望远镜与光学测试
测试光学We all know that various errors are likely to be present in mass-produced telescopes from Synta, Meade, and Celestron. Higher end scope makers do not have as many problems. The good APOs and some of the better SCTs, MAKs, and DOBs have good quality control. They test their optics, and only very slight errors are allowed in production (less than 1/8 wave and most of the time 1/10 wave or better). Don't think that a $10,000 scope is perfect. No scope is perfect because the nature of light will not allow for perfection. There are many variables, and when one part is made perfect the cost is at the loss of another part. It's give and take. You gain here but lose there. But we can get very close with exotic materials and complex design, at a high cost in man-hours and materials. Wonder why those good APOs cost so much? Now you know.
我们都知道，各种像差在望远镜规模生产中都有可能出现，包括信达，米德还有CELESTROM。更高端的望远镜生产厂家就没有那么多问题。好的APO，一些更好的SCT，马卡，DOB等，都有好的质量控制。他们测试他们的光学产品，只允许有很小的误差（小于八分之一波长，大多数时间小于1／10波长甚至更低）。不要认为10000美元的望远镜就是完美的，因为光线物理性质的原因，没有什么望远镜是完美的。有很多的变量，当一部分被做得完美时，代价就是损失了另一部分的质量。这些变量是互换的，顾此失彼。但是我们可以使用一些特殊的材料和复杂的设计，用高成本的人工和材料进行生产，这就是为何好的APO价格那么昂贵的原因了。
Let's start with the most common aberrations of low to mid-range scopes. COMA is caused by misaligned optics, and good collimation will cure it. It will make images look like comets or meteors, with a little tail to one side. ASTIGMATISM, which is common in the human eye, will cause the image to look asymmetrical in out-of-focus images. A good test is to defocus just a bit and see if the image is nice and round or egg-shaped. If it is egg-shaped, it will look the same on both sides of defocus, just turned around. If the inside focus is vertical, then outside will be horizontal. If it's at an angle, like 8 o'clock and 2 o'clock, then the other side of focus will be 10 o'clock and 4 o'clock. It just depends on the figure of the lens or mirror and where the astigmatism is. Poor collimation or using wide angle EPs and poor seeing can give a false result which might look like astigmatism, so use narrow field EPs and make sure your optics are collimated perfectly and the seeing is good. The amount of astigmatism can be calculated by how long the image is stretched or elongated before snapping to a symmetrical image. If it snaps into a symmetrical circle in the first 0.26mm of defocus, you have about 1 wave astigmatism. Defocus of 0.14mm is 1/2 wave or very close. (This is based on a 150mm at F8. As the F ratio gets faster, the defocus to one wave gets smaller. Slower F ratios get longer. If the scope was F10, then 1/2 wave would be 0.22mm defocus, and F6 would be 0.10mm.)
现在让我们来讨论一下从低端到中段镜子的最常见的像差问题。慧差是由于光学不同轴引起的，因此好好的较准可以对其进行修正。慧差会令图像看起来像慧星或者流星，在一边出现一条小小的尾巴。像散，普遍存在于人眼中，虚焦后（焦外）令图像变得不对称。一个好的测试方法是虚焦一点点，看图像是不是圆形或者是蛋型的。如果是蛋型的，那么焦内焦外看起来是一样的。如果焦内是平行的，那么焦外就是垂直的。如果是一定的角度，如8点和2点的夹角，那么另一面就是10点和4点的方向。这个现象是由像散造成的，主要与镜头或者反射镜片有关。光轴不准，或者使用大广角目镜，或者天气不好，出现的问题非常像像散，因此你应该选用窄视场的目镜，确保光轴准确，选择一个好的天气进行测试。像散的程度可以根据图像到对称前被拉长的程度来计算。拉伸0.26毫米说明你的像散是一个波长，0.14毫米是1／2波长（这个是在150F8的镜子上作出的测试。如果镜子焦比是10，那么0.22毫米相当于1／2波长，而在F6的镜子上，0.1毫米相当于1/2波长）
SPHERICAL errors. Lower spherical aberration (LSA) is more common and unfortunately worse. LSA will cause the image to shift light energy to one side of defocus, and the other side will be dimmer. Higher spherical aberration (HSA) will do the same but on the other side of focus from LSA. LSA is known as undercorrected and HSA is overcorrected. LSA is more damaging to images, and not much can be done to fix this problem except a corrector like the CHROMACOR, which will be corrected to compensate for LSA or HSA. If your scope is undercorrected the corrector will be overcorrected, resulting in a very well-corrected scope. TUBE CURRENTS are no big deal. It's the same as turbulence in the atmosphere, just in the scope tube. Let the scope cool down to the same temperature as the outside and they will be gone. TURNED DOWN EDGE refers to an area where the primary mirror slopes at the edge and is not figured true with the rest of the mirror. A small mask at the edge will fix this at a slight loss of aperture. A turned edge shows a defocused star image with a cloudy or smeared edge, along with a loss of crispness at the edge of image. ZONES, imperfections in the glass or figure of the lens or mirror, can make a scope useless. Zones can take on many forms: blobs, rings, or blurry spots that just won't go away. Slight zones can seem to clear up a bit as the scope cools down, but sometimes they may get worse, depending on the nature of the zone. Both mirrors and refractors lenses can suffer from this.
