光学测试

我爱祖祖

中文翻译请见http://blog.sina.com.cn/u/4c6ee55101000a69，不对之处希望不吝指教。


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.
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.)
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.
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.
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. 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. 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. 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. 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.

skystar

看了。

夜猫子

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。

skystar

祖祖怎么只增加你博客的人气，不为论坛添点呀。