目镜
维基百科，自由的百科全书
- 與照相機的設備請參見反光鏡。
目鏡，又称接目镜，通常是一个透镜组，可以連接在各種不同光學設備，像是望遠鏡和顯微鏡，的後端。所以如此命名，是因為當設備被使用時，它常是最接近使用者眼睛的透鏡。物鏡的透鏡和面鏡收集光線並引導至焦點生成影像；目鏡被安置在焦點，主要的功能在放大影像，放大的倍率則與目鏡的焦距有關。
目鏡通常會包含幾個組裝在一起的" 透鏡元件"，裝在一個筒狀物的後端。這個筒狀物則會塑造成適合儀器的特別開口，影像可以經由移動目鏡和物鏡焦點的位置而聚焦成像。多數儀器都會有一個聚焦的裝置，允許目鏡在軸上移動，而不需要直接去操作目鏡。
雙筒望遠鏡的目鏡通常是永久固定在鏡筒上，因此它們的視野和放大倍率都是預先就被設定好的。望遠鏡和顯微鏡，目鏡通常都可更換，而通過目鏡的更換，使用者可以調整視野和倍率。例如，望遠鏡就經常以更換目鏡來增加或減少倍率；目鏡也為使用者提供提供不同視野和適眼距的調整。
現在用於研究的望遠鏡已不再使用目鏡，取而代之的是裝置在焦點上的高品質CCD感測器，而影像就可以直接在電腦的顯示器上觀察。有些業餘天文學家也在個人的望遠鏡上安裝了相似的設備，但普遍的仍然是直接使用目鏡來觀察影像。
除了伽利略式望遠鏡的目镜采用凹透镜以外，大多数望远镜的目镜都可以等效为凸透镜。一个好的目镜应该尽可能消除色差、像差、提供优良的像质，提供较大的表观视场，较长的適眼距以方便人们使用，提供较好的目镜罩以减少杂光干扰。设计优秀的目镜还考虑了戴眼镜的人使用，使用了橡皮可翻目镜罩或者可调升降目镜罩。目镜的光学系统的设计有多种形式，如：惠更斯目镜（H式或HW式）、冉斯登目镜（R式或SR式），这些属于第一代目镜。第二代目镜具有代表性的有四种：凯尔纳目镜（K式）、普罗素目镜（PL式）、阿贝无畸变目镜（OR式目镜）、爱尔弗广角目镜。第三代目镜最著名的目镜是Nagler目镜，它拥有更加出色的表现，特別是在視場修正技術方面。在小型天文望远镜中，大部分目镜的接口遵循三个标准，即外径为0.965英寸（24.5毫米）、1.25英寸（31.7毫米）和2英寸（50.8毫米），具有相同接口标准的目镜可以互相替换使用。
目录 |
[编辑] 目鏡的性質
目鏡的一些性質對光學產品的功能非常重要，需要比較以決定最適合需求的目鏡。
[编辑] 入射光瞳的距離設計
目鏡的入射光瞳永遠不變的被設計在目鏡的光學系統之外，它們必須被設計在特定的距離上有優異的性能（即在這個距離上的變形極小）。在折射式的天文望遠鏡，入射瞳通常很靠近物鏡的位置，與目鏡通常有數英呎的距離；在顯微鏡，入射瞳通常緊靠著物鏡的後焦平面，與目鏡只有幾英吋的距離。因此顯微鏡的目鏡與望遠鏡的目鏡性質不同，不是互換就能獲得適當的表現。
[编辑] 元素和群
每一個獨立鏡片稱為元素，通常是簡單的透鏡，可以組合成單鏡、膠合的雙鏡或是三合鏡。當這些元素被兩個或三個黏合在一起時，這種組合就成為群。
第一個目鏡只是單片的透鏡元素，得到的影像有高度的變形。二或三個元素的設計發明之後，由於改進了影像的品質，很快就成了標準的設計。今天，工程師在計算機協助規劃下的設計，以七或八個元素提供了絕佳的影像。
[编辑] 內部反射和散射
內部反射有時也稱為散射，導致穿過目鏡的光線不僅分散還降低了目鏡產生影像的對比。當影像的效果很差時就會出現"鬼影"，稱為幻像。多年以來，設計時玻璃與玻璃之間製造很小的空氣隙，就能有效的改善這個問題。
對薄透鏡可以採用在元素表面鍍膜的方法來解決這個問題。這一層厚度只有一或兩個波長的膜，可以改變通過元素的光線折射來減少反射和散射。有些鍍膜可經由全反射的過程吸收這些光線以低淺角度射入的光線，使它們不會穿過透鏡。
[编辑] 側向色差
色差的產生是因為不同的顏色（波長）由一種介質到另一種介質時，有不同的折射率。對目鏡而言，色差來自穿越空氣和玻璃之間的界面。藍光和紅光在經過目徑的元素之後不能距焦在同一個焦點上，這種現象對點光源 的結果是可能產生一個圍繞著焦點的模糊色環，通常的結果是造成影像模糊不清。
有幾種方法可以減緩這個問題，一種是利用薄膜來改正目鏡的元素。較為傳統的方法則是利用多個不同玻璃和曲度的元素來消減變形。
