希望此文对初学者的目镜选择有所帮助。本文翻译注重内容，翻译不妥之处敬请谅解，内容曲解之处肯请纠正。
此文选择目镜的出发点基于 视场光阑，与国内同好关注的焦距和视角不太一样。
Choosing Eyepieces 目镜选择
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MAGNIFICATION
Eyepieces determine your telescope's magnification. To calculate the magnification of an eyepiece in your telescope, divide the focal length of the telescope by the focal length of the eyepiece:
Magnification = telescope focal length ÷ eyepiece focal length
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放大倍率
目镜决定你的望远镜的放大倍率。计算一只目镜在你的望远镜中的倍率，是将望远镜的焦距长度除以目镜的焦距。
倍率 ＝ 望远镜的焦距 ÷ 目镜的焦距
TRUE FIELD OF VIEW
Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3:
True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3
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真实视场
目镜也决定你所看到天空的实际区域范围。计算实际视野区域（度数），目镜的视场光阑直径除以望远镜的焦距再乘以57.3：
真实视场（度数）＝ 目镜的视场光阑直径 ÷ 望远镜的焦距 x 57.3
本人举例： 物镜焦距 ９００ｍｍ，66度 广角目镜：目镜焦距２０ｍｍ,视场光阑 23.5mm
真实视场（度数） = 23.5 ÷ 900 x 57.3 = 1.495度 (月亮直径占视场直接的三分之一)
倍率 ＝900 ÷ 20＝ 45倍
该目镜的表现视场为 66度
THE FIELD STOP AND APPARENT FIELD OF VIEW
The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50º, Radians have 60º, Panoptics have 68º, Naglers have 82° and Ethos eyepieces have 100°.
视场光阑和表现视场
视场光阑是安装在目镜镜筒内的金属的圆环用来限定范围的大小。它是由目镜投影的，当你在目镜中观察时它呈现为一个圆形区域。这个圆形区域的角度直径称为表现视场 (AFOV)并且是一个固定的属性由每只目镜的设计决定。例如Plössl 目镜的表现视场（AFOV）为50º, Radians 为 60º, Panoptics 为 68º, Naglers 为 82° ， Ethos 为100°.
LOW-TO-MEDIUM POWER VIEWING
For low-power viewing of large objects, or to use your telescope as a low-power finder, you will want an eyepiece that delivers close to the maximum possible true field of view (note that for 1.25" eyepieces, the maximum field stop diameter is 27mm; for 2" eyepieces, it's 46mm). Then add eyepieces covering uniform increments in smaller field stops. For example, if your widest field eyepiece has a 40mm diameter field stop and you choose a decreasing increment diameter factor of 2 (which results in a 4x decrease in area size), you'll end up with eyepieces having field stop diameters of approximately 40mm, 20mm and 10mm. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm. Of course, avoid duplicating focal lengths. For example, if you use a 31mm Nagler (with a 42mm field stop diameter), you would not need a 32mm Plossl (with a 27mm field stop diameter).
In general, for each field stop size, choosing eyepieces with shorter focal lengths and larger apparent fields of view will allow you to see more detail and fainter stars. In addition, you'll have a smaller exit pupil to better match your eyesight.
低－中倍观察
低倍观察一个大目标，或利用你的望远镜来进行搜寻，你将需要一个尽可能接近最大实际视场的目镜。（1.25寸目镜，视场光阑最大是27mm,2寸目镜的是46mm）然后所填加目镜按照一定的间隔进行缩减。例如，如果你的最广视野的目镜拥有40mm的视场光阑直径并且你选择间隔倍率为２，（导致４倍减少区域面积），你最终选择的这组目镜的视场光阑为40mm,20mm和10ｍｍ。为了填补中间的空余，可以增加的目镜视场光阑为28mm和14mm。当然，也要避免焦距重复，例如你使用一个31mm的Nagler (42ｍｍ视场光阑)，你不需要一个32mm Plossl (with a 27mm视场光阑)。
通常，针对每一个尺寸的视场光阑，选择短焦距和大表现视场的目镜将允许你看见更多的细节和暗淡的星光。另外，你还将拥有一个较小的出童直径来更好的配合你的视力。
EYEGLASSES AND EYE RELIEF
If you do not need eyeglasses to correct astigmatism, don't use them when observing. If you wear glasses to correct astigmatism, make sure they're multi-coated, and try to choose eyepieces that have at least 15mm to 20mm of eye relief, to minimize any field reduction (vignetting).* You may find that with small exit pupils such as 1mm or less, you probably will not need eyeglasses, and can therefore use eyepieces with less eye relief.
