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本节翻译作者为同好@DOB-YIN，他的英文水平比我不知道高到哪里去，很高兴有这样一位高手一起翻译这本书。 我擅自补充一句：如有错漏，敬请指出，谢谢！
Chapter6 TheSecondary Cage 副镜框 Ifconstructing the upper end of your telescope before the rest seemscounterintuitive, consider this: The whole purpose of a telescope is to let youaim some big optics at the stars. Logically, then, your telescope begins withthe optics and you build the rest of the telescope around them. The function ofthe secondary cage is to provide a rigid supporting framework for the secondarymirror and eyepiece. After making the mirror cell, constructing the secondarycage is the next logical step. 将位于望远镜顶部的副镜框的制作，安排在（除主镜支撑单元之外的）其他部分之前，这个顺序看起来似乎有悖常理。其实很好理解，制作望远镜的目的是要将巨大的光学系统瞄准星星，因此望远镜的制作也是从光学系统开始，再做周边部件。副镜框的功能是为副镜和目镜提供一个刚性支撑框架。制作完主镜支撑单元后，接下来开始副镜框的制作，是合乎逻辑的步骤。 Itis extremely important to keep the secondary cage as lightweight as possible, consistentwith being reasonably strong and very rigid. A lightweight upper end is thesecret to keeping the rest of scope from becoming too heavy because, in a f/5telescope, every extra pound you add to the secondary cage adds up to sixpounds to the total weight of the telescope. One pound goes on the secondarycage and five pounds go behind the mirror to balance it. 对副镜框的要求除了坚固和刚性好之外，很重要的一点是尽可能轻便。减轻位于望远镜顶端的副镜框的重量，是减少整个望远镜重量的秘诀，因为在焦比f / 5的望远镜中，副镜框重量每增加1磅，整个望远镜总重量就会增加6磅。副镜框的1镑重量需要主镜端的5磅来平衡它。 The secondary cage consists of twolightweight plywood rings separated by four short aluminum struts and a focuserboard. The secondary is carried on a spider, andthe rings are lined with thin, tough Kydex plastic to block stray light. Thesecondary cage for a 20-inch telescope weighs in at about 15 pounds, complete. 副镜框由两个轻质胶合板环组成，由四个短铝支杆和一个调焦座板分隔。副镜通过十字支架（蜘蛛架或副镜架）安装，两环之间衬有薄而坚硬的Kydex塑料以阻挡杂散光。 用于20英寸望远镜的副镜框总重量只有大约15磅。 Intelescopes of 25 inches and under aperture， you can attach the secondary cage directlyto the truss poles. When you assemble the telescope, you climb the laddercarrying the secondary cage and place it atop the poles, and clamp the polesinto the tube sockets. To make telescopes larger than 25 inches aperture easyto set up, before you set the secondary cage atop the telescope, attach aconnecting ring to the top of the truss. The truss ring is light and easy tocarry and attach to the poles. You then lower the telescope and attach thesecondary cage to the truss ring. This extra step makes assembling "TheBig Ones”no more difficult than assembling the smaller sizes. 对于口径不大于25英寸的望远镜，你可以直接将副镜框安装到桁架杆上。 组装望远镜时，你需要拿着副镜框爬上梯子，并将其放在杆子顶部，并将杆子夹入管座。而对于口径大于25英寸的望远镜，为了便于安装，在你将副镜框安装到望远镜顶部之前，需要先把连接环安装到桁架的顶部。桁架连接环轻便，易携带，并容易与杆子顶部连接。然后，将望远镜拉低，并将副镜框安装到桁架环上。通过这额外的步骤，使组装“大块头”望远镜并不比组装更小尺寸的望远镜更困难。 Ifyou build the secondary cage as we recommend in this chapter, your scope willhave a low-profile mirror box and rocker. This is desirable because theshallower the mirror box and rocker are, the more rigid they are turned inazimuth, the mirror box and rocker are get. When large telescope are turned inazimuth, the mirror box and rocker are the sources of most of the flexure. Thelow-profile mirror box and rocker may look strange, but they work great. Whenyou follow an object, the telescope responds, and when you let go, the objectremains in the center of the field. There is no rebound. These benefits followas direct consequences of the lightweight secondary cage. 