縱向色差在光學望遠鏡中，因為焦距很長而成為很顯著的效應；顯微鏡，因為一般的焦距都很短，就不受這種效應的影響。
通常，目鏡在改善色差時，這兩種都需要做修正。
[编辑] 焦長（焦距）
焦長是平行的光經過目鏡後匯距的點與目鏡主平面的距離。在使用時，目鏡焦長和物鏡焦長的結合，確定了附屬的放大倍率。當單獨提到目鏡時，他的單位通常是毫米（mm）；而當在一架可以更換目鏡的儀器上使用時，有些用戶喜歡使用經過目鏡後所能得到的放大倍數做為單位。
對望遠鏡，一些特殊的目鏡可以產生不同的角放大率，並且望遠鏡和顯微鏡的組合倍率可以用下面的慣例式來計算：
此處：
- MA是要計算的角放大倍率，
- fO是望遠鏡物鏡的焦長，
- fE是目鏡的焦長，要用同樣的測量單位來表示。fT.
對一個複合式顯微鏡的慣用式是：
此處：
- D 是距離最接近的明視距離（通常是250mm），
- DEO是物鏡的後焦面和目鏡的後焦面（稱為筒長）的距離，在現代的儀器上這個距離通長是160mm
- fO是物鏡的焦長，FE是目鏡的焦長。
因此，要提高放大倍率，可以將目鏡的焦長減短，或是將儀器本身的焦長加長。例如，焦長25mm的目鏡用在焦長1200mm的望遠鏡上，放大倍率是48倍；焦長4mm的目鏡用在相同的望遠鏡上，放大倍率是300倍。
業餘天文學家使用的望遠鏡的目鏡傾向於將焦長標示出來。在天文學，焦長的表示單位通常是毫米（mm），範圍則在3至50毫米之間。實際的放大倍率則依使用的望遠鏡的焦長來決定。
但是當描述觀測現象時，天文學家對於目鏡的標示，卻又慣用放大倍率，而不是標示目鏡的焦長。在觀測報告上使用放大倍率是比較方便的，因為它更直接的提示了觀測者實際上看到的是甚麼的看法。由於放大倍率是依賴所使用的望遠鏡決定，因此單獨只提放大倍率對望遠鏡的目鏡是毫無意義的。
依據協議，顯微鏡的目鏡通常標示具體的倍率來取代焦長。顯微鏡的倍率 PE和物鏡的倍率PO的關係如下：
因而對一個複合式的顯微鏡前端角放大率的表示是：
倍率的定義是依據儀器對任易分離角度在目鏡和物鏡之間被放大的能力。不同於歷史上對顯微鏡目鏡的分析，是依據目鏡對角度的放大倍率，和物鏡原本的放大能力。這對光學設計師是很方便，但從顯微鏡學實用的觀點上看卻缺乏便利性，因此便被摒棄了。 一般目鏡的放大倍率是8X、10X、15X、和20X。這些倍數是與正常人的能看清楚的最短明視距離，D250mm，比較得到的，所以目鏡的焦距可以用250mm除以放大倍率而計算出來。雖然被接受的標準距離是250mm，但現在的顯微鏡會設計成只有160mm的焦距，使得儀器變得非常的緊湊。現在的儀器也許還會被設計成管子實際上是無限長的(在鏡筒內使用一個輔助透鏡)。 顯微鏡影像整體的角放大率是目鏡放大率與物鏡放大率的乘積。例如，10X的目鏡與40X的物鏡組合就會得到400X的放大倍數。
[编辑] 焦平面的位置
有一些目鏡，像是冉斯登目鏡 (在下面有詳細的說明) ，焦平面的位置在目鏡之外的場透鏡前方，因此很適宜做為標線或測微表等十字線安置的位置。在惠更斯目鏡，焦平面的位置在眼睛和在目鏡內的場透鏡之間，是不容易接近的位置。
[编辑] 視野
視野，經常會使用縮寫FOV，描述的是經由目鏡能看見的目標 (從觀測者所在地測量得到的角度) 。目鏡的視野範圍會根據各自所結合的望遠鏡或顯微鏡的放大率而有所變化，也和目鏡本身的性質有關。目鏡由他們的視野闌做區分，這是進入目鏡的光線抵達場透鏡前所經過的最狹窄孔徑。
由於這些可變的因素，"視野"這個名詞通常有兩種意義，並且總是只表示其中之一。
- 實視野是使用某一架望遠鏡時，由於具體的放大效果，通過目鏡能看見的真實天空的角度大小，它的範圍通常在0.1度至2度之間。
- 視視野是被測量的目鏡所有的一個恆定值，範圍從35度至80度以上。它本身，明顯的是一個抽象的數值，但是可以經由望遠鏡與目鏡結合所得到的的放大率測量出實視野。目鏡的視視野通常都會作為目鏡的特性標示出來，為用戶提供一個方便的方法，計算在自己的望遠鏡上使用時的實視野。
目鏡的使用者通常都需要計算實視野，因為這表示出目鏡與望遠鏡結合時，實際上能看見的天空大小。計算實視野最方便的方法取決於是否知道視視野。