球差。较低的球差存在非常普遍。球差导致像点的能量不能集中，一边亮一边暗。较高程度的球差也有同样的特征，但是只有一般不合焦。通常认为，低程度的球差是校准不够造成的，高程度球差是校准过度造成的。低程度的球差对像质的损害更大，而且不容易校正，除了一种专门的校正装置。如果你的球差是因为校准不够，这个校正装置可以提高校准，是你的镜子被校正的非常好。镜筒内的气流不是一个大问题，就和空气的气流是一样的。使用前先放在室外进行热平衡。边缘曲率（TURNED DOWN EDGE）是指主镜的边缘和镜子的其他部分表现不一样。加一个小的光澜，稍微牺牲一点口径可以校正。边缘变形会使星点衍射圈边缘像云或者油污的样子，失去清晰度。（ ZONES？）,质量不好的玻璃，镜头和反射镜的形状，都会让镜子不好用。ZONES有很多表现形式：斑点，环状物，模糊等等，这些都挥之不去。
Take the time to get the collimation DEAD ON -- not close but RIGHT ON. I can't help but drill you about getting the collimation perfect. I MEAN PERFECT. It makes a big difference in the scope's use of every mm of aperture. A slight turn of a screw can make an image at high power go from a fuzzy blur or smear to a very shiny pinpoint with detail. For example, I was looking at the Ring Nebula and the central star was in and out and faint at that. After tweaking the collimation to "absolute dead on," which was just a touch from "good," the star was very plain to see and the rings and color were impressive.
花一些时间去调焦你的镜子精确，希望你调教得尽量完美。用好每一毫米口径，效果会非常的不同。轻轻转动一个螺丝，可以让你的画面消除斑点或者云雾状，更加锐利和充满细节。比如，我看环状星云，中心星点有变形和闪烁。当调整螺丝进行精确的校准后，星点特别平整清晰，环状和颜色令人记忆深刻。
Here's how I collimate a scope. First, I use a steel ball (around 2 to 4mm) set up outside in the sun about 80 feet from the scope. Use the sun's reflection on the ball as a star. A green filter will help with the image of the ball dancing around. Take a high power EP and start collimating on both sides of defocus.
下面是我如何调教光轴。首先，我用一个钢株，大约2－4毫米，放在80英尺远的户外太阳底下，用太阳反射光作星点。绿色的滤镜可以帮助你更清楚的识别这个星点。用高倍的目镜，焦内焦外的调整光轴。
If you think it is perfect, it's not -- when you get outside under real skies, bright stars at high power will be a new task to make your ultra fine adjustments. I use a star like Deneb or Altair as they are not too bright (like Vega). I defocus around 1 to 1-1/2 wave and look for where the circle or disk wants to start out from. It is hard to see, but the image always wants to start out at one side or the other.
如果你觉得已经完美了，其实不是。当你在户外看星空，亮星在高倍下调整后，还需要作进一步的校正。我用像天津四这样不是太亮的星作调教。我散焦1－1／2波长，然后看圆盘或者圆环哪一边不规则。看出来有些困难，但是图像通常会在某一边出现不规则。
We want to get the scope to go right to a circle from the smallest defocused point without pinching or squishing out one side. Then I make a micro adjustment and go to the other side of focus and see if it has gotten better or worse for that side. Remember what I said about variables and no free rides? Well, split the difference for the figure of your optics. Sometimes you get a perfect image on one side but just a tad off on the other, so find the best compromise to get all the scope can give for the errors that might be present in the optics.