如果按照本章中的建议制作副镜框，则你的望远镜将会有很低的主镜箱和侧转轴。这是有好处的，因为主镜箱和侧转轴越低，望远镜水平转动时，刚性越好。当大望远镜水平转动时，主镜箱和侧转轴是大部分弯曲形变的来源。主镜箱和侧转轴低，可能看起来很奇怪，但它们工作得很好。当你推动望远镜追踪一个目标时，望远镜会马上作出反应，当你放手时，目标会留在视场中心，没有反弹。这些好处都是由副镜框的轻便带来的。 6.1Dimensioning the Secondary Cage 确定副镜框尺寸 Thesecondary mirror sits in the middle of the converging cone of light front the primaryand reflects it to the eyepiece on the side of the telescope. The main purposeof the secondary cage is to hold the secondary mirror and eyepiece in theirproper positions. (The secondary cage is also a handy place to put a Tetradfinder and a digital setting-circle readout,since these are often used near the eyepiece.) 副镜位于主镜反射汇聚光锥的中间，并将光锥反射到望远镜侧面的目镜上。副镜框的主要作用就是将副镜和目镜保持在适当的位置。（副镜框也是放置Tetrad取景器(寻星镜)和数字设置圈读数(角度显示屏)的便利场所，因为它们通常用在目镜附近。） The distance from the secondary to thefocal plane is the sum of one-half the inside diameter of the upper tubeassembly, the thickness of the board that supports the focuser, and the heightof the focuser. The inside diameter of the secondary cage is set by the optics:the secondary cage must not block any incoming light. You need to find aproperly baffled focuser, and the focuser board should be as thin as possible. 从副镜到焦平面的距离，是副镜框内径的一半与调焦器支撑板的厚度以及调焦器高度之和。副镜框的内径由光学系统决定：副镜框不能得阻挡任何入射光线。需要选用有合适挡板的调焦器，调焦器支撑板(调焦座安装板)应尽可能薄。 6.1.1The Truss-Tube Advantage 桁架的优势 Inthe standard Newtonian telescope with a solid tube, it is usually considereddesirable to allow several inches clearance between the inside of the tube andthe incoming cylinder (actually, a truncated cone) of light from the stars. Ifthe tube is too close to the light path，so the thinking goes，aircurrents from the warm tube wall degrade the image. 在采用整体镜筒的标准牛顿望远镜中，通常认为在镜筒内壁和来自恒星的入射圆柱体（实际上是截头圆锥体）之间应该留有几英寸的间隙。如果镜筒太靠近光路，一般会认为，来自温暖的镜筒(筒壁)的气流使图像劣化。 Becausethe truss-tube design is open, tube currents do not exist. Air flows throughthe tube, or if you have a cloth shroud over the truss, the shroud and tubecomponents quickly reach air temperature and hence generate no air currents. Inthe traditional cardboard-tube design, a 20-inch mirror needs a tube at least24 inches inside diameter, and will almost certainly be subject to tubecurrents. However, in a truss-tube telescope the inside diameter of thesecondary cage can just barely clear the path of theincoming light. Half an inch on either side is plenty. A secondary cagewith an inside diameter of 21 inches is sufficient for a fast 20-inch mirror,and because there is no tube, there are no tube currents. 由于桁架设计是开放的，并不存在镜筒内的气流。空气会穿过镜筒，或者如果在桁架上有布套，布套和管子部件会迅速达到的空气温度，因此也不会产生气流。在传统的纸板管设计中，20英寸的镜子需要至少24英寸内径的镜筒，并且几乎肯定会受到镜筒内气流的影响。 然而，在桁架望远镜中，副镜框的内径只要刚好能让入射光通过就可以了。 每边留半英寸就足够了。 内径为21英寸的副镜框足以用于快速(短焦比)的20英寸镜子，并且由于没有镜筒，所以不存在镜筒内气流。 6.1.2The Inside Diameter of the Secondary Cage 副镜框的内径 Ina little while, we will argue (and hopefully convince you) that the field ofview for a large-aperture telescope at low power is about 1/2°. This means that theincoming light paths from individual stars are cylinders that can deviate by 1/4° from the optical axis of the telescope, and that the diameter ofthe incoming beam at the position of the secondary cage equals the diameter ofthe primary plus the sine of 1/4° times the distancebetween the primary mirror and the end of the telescope.