如果已經知道視視野，實視野可以經由下面的近似公式計算：
- 或
此處：
- FOVC是實視野，計量的單位是以FOVP時所提供的角度單位來測量。.
- FOVP 是視視野。
- mag是放大倍數。
- fT是望遠鏡的焦長。
- fE是目鏡的焦長，用與fT相同的量度單位來標示。
望遠鏡物鏡的焦長是物鏡的口徑乘上焦比的值，他代表鏡子或透鏡將光線聚集在一個點上的距離。
這種形式的精確度可以在4%以內，或視視野達到40°都是良好的，而在60° 時的誤差為10%。
如果不知道視視野，實視野可以使用下面的方法來概估：
此處：
- FOVC 是實視野，以度為計算單位。
- d是目鏡視野闌的直徑，單位為mm。
- fT式望遠鏡的焦距，單位為mm。
第二個公式比第一個來得精確，但是多數廠家通常都不會告知視野闌的大小。如果視場不是平坦的，或是對設計的角度大於60°的超廣角目鏡，第一個公式就會不準確。
[编辑] 筒徑
望遠鏡有三種不同標準的筒徑，而筒徑的大小習慣用英吋標示。
- 最小的標準筒徑是0.965 英吋 (24.5mm)，但幾乎已經被摒棄了。仍然使用這種筒徑的望遠鏡不是玩具店內的商品，就是通常只在商城 (大賣場) 內仍然充斥的品質較差的望遠鏡。許多在這種望遠鏡上的目鏡都是塑膠製造的，有些甚至連透鏡都是塑膠的。高品質的望遠鏡早已不再種尺寸的目鏡了。
- 大部分的目鏡筒徑都是1¼ 英吋 (31.75mm)，這種筒徑的目鏡在實用上的焦距上限大約是32mm。焦距更長的目鏡，焦距比32mm更長的目鏡，筒徑的邊緣限制了視視野的大小不能超過50°，而多數的業餘者認為這是可以接受的最小視野。這種筒徑的螺旋可以置入30mm的濾鏡。
- 2 英吋 (50.8 mm) 筒徑的目鏡經常被使用。2英吋目鏡的焦距極限大約在50mm，大於2英吋 (50.8 mm) 的筒徑主要在協助延伸目鏡焦距的極限。這種目鏡的價值通常都很昂貴，並且可能重得足以傾覆望遠鏡。這種目鏡的螺旋適用48mm的濾鏡 (或是49mm的)。
顯微鏡的目鏡使用mm為單位，標準筒徑為23.5mm和30mm，都比望遠鏡的筒徑小一些。
[编辑] 適眼距
眼睛需要在目鏡後方的一段距離內觀看經過目鏡形成的影像，這段適當的距離稱為適眼距。有著較大的適眼距，意味著目鏡的品質越佳，也越容易觀看到影像。但是如果適眼距太大，要讓眼睛長期處在正確的位置上，它會造成眼睛的不舒適。基於這個原因，有些有著長適眼距的目鏡，在目鏡透鏡的後方有眼罩杯的設計，可以幫助觀測者能長時間的在正確的距離上觀測目標。出射瞳的大小應該與拉姆斯登盤的大小相符。在天文望遠鏡的情況下，入射光瞳的影像對應於物鏡的大小。
適眼距的典型範圍在2mm至20mm之間，依據目鏡的構造來決定。長焦距的目鏡通常都有較寬裕的適眼距，但短焦距目鏡的適眼距就有問題了。直到最近，這仍然是相當普遍與共通的，短焦點目鏡的適眼距就較短。好的設計指南建議適眼距至少要有5-6mm，以避免睫毛造成的不舒適。現代的設計可以增加許多透鏡元件，不僅在這方面獲得改善，還可以在高倍率的觀測上變得更加舒適。特別是對於帶眼鏡的觀測者，他們至少需要20mm的距離才能容納德下它們的眼鏡。
[编辑] 目鏡設計
Technology has developed over time and there are a variety of eyepiece designs for use with optical telescopes. They vary in their internal lens configuration and different designs are sometimes more appropriate both for different types of viewing, and for different types of telescope. Eyepiece designs include Huygens, Ramsden, Kellner, Orthoscopic, Erfle, König, Plössl, RKE, and Nagler. These are described in more detail below.