我们希望散焦最小的程度下，衍射圆都是规则的，那么望远镜调教就不错了。然后我们再做微小的调整，到焦点的另一边轻微散焦后，看衍射圈质量是否有明显的提高。有时，你在一边获得了高质量的图像，在另一边却有一点不良，所以请寻找最好的一个程度，使各种误差都缩小到最小，达到一个折衷的方案。
Favor the inside focus if possible; it is the preferred side and HSA is better than LSA. I adjust inside and check outside and repeat until I have the best pinpoint image I can get out, regardless of what the out-of-focus image might look like on one or both sides of focus. The key is to get a nice image at focus. That is what we use the scope for -- not to dial in a defocused image for collimating and have a less-than-optimum final image.
有可能的话，先让焦点内作到最好。我一般都是调教焦内，然后焦外进行验证，反复这个过程，直到图像质量最佳。这个关键就是得到焦点内完美的图像，这也是我们不断调教焦内焦外获得一个最适当的图像的目的。
Scopes with very good optics will almost always have near perfect out-of-focus image patterns on both sides, but those with less-than-very-good optics or multiple errors need to be adjusted to make the best of what they have. That might mean a very nice inside focus image and a slightly skewed outside focus image if that's what gives the best focused image. Always go for final image at focus, not the perfect defocused image for collimating.
光学良好的望远镜，焦外的图像也近乎完美。一般来说，比较好的调教都是焦内图像完美，焦外有很轻微的云雾状。我们追求的还是焦点上的图像质量，而不是散焦后的图像质量。
[ 本帖最后由 我爱祖祖 于 2007-6-21 20:58 编辑 ] 器材知识：光学衍射极限
Diffraction Limited Optics
I continually see statements of 揹iffraction limited optics?in the industry and this generates an astounding amount of discussion to exactly what this is and what it means. Therefore, I write this paper to try to explain what this is and what it means to you ?the telescope user.
我不断的看到有关光学衍射极限的评述。光学衍射极限是什么？有什么意义？我写这篇文章就是尝试对望远镜爱好者解释是光学衍射极限是什么以及有何意义的问题。
In order to first understand what the statement 揹iffraction limited?means, we need to understand what diffraction is and how it effects image formation in a telescope.
要理解衍射极限的意思，首先我们要搞明白衍射的定义以及衍射如何影响望远镜的成像。
From Wikipedia, the free encyclopedia
Diffraction is the bending, spreading and interference of waves when they pass by an obstruction or through a gap. It occurs with any type of wave, including sound waves, water waves, electromagnetic waves such as light and radio waves, and matter displaying wave-like properties according to the wave杙article duality.
衍射就是指当波通过一个小孔，或者一个障碍物体，发生的波的弯曲、扩散和干涉的现象。衍射发生在任何形式的波上，包括声波，水波，电波以及光波，表现出来的波浪形的特性都遵从波粒二象性。
From this definition we can see that as light waves pass by an obstruction or EDGE of an optic, they are bent. This process is diffraction. Since our telescope mirror has an edge (round) the impinging starlight gets diffracted at this edge of our telescope mirror (refractors do this to!). Therefore, instead of incoming light being focused to a perfect geometric point, the diffraction effect spreads the light into a disc with rings surrounding it. This disk is known as the Airy disc.
从概念中可以看出，光波通过障碍物体时或视觉边缘，会发生弯曲，这个过程就是衍射。同理，我们镜子的反射镜也有一个边，在这里也会造成衍射（折射也同样）。因此，由于衍射存在，我们很难把入射光线集中在一点上，衍射使最终形成一个圆盘或者在外圈形成一个圆环。这个圆盘就叫做爱里克斑。
If we examine 12 inch f/4 parabolic optics with ray tracing software we can explore this diffraction effect. Below is a ray trace of the system layout.
