星点测试技巧

中文翻译请见http://blog.sina.com.cn/u/4c6ee55101000a6a，不对之处希望不吝指教。

突然发现，这一片文章好像是TESTING OPTICS的续文阿，大家应该再关注一下。
Star Testing Tips
Cloudy Nights ask me to explain what the spherical aberrations undercorrected and overcorrected mean and how to tell what you have as you almost always have at least one. Even high end APOs have this condition most of the time but to what degree and over or under makes a big difference.
论坛让我再解释一下球差较正不够和过度是什么意思，以及如何辨别。甚至是高端的APO，大多数情况下也存在这样的问题，只是程度不同而已。
First lets deal with astigmatism which can be seen very easy unless it is so slight then it will not be of importance. To test for astigmatism which is the same as the human eye problem where the focus is stretched vertical on one side of focus and horizontal on the other side of focus. When testing for astigmatism DO NOT use a wide angle EP as it will almost always show astigmatism, which may not be there. Use a moderate to high power EP around 150X for a smaller scope 80 to 90mm and 250X or near that for a 5" or 6" scope. and I am referring to refractors but the same rule for SCTs and Maks and DOBs.
首先让我们看像散，像散很容易被看到，除非特别轻微，那么对像质也就没有太大的影响了。测试像散，看是否焦点的一侧有垂直拉伸，而另一侧有水平拉伸。测试像散不能用广角目镜，因为广角目镜本身也有色散。如果测试小于80-90MM的镜子，你应该选择150倍的目镜，如果是5-6寸的镜子，就选择250倍的目镜。这对折射、折返、DOB都适用。
Bring a star or an artificial star to sharp focus. Seeing must be good. Then defocus inside and outside and look for the star to pinch like an egg. Not round and on one side it will be elongated vertical and the other side horizontal then as you go more defocus it will snap to a perfect symmetrical circle. When it is not round but elongated that is astigmatism. If you have just a tad it will go from pinpoint focus to an egg shape just for a very small touch of focus then go round. That is a small amount of astigmatism, which will not be a problem for good views. But if it stays elongated for significant amount of defocus then you have allot of astigmatism and the scope will loose performance more so on high power viewing. The scope is a good low power deep sky and that's all。
找一颗星，或者制造一个人造星点，要比较尖锐的。选择一个好的天气。焦内焦外进行散焦，看衍射圈是否变成蛋型。如果在一面是垂直拉伸的，在另一面水平方向，如果你继续散焦，会变成对称的圆环。只是因为拉伸而造成衍射圈不圆，这就是像散造成的。如果从蛋型到圆形的变化只是移动调焦极小的一部分，那么像散的程度就很低，对像质的影响不会很大；如果需要移动较大的距离，那么说明像散比较严重，高倍下的像会比较松散。那么这个望远镜只适合低倍深空，仅此而已。
Now for the spherical errors there are two of importance they are called the LSA 3rd and the HSA 5th, which is over and under corrected. When you defocus outside if the outer ring is brighter and sharper than the inside focus you are overcorrected and a ronchi test will confirm this. If the Inside focus is brighter and sharper on the outer ring then you are undercorrected. If both sides look the same then you have good optics with good correction. NOTE there can be misleading problems that will give false test as poor collimation and cheap EPs or focuser out of alignment. The test are a subjective test to get a good estimation of the optics and are not 100% as there are many factors that can make the test weak or outright wrong. Good seeing and collimation are the most important also using the Ronchi test will show more precise figures. But this simple eyeball test works pretty well.
再看球差。有两种很重要的球差，是因为调教过度或者不够造成的。焦外散焦，如果外圈环比内圈环明亮并且锐利，说明调教过度了，如果内圈比外圈明亮和锐利，则是调教不足。如果内外一样，那么说明光学调教已经到位了。需要注意的是，有一些因素，如光轴偏移严重，使用低端目镜，或者调焦座不同轴等原因，都会产生类似球差的现象，对调教起到误导。测试一般主观的成份大一些。好的天气和较准在测试中非常重要。