[编辑] 惠更斯目鏡
The two-elements Huygens eyepiece was invented by Christiaan Huygens in the 17th century. This optical design is now considered obsolete. Their main use in optics is as an example of the simplest possible compound lens design.
Despite being depricated, these eyepieces are inexpensive to make and so are often sold with the cheapest telescopes and microscopes. Huygens eyepieces suffer from short eye relief, high image distortion (especially on short focus telescopes), chromatic aberration and have very narrow apparent field of view.
Essentially their only good use is for projection of a solar image onto a screen. Because Huygens eyepieces do not contain cement to hold the lens elements, they are less likely to be damaged by the intense, concentrated light of sun. Lens cement can overheat and either dissolve or burn.
Huygens eyepieces consist of two plano-convex lenses with the plane sides towards the eye separated by an air gap. The lenses are called the eye lens and the field lens. It is usually designed for zero transverse chromatic aberration. The focal plane is located between the two lenses. If the lenses are made of glass of the same refractive index, to be used with a relaxed eye and a telescope with an infinitely distant objective then the separation is given by:
where fA and fB are the focal lengths of the component lenses.
[编辑] 冉斯登目镜
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 is not possible to correct the Ramsden design completely for transverse chromatic aberration. The design is slightly better than Huygens but still not up to today’s standards.
It remains highly suitable for use with instruments operating using near monochromatic light sources e.g. polarimeters.
[编辑] 凯尔纳目镜或"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.
[编辑] 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.
[编辑] 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 are Plössls with extra lenses.
Erfle eyepieces are designed to have wide field of view (about 60 degrees), but they are unusable at high powers because they suffer from astigmatism and ghost images. However, with lens coatings at low powers (focal lengths of 20mm and up) they are acceptable, and at 40mm they can be excellent. Erfles are very popular because they have large eye lenses, good eye relief and can be very comfortable to use.
[编辑] 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 adaption 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°.
[编辑] 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 4 optical elements. Usually consisting of two sets of doublets, a convex and concave 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.
[编辑] 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 is based. This arrangement makes the design similar to a widely spaced version of the König design.
[编辑] Nagler
Invented by Albert Nagler and patented in 1979, the Nagler eyepiece is a design optimized for astronomical telescopes to give an ultra-wide "flat field" field of view. It also gives the viewer an apparent field of 82° that has good correction for astigmatism and generous eye relief. This is achieved using up to seven elements in 4 or 5 groups; there are at least 3 similar designs called the Nagler, Nagler II, Nagler type 5, and so on.
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 exceeding 0.5kg, 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. [1]
[编辑] 參考資料
- A. E. Conrady, Applied Optics and Optical Design, Volume I. Oxford 1929.
- R. Kingslake, Lens Design Fundamentals. Academic Press 1978.
- H. Rutten and M. van Venrooij, Telescope Optics. Willmann-Bell 1988, 1989. ISBN 0-943396-18-2.
[编辑] 相關條目
[编辑] 外部連結
- A list of eyepieces with some details of their construction.
- A list of eyepieces with some details of their construction.
- A list of eyepieces with some details of their construction.
- Demonstrates the effect of eyepieces