如果我们用12英寸F4的抛物线镜子，使用光线跟踪软件，就可以揭示衍射的效果。下面就是光线跟踪的示意图：
http://www.cloudynights.com/images/diffraction/image002.gif
Next we examine the resulting spot diagram for the exact optical center of the mirror.下面我们检查镜子中心的结果：
http://www.cloudynights.com/images/diffraction/image003.gif
You抣l note that you will find a small spot centered in a larger circle. Along side is a scale bar of 10 microns (10/1000ths of a millimeter). The small spot represents the size of a star image as if it were perfectly geometrically focused without the effects of diffraction. The outer circle represents the Airy disk of the REAL star image. So even though the geometric focus is smaller than the diffraction disc, the star image 揵lows?up to the size of the outer circle. In this case, one could say that the performance of this optic is limited by the diffraction effects, hence the term 揹iffraction limited? However, AS USUAL, there is more to the story. The first question that arises is what does it take in terms of surface error, to obtain diffraction limited performance. The second part of the story is over how much field of view is the performance considered diffraction limited?
你会注意到，在大的圆环中有一个很小的点。旁边的刻度是10微米。这个小点代表星像，好像光都聚集到了一个点上，没有收到衍射的影响，但是外圈的大圆其实就是爱里克斑。因此，尽管光点比爱里克斑小很多，但是真正的星像仍然应该是外圈的大环。因此我们可以说，这样的结果就是受了衍射的影响，也就是衍射极限。这个实验还说明其他的问题。问题就提出来了：根据镜子表面的误差，衍射如何起作用？对视场的影响上，起多大的作用？
Before we get too deeply into thiese subjects, I have to stress that we are talking about only the extreme center of our parabolic mirror and once we start examining the field of view (which we will) things change.
在我们深入研究这个问题前，我们要强调我们讨论的仅限于抛物面镜子的正中央部分，来看有关的变化。
So how much surface deviation can we tolerate before our optic is no longer diffraction limited? Below you will find a repeat of our spot diagram showing the geometric focus as well as the diffraction disc (the circle). This time the geometric focus is a bit of a blur. This is due to the application of tolerance calculations in the ray trace software. I allowed the resulting wavefront to vary by ?wave and applied a Monte Carlo algorithm. The resulting geometric focus blur still just falls within the diffraction disc and again the performance is 揹iffraction limited?
那么多大的镜面误差是是我们能够容忍的呢？下图你可以看到一个焦点处的点状图。这一次焦点显得有些模糊，这是由于光线跟踪软件的误差造成的，这个误差是允许的。这个模糊的焦点仍然落在了大的衍射环内，这也是衍射极限限制的结果。
http://www.cloudynights.com/images/diffraction/image004.gif
Next we adjust the tolerance level to ? wave on the wave front and we now see that the geometric focus starts 搒pilling?out of the diffraction disc. It becomes obvious that the performance is no longer limited by the diffraction of the system. This optic is not 揹iffraction limited?
当我们继续放大可以容忍的误差，下面这张我们可以看到，焦点处的斑点已经散落在衍射圈的外面。很明显，整体的表现已经不受衍射极限的限制。
http://www.cloudynights.com/images/diffraction/image005.gif
Now we should have a good understanding of what the term diffraction limited means, however as I mentioned earlier there is a bit more to the story. When we use the term diffraction limited, we must also qualify where in the telescope field of view this is happening. Is it happening at the exact center of the field of view only? Does this extend to the edge of our eyepiece or camera field of view? How big is this area? If we are to truly understand the performance of our telescope we must answer these questions.
现在我们可以很好的理解衍射极限的意思了。当我们使用衍射极限这个概念时，我们必须限定是在望远镜的哪一个部分。这个现象只会发生在镜子的中心部分么？会延伸到目镜的边缘或者是像机的视场内么？这个区域有多大？如果我们要真正了解我们望远镜的表现，我们必须回答这个问题。
First, when we talk about the image performance, we have to qualify how big the field of view is. We talk about this in terms of angular field.
首先，当我们讨论像质表现时，我们必须限定视角的大小。我们依照角度来讨论。
To help understand this, we can imagine viewing a single star in the exact center of the telescope eyepiece (field of view). This star is at the zero field angle and, as in our example above, we are enjoying a diffraction limited view of this star. But what if we view more than one star at a time? What if we view a cluster, a galaxy or the moon? These objects extend outward from the center of the view towards the edge of the filed of view provided by our eyepiece or camera.