As for turned edge that is more problematic with mirrors and not so much with refractors but refractors can have it too. A turned edge is when the mirror or lens is not ground straight at the edge following the curve of the mirror or lens. In other words the edge is taking a dive down or up not crisp like a knife-edge. This will look like a frosty or degrading sharpness at the edge of field where the light is defused off axis to the correction of the lens or mirror resulting in poor performance. A small mask of black paper or cardboard about 1/8 inch around the mirror edge will fix this but you loose a tad of aperture but performance increases. DO NOT GLUE OR STICK mask to mirror use tape on outer side of glass to affix the mask and if there is no improvement remove it. If you put tape or glue on the surface you will destroy the coating or make it a mess. Never touch the coating on a mirror or lens if possible. The salt and sweat or oil from your skin will cause premature coating degrading.
边缘弯曲对反射镜比较常见，折射很少出现，但是也有。边缘弯曲就是镜面或者镜头不再一个水平的平面上，边缘有卷曲的现象，换句话说，边缘不像刀口那样锋利，因此导致边缘的画面不清晰从而影响画质。用一个大约1/8英寸小纸版把反射镜的边缘蒙上，可以提高像质，但是会损失一点口径。不要用胶带把纸板粘在镜子上固定，这样会很难拿下来，而且还会损伤镀模。
Back to spherical errors lower errors are worse than higher errors Lower being the 3rd and higher being the 5th .Hence the name LSA and HSA Now LSA will cause a washing out of detail and a blurring of image depending on how much error is present.HSA is not as bad and not so destructive to image unless very high HSA error.
回到球差的问题。低程度的球差比高程度的球差更容易影响画质。低程度球差将引起细节损失，图像模糊，高程度的影响则小得多，除非太高程度的球差了。
The last error of mention is that horrible no one one like to see is ZONES. Zones are the worst in my opinion unless very slight. Zones looks like colored rings or blobs where there shouldn't be anything but nice even pattern out to the edge except for the dot in center or secondary shadow on SCTs MAKs and DOBs. Looks like something on the lens or chunk of dirt on the EP. If it's bad the scope is not of much use except low power. Most of the errors mentioned are very common but the big question is how much is there. A tad or loads. Both of my scopes the 120mm and 150mm Refractors are undercorrected LSA but just a tad on the 120 and a tad of a tad on the 150mm which has very good correction for a SYNTA only about 20% are this well corrected and my 9.25 is so good I cant tell without a Ronchi test or interferometer.
ZONE (如何译？)是大家都不愿意看到的令人讨厌的像差。这种像差使我印象中最严重的一种。这种像差看起来像一个彩色的环或是斑点，看起来好像是镜头或是目镜上的灰尘。如果镜子本身品质较差，那么就只能用在低倍上面。所有提及的像差都非常普遍存在，但是最大的问题是到底到什么程度。



[ 本帖最后由 我爱祖祖 于 2007-6-20 15:05 编辑 ]

照片拍的不错昂，呵呵

原帖由 fkm2000 于 2007-6-19 15:40 发表

                               
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没看到翻译啊

不好意思，一直没有时间，别着急，我尽快昂

原帖由 赵垒 于 2007-6-19 15:54 发表

                               
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照片拍的不错昂，呵呵

嘿嘿，你哥们给拍的，已经到了直接出片的水平。

Star test （星点测试）是一个非常复杂的问题， 我每次除去观测， 遇到有经验的人， 都会向别人请教。 国外有一本书不错， 是专门讲如果做star test的：

http://www.company7.com/books/products/testing.html

有兴趣的可以去看看。

呵呵，那本书我早就很想买了，只是没有渠道。老兄那里方便帮着弄一本么？

不错！有价值的好文章！

嗯，很实用的东西！楼主把好东西都统统发上来吧，分享

原帖由 Alone狼 于 2007-6-22 08:20 发表

                               
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不错！有价值的好文章！

老兄怎么最近失踪了呢？几天不见，有些想念

顶下好文 收藏了

不错,这个发表的很好哦