [) M  J" i2 I5 C天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪B)  Borosilicate Crown                          BK7天文家园欢迎您！3 p( J  h( i% @: x' r4 C% A/ W
C)  Hard Crown                                  K5www.astroview.com.cn7 q! W5 |( u7 d$ W2 Q! q
    Zinc Crown天文家园欢迎您！4 r9 H6 l# G6 k0 {% W
D)  Soft Crown
9 r! }. |* {& v! [, W- F* l(C, D: Crown)www.astroview.com.cn$ d( ~% A0 U  p
E)  Crown Flintwww.astroview.com.cn" K4 Y. ]% A; Q2 O2 N1 r' S
    Telescope Flintwww.astroview.com.cn) F- U: ]/ q3 Q# }& J9 }/ M
    Short Flint
8 L  m2 Q' I$ s3 t' s天文望远镜器材评测和交流论坛F)  Extra Light Flint (or E and F)              LLF6
: y8 g% F0 N2 Q. V天文家园欢迎您！G)  Light Flint                                 LF5
) r) n4 R# b+ d) wH)  Dense Flint                                 F2www.astroview.com.cn& x9 h' T' m' K! Y$ x. I
I)  Extra Dense Flint (or I and J)              SF8
; E+ Y( `+ D  m% m9 S  P& E天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪J)  Double Extra Dense Flint                    SF10, SF6
' w( u( F* Z& j5 |  z. h. ]3 |天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪www.astroview.com.cn7 H' y. o% }1 \! @
K)  Light Barium Crown4 h& Y6 m# q# ^, _: r9 [7 }
L)  Medium Barium Crown                         BaK4www.astroview.com.cn$ V* O$ Z3 {5 [- c) P, |& P% i/ H" n
(K, L: Barium Crown)
) W- o. b( M3 y6 D天文望远镜器材评测和交流论坛M)  Light Barium Flint                          BaLF4
$ W' \( L% Z# {0 sN)  Barium Flint (or Q and R)                   BaF N10
' K) Z0 }- N2 ?$ B# H天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪O)  Dense Barium Flint
. A; R/ u' ?# A0 w天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪    Borate Flint
$ t; _! _( Z+ d% t& A( ]天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪www.astroview.com.cn6 H0 A3 E& w; ?7 }
P)  Dense Phosphate Crownwww.astroview.com.cn. Z; n3 N" K# h* A
Q)  Dense Barium Crown (or L and M)             SK2www.astroview.com.cn( O4 w1 O1 t' f5 I) t
R)  Extra Dense Barium Crown                    SSK N5
' }3 Y1 l+ C4 q5 z$ F3 q国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材www.astroview.com.cn: K) H9 t% k0 e$ ^& l1 w
S)  Lanthanum Crown
- J8 l( m3 z# R- Y3 o国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材T)  Lanthanum Flint                             EK-325天文家园欢迎您！$ A: k# M+ {* J" `4 j# Q
U)  Dense Lanthanum Flint
& C: l0 H" _! b! V  h4 W国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材    Phosphate Flint
1 v2 C4 T9 O: A! y, X天文望远镜器材评测和交流论坛V)  Dense Lanthanum Crown天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪3 _6 E. Q: K, I7 ~1 W

5 ~$ @4 Y3 f" R/ ^* b/ |* x4 z: C5 D天文家园欢迎您！W)  Fluorite Crown                              FK51
8 W* _% E) r2 S3 j! UX)  Titanium Crown国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材9 ?1 S0 n# a1 j/ a1 W) U
Y)  Extra Light Titanium Flint
& j, T% o' K5 F8 o( TZ)  Light Titanium Flint
' g7 v1 Y  e! h% ^4 c1 A0 s+ z/ K1 L天文望远镜器材评测和交流论坛AA) Medium Titanium Flint
  t. t" w- b6 G% P天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪BB) Dense Titanium Flint

Note that the area from A to J includes many glass types, including many of the earliest optical glasses available. However, this region should not be confused with the "glass line", an area in which most optical glasses reside with respect to a parameter not visible on a chart of index of refraction versus dispersion, specifically partial dispersion.
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& |" f* G8 p2 o* V1 j( e天文望远镜器材评测和交流论坛Only a very few glasses, as well as substances like the mineral fluorite, and to a lesser extent some non-glass transparent materials such as plastics and water, are a significant distance from the glass line, making them useful in designing apochromatic lenses.
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0 t1 T3 n" F8 D  l天文论坛,天文,望远镜,天文望远镜,apo,ed,televue,tv,tmb,高桥,ap,tec,望远镜评测，望远镜测评,双筒望远镜，天顶，目镜，望远镜配件，赤道仪Because of concerns about people throwing their Kellners, if not their Naglers, into the trash, and also due to the problem of disposing of waste materials produced during manufacture, the optical industry is currently in a time of transition, where many optical glasses using lead oxide to increase their refractive index are being replaced with glasses having different compositions not including lead. Titanium oxide, which also allows producing glasses with lower indices of refraction for a given dispersion, is one material used for this purpose.
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7 G9 b6 Y1 e  q+ ^, v8 [4 `天文家园欢迎您！In this connection, it may also be noted that some optical components made during the Second World War used thorium oxide, or even uranium oxide, which are radioactive, to achieve exceptional optical properties. Such glasses have been abandoned; such is the current advanced state of the optical industry that there are now available glasses having a similar range of optical properties as they had posessed.天文望远镜器材评测和交流论坛( S1 _4 C2 c1 A" X& S6 A$ E+ @1 u
国内专业的天文望远镜器材评测和交流论坛，致力于如高桥、televue、tmb、ap、tec等顶级望远镜和附属器材- y- t& }- I6 w) F* o* @
Except for the historic EK-325, from Kodak, the example glasses are all from Schott, whose distinctive designations for glasses are popular.
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. `3 V, Y# x" ]4 u# k; D天文家园欢迎您！2002 John J. G. Savard

Monocentric eyepiece.... aimed.... at providing the highest-quality images 天文望远镜器材评测和交流论坛" Y( Z* Y: |" Q# }7 X4 q( Y# s! S

! T! J: ^# O/ ^# j8 r; x; N天文望远镜器材评测和交流论坛这点我絶对同意，在我试过的目镜及望远镜对照比较上，Mono只会相若或较好。

还有一片的，很酷，，，，# W2 o$ m  F- H  O, u7 _
单纯从美不美观上来看，个人感觉类似pl结构的目镜都很漂亮，前置巴罗的都很难看，不够紧凑，，天文望远镜器材评测和交流论坛1 V( q/ t" E, U* C3 }. \! N2 b
有个问题，，是不是应该避免那种空气界面相邻两个表面的曲率太过接近，，？感觉konig那个从阿贝结构分出来的部分太难看了，，
1 Y  e# S% {, I1 W  S, G3 _天文望远镜器材评测和交流论坛另外，镜片组中，近距离连用两片相近的凸透镜感觉也不好看，，，，像bertele的，，www.astroview.com.cn( A1 R1 M  c' T# o8 U1 S* }* ]1 H, K
接目镜的部分，凹的最漂亮，因为感觉和人的眼球匹配，平的也不错，凸的感觉就差多了，，虽然可能和实际能给人带来的舒适感觉相反

呵呵 很期待那个单片的能是什么效果