为了有助于理解这个问题，我们可以想象，在目镜的正中央，看一颗星。这颗星的角度就是0度，我们当然很乐意进行这样的单星测试。但是，如果同时有多颗星存在怎么办？如果我们看的是星簇，星系或者月亮怎么办？这些物体都会从目镜的中心延伸到边缘。
Next we present a spot diagram with our original star in the center of the field of view (left), but this time we add a star at the edge of a ? degree diameter field of view (at right). Here we see that the central portion of the view is diffraction limited, but the edge of the field is far from diffraction limited. The coma blows the star image up way beyond the Airy disc, which is the black circle.
下面我们再提供一个点图，左面是我们一开始提供的星点，只在视场的中心位置；右面是我们在视场的边缘增加了一颗星。我们看到，中心部分是衍射极限的作用，但是边缘已经远远超出了衍射极限，慧差使得星点的图像超出了爱里克斑的限制。
http://www.cloudynights.com/images/diffraction/image006.gif
The obvious question becomes: is this diffraction limited or not? The answer is both. This optic is diffraction limited at the center field but not at the edge. This in turn leads up to asking the question of exactly how large is the diffraction limited field of view.
现在问题变成：这个是不是衍射极限的作用？答案是共同的作用。衍射极限只在中心部分起了作用而不是边缘。反过来，又引出一个新的问题，衍射极限的视角到底有多大？
In turn we present another spot diagram with several points in the field of view to demonstrate the portion of the view that is diffraction limited as well as what is not.
我们再提供另一个点图，这个图是在视场内提供了几个光点，来说明刚才所说的共同作用的道理。
http://www.cloudynights.com/images/diffraction/image007.gif
What we see from this spot diagram is that a small central portion of the field of view is diffraction limited (extreme upper left) and then at some point, the diffraction limit is exceeded (lower left field position). From the above example we see that up to about 5 times the diameter of Jupiter is diffraction limited and then the performance starts 揵reaking down?on objects larger than this size.
我们看到，左上图，衍射极限只是集中在中心很小的部分，在用多点的测试中（左下），衍射极限超过了爱里克斑。从上面的例子我们可以看出，其他作用的影响远远大于衍射极限的影响。
Finally, we will see that the diffraction limited area in the field of view is proportional to the focal length. The longer the focal length, the larger the diffraction limited field of view is. Again we present a spot diagram with the same fields as above except that we抳e changed the focal ratio of our sample parabolic mirror to f/8, doubling the focal ratio.
最后我们将看到，衍射极限影响的区域与焦距是成比例的。焦距越长，衍射极限作用的区域越大。我们还是用上图，只是改变焦比，把焦比改成F/8：
http://www.cloudynights.com/images/diffraction/image008.gif
From this spot diagram, we can see that the diffraction limited field of view has grown by almost a factor of 5 times and the diffraction limited field of view is almost ?degree!
从这个点图，我们可以看出，衍射极限作用的区域随之增长了5倍（我理解的是，更多的斑点被限制在爱里克斑内，也即发散出去的斑点面积反而小了）。 谢谢老毛兄::28:: 谢谢,辛苦了.::41:: ::41:: 等着再找一些有关双筒的器材介绍翻译一下::42:: 支持一下，给老兄顶顶！::32:: 了不得，顶起来。支持
[ 本帖最后由 jnhanjun 于 2007-6-24 21:57 编辑 ] 请教:啥样的镜子可以自行校调那么多参数?市售的双筒,好像给用户校调的只有焦点和屈光度吧? 请教一下,折射式望远镜的目镜用凸透镜还是凹透镜. 强烈要求祖祖上双筒的英文资料，想看看关于88、100的资料，谢谢 楼主辛苦, 不顶不行, 呵呵.. 不帮顶不像话啦！::42:: 谢谢谢谢::070821_14.jpg:: 好帖,学习了!!!yct53.gif 把支持原创资料翻译进行到底。 不容易啊，支持，::070821_18.jpg:: 祖祖，不错啊，看来是放毒放到底，连理论科都有了，以后实践加理论，燎咋了！！::42:: 中英文对照，楼主辛苦你了，这片帖子也很好呀，好好学习一下，另外锻炼一下英文，呵呵
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