*Many Tele Vue eyepieces accept DIOPTRX, the unique addition that replaces prescription eyeglasses that correct eyesight astigmatism.
眼镜和出童距离
如果你不需要眼镜来修正散光，观察时不要佩戴眼镜。如果你佩戴眼镜来修正散光，确认眼镜是多层镀膜的，尽量选择出童距离在15mm和20mm的目镜来避免视场的缩小。*你会发现一个小的出童甚至比１ｍｍ还小，你或许不需要眼镜即可使用出童距离小的目镜。
*许多Tele Vue的目镜允许屈光的（近视或远视），唯一需要佩戴眼镜的情况是你需要眼镜来修正散光。
EXIT PUPIL
The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope's focal ratio:
Exit pupil = eyepiece focal length ÷ telescope f/#
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For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.
出童
出童是目镜形成的目标的影响。眼睛在那可以看到全部的视场。你可以计算出童直径通过以下公式：
出童直径 ＝ 目镜焦距 ÷ 望远镜的焦比 (F/# )
本人举例： 物镜口径 １００ｍｍ，焦距 ９００ｍｍ，目镜焦距２０ｍｍ
(原文算法）出童直径 ＝ 目镜焦距 ÷ 望远镜的焦比 (F/# ) ＝ 20 ÷(900÷100 ) ＝ １.889mm
（通常算法）出童直径 ＝ 物镜直径 ÷ 放大倍数 ＝ 100 ÷(900÷20) ＝ １.889mm
IMAGE AMPLIFIERS (BARLOWS AND POWERMATES)
You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power. Tele Vue makes Barlows and Powermates (an improvement to the Barlow-type design) in magnifications factors of 2x, 2.5x, 3x, 4x and 5x.
图像放大器（巴罗和POWERMATES）
你可以选择一个长焦的舒适出童的目镜同时使用一个图像放大器来增加倍数。Tele Vue制造巴罗（Barlows）.和 Powermates （一种改进的Barlows结构设计）放大倍数有2x, 2.5x, 3x, 4x 和 5x。
Paracorr (Parabola Corrector)
If you have a Newtonian or Dobsonian reflector that's f/5 or faster, you should seriously consider using the Paracorr to eliminate coma, so your full field eyepiece sharpness is not compromised. Paracorr also acts like a 1.15x Barlow, so for example, a 1000mm f/4.5 scope becomes 1150mm f/5.2.
抛物修正 Paracorr (Parabola Corrector)
如果你拥有一台牛顿或道布森反射镜焦比在 f/5 或更大，你应该认真的考虑使用一个Paracorr 来修正慧差，因此你的目镜的整个像场的锐度将不妥协。Paracorr 同时但当一个1.15x 巴罗的角色，例如，一个 1000mm f/4.5成为 1150mm f/5.2.
HIGH-POWER VIEWING
Once you’ve selected an eyepiece set based on field stop sizes, calculate the magnifications produced with your scope. For planetary or double star observing, you’ll want an eyepiece in at least the 150x range. For determining maximum power, a good rule of thumb is to use no more than 60x per inch of aperture for scopes with apertures up to 6". Higher magnifications may still be pleasing but will not likely reveal any additional detail. Realistically, the atmosphere will usually limit your planetary observing to a maximum magnification of about 300x, no matter how large your telescope aperture.
Basically, you’ll be choosing low and medium power eyepieces by field stop increments to "frame" the subject, and high power eyepieces by magnification increments (based on your scope’s aperture), to reach the optimum contrast and resolution for viewing planets and double stars.
Thank you for considering Tele Vue eyepieces. Every Tele Vue eyepiece is Q.C. tested at f/4.0, and can be serviced or repaired by Tele Vue. For more individual advice on specific applications, you can always call Tele Vue during east coast business hours.
高倍观察
当你基于视场光阑选择了一只目镜，计算它与望远镜配合的放大倍率。行星观测和双星观测，你将需要一个最少１５０倍的目镜。决定最高的放大倍数，有一个好的简便算法，在６英寸以下的镜子，每英寸不会超过60倍 。（每ｃｍ不超过２3.6倍，适用于153mm以下口径）。更高的倍数令人高兴，但是不太可能展示额外的细节，实际上，大气通常会限制你的行星观察最高只能到大约300倍,无论你的望远镜拥有多大的孔径。
基本上，你可以选择低倍中倍的目镜通过视场光阑来“圈定”你要观察的目标，选择高倍目镜取决于你望远镜的孔径，以获得最佳的反差和解像力用于观察行星和双星。
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