稍后，我们将讨论（并希望说服你）大口径望远镜的低倍率下的视场大约为1/2°。 这意味着来自单个恒星的入射光路是与望远镜的光轴偏离1/4°的圆柱体，并且在副镜框的位置入射光束的直径等于主镜直径加上主镜与望远镜顶端距离乘以1/4°的正弦。 Supposethat you have just one star centered in the eyepiece. the light entering thetelescope is a cylinder with the same diameter as the mirror. The light from astar at the edge of a 1/2° field of view is angled at 1/4° to theoptical axis, so that the off-axis beam diverges from the on-axis cylinder by 1/4°. When it reaches the secondary cage, a distance just a little lessthan the focal length of the mirror, it willdeviate from the on-axis beam by the distance times the sine of 1/4°. Since we need toaccount for the 1/2°-distance across the whole field,we'll fudge a bit and say the divergence of the beam is the focal length timesthe sine of 1/2°. 当只有一颗恒星位于目镜视场中心时，进入望远镜的光线是与主镜直径相同的圆柱体。而来自1/2°视场边缘的恒星的光线与光轴成1/4°角，因此离轴光束与同轴圆柱体光束的偏离角度是1/4°。在副镜框处，光束偏离范围是：主镜与副镜框的距离（略小于主镜焦距）乘以1/4°的正弦。 由于我们需要考虑整个1/2°视场角，并且这里模糊处理一下，将主镜和副镜框的距离直接用主镜焦距来代替，因此则得到：在副镜框处，光束的偏离范围是主镜焦距乘以1/2°的正弦。 Theinside diameter of the secondary cage is thus the sum of the diameter of theprimary mirror plus the divergence, or: Tse=D+sin(Φ)F inwhich: D =diameter of the primary mirror F=focal length of primary mirror Φ= angular field of view. 因此副镜框的内径是主镜的直径加上光束偏离范围， Tse=D+sin(Φ)F 这里: D = 主镜直径 F= 主镜焦距 Φ= 视场角 Thesine of 1/2° is 0.00873. We recommend leaving a bit more clearance around thelight path, so use a value of O.01. The equation is thus: Tse=D+ F/100 1/2°的正弦为0.00873。我们建议在光路周围留出更多空间，所以使用O.01值。因此上式变为： Tse=D+ F/100 Therefore， for any open trusstelescope the rule is that the inside diameter of the secondary cage is thediameter of the mirror plus the focal lengthof the mirror divided by 100. For a 20-inch f/5scope, the rule works out as follows: Tse=20+100/100 Tse=21 inch 因此，对于任何开放式桁架望远镜，规则是副镜框的内径是主镜直径加上主镜焦距除以100.对于20英寸f / 5的镜子，应用该规则得到： Tse=20+100/100 Tse=21 inch Thisrule works because it is dead simple to remember and it adds a decent fudgefactor automatically. 这个规则是有效的，因为它简单易记，并且自动添加了一个体面的模糊因子。 Sampleproblem: You are a traditionalist who has always wanted to build a 24-inch f/8telescope. Forgetting momentarily about the massive lead counterweights youwould need in the bottom of the mirror box to balance such a long telescope,determine the inside diameter of the secondary cage. 示例问题：你是一位一直希望制造24英寸f / 8望远镜的传统主义者。暂时忘掉因要平衡如此长的镜筒而需在主镜箱底部放大量的铅配重，确定副镜框的内径。 Theanswer: Tse=24inch+192/100 Tse=25.92 inch Youwould probably round this up to an even 26 inches. 你可能会四舍五入取26英寸。
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