翻译CN上一篇关于星特朗EdgeHD 8的文章

  

Home / Celestron EdgeHD 8" SCT by Ed Moreno 03/19/10 | Email Author

  

   我最近刚收到新买的星特朗Edge HD800,虽然之前对它也有了一个初步的印象，但是现在，我有更多的时间来好好审视这台望远镜。我想我将能够分享更多更丰富的观测感受。Edge HD是业余天文级施卡发展史上的一个里程碑。在传统施卡的设计中，主镜和副镜都是球面镜，这种传统设计的施卡会有慧差和像场不平的毛病。必须明白，慧差和场曲是不可避免的，因为这是老设计。这种设计出现时， 1.25英寸的目镜才刚刚开始流行。业余天文级施卡出现之前，许多望远镜还在使用0.95英寸的目镜，一个焦距为15mm或者20mm的尔弗利目镜，其视野只有60到65度。而一个焦距为32mm的PL目镜其视野只有50度。设计者当时就针对1.25英寸的目镜对施卡的设计进行了优化，以使在使用一个1.25英寸的高质量目镜时，望远镜的场曲和慧差都在一个可以接受的范围内，并在口径尽可能大的同时，其重量较轻，价格也适中。虽说什么都在发生变化，但是究竟什么发生了变化呢？我的意思是自星特朗40年前推出我们所熟知并喜爱的C8以来，发什么什么变化。想想这40年来所有在技术上发生的变化。比如计算机、网络、超广角目镜和50Bigabit CCD相机（这个不知道中文相应的词汇）。天啊，世界真的已经变了。
但是，尽管如此，施卡还是以前的施卡。是的，有小的调整和改善，甚至有些特殊的改进，但是大多数设计还是没有变化。只是近年来超广角目镜和高像素相机的出现才慢慢显露出以前设计的不足之处。这是拿一个适用于1.25英寸目镜的望远镜来配2英寸目镜。人们已经开始注意到这一点。美德以ACF走出了第一步，ACF设计是为了改善离轴慧差。现在，慧差在施卡的设计上已经不再是问题，但是这并不是畸变的唯一因素。在施卡里，慧差经常是和场曲同时出现。如果已经有了一个原始的慧差，那么它并不容易被发现。大多数来自慧差的光线都集中在慧差的头部，只有在观察明亮的星星时才有足够的亮度使之出现。但问题是，在施卡的设计中慧差并不是一个纯理论的、独立的畸变。观测者在失焦的部分也能发现慧差，which expanded the head of the comet to the point that the brighter parts of the “V” shaped tail very close to the central part of the comatic blur suddenly became enlarged angularly to the point that it could be easly
resolved（这段确实不知道怎么翻才好）。再加上你的2英寸广角目镜，图像将变得更加丑陋。施卡中一个明亮的星星在Negler 31mmT5的边缘将变成“哈雷彗星”。
美德的ACF解决了这个问题的相当大一部分。在ACF的设计中，望远镜更好的修正了慧差。现在当你把一颗亮星放在31mm的Nagler T5的边缘时，尽管那颗星可能并不完美切合望远镜的曲率，但是没有合焦的星点仍然是圆的，尽管它实际上可能实际上是一个在亮星失焦的点，但是也比拖着慧尾让人赏心悦目。不过，尽管ACF有了进步，但是场曲仍然存在。
公平地说，年纪稍小的人可以通过他们视觉上的调节，尽量减少ACF设计中焦平面弯曲带来的影响。为了补偿目视视野，观察者将望远镜的焦点放在一颗靠近视野边缘以外的星点上。这样，当他们再回过头来看在视野中心的星点时，他们的眼睛将会试着将视场中心解析成完美的合焦。如果曲率是微小的并且观察者的视觉适应力很好，那么一个ACF的望远镜确实能够提供一个非常锐利的视野表现。不过，当使用82度广角目镜，这些目镜与其他窄视场的目镜相比要宽得多，即使是年轻人也不一定能经常能够舒适得调节适应。眼睛调节也会使视觉疲劳，这也会成为长时间观察的瓶颈。    CCD芯片的宽容度比眼睛更小，他不能进行任何的自我适应调节。如果视场是弯曲的，星点离轴越远差，这样的结果是，如果不使用平场镜，将限制CCD芯片的大小。不过，这也是一个很好的方法，对于那些喜爱照相的人来说。此外，平场镜也能用在望远镜上给那些不能完全适应长焦距带来的场曲的目视观察者。
市场的竞争使出现了解决视力下降的人使用广角目镜，和由此带来的场曲和离轴慧差的问题的方法。也同时找到了解决摄影者想要以大口径长焦距来拍摄小目标方法。
星特朗的相应产品是EdgeHD，EdgehdDE的设计中加入了一组修改镜来改正慧差和平场。EdgeHD为那些需要使用超广角目镜的目视着和希望以长焦距大口径来拍摄小目标的摄影者同时提供了足够好的慧差校正和平场，即使是使用最广视场的目镜和最大画幅的CCD相机，这台施卡也能满足。
原文出处：http://www.cloudynights.com/item.php?item_id=2411 第一次翻译文章，其中有很多的专业名词不知道标准应该怎么翻，凭着自己的理解去翻译的，翻得不好见笑了，有错误的地方希望大家能够指正。

附原文如下：
I recently received a new Celestron EdgeHD C8 OTA and have already shared a initial impression, however now that I have had a more opportunity to view with the telescope, I thought I would share a more complete review with more extensive observing experience.

The EdgeHD marks a milestone in commercial SCT development. In the standard SCT design, the primary and secondary mirrors are both spherical. As designed, the standard SCT has inherent off axis coma and a fairly steeply curved focal plane. To put these off axis aberrations in perspective, it is vital to understand that this is a VERY old design. The original formula was decided in an era when 1.25” eyepieces were JUST coming into mainstream. Prior to the first commercial SCTs, many telescopes were still equipped with .965 eyepieces, and a “Wide Field” eyepiece was usually a 15mm or 20mm Erfle with 60 to 65 degrees of apparent field, or a 32mm Plossl with a 50 degree apparent field.

The designer(s) rightfully fixed the design to be optimized for these 1.25” eyepieces and when used with high quality eyepieces of this type, the coma and field curvature of the standard SCT design is a VERY acceptable tradeoff to the goals of having a compact, light, and affordable, but large aperture telescope.

What has changed though is EVERYTHING! I mean it has been 40 YEARS since Celestron first introduced the C8 that we know and love. THINK about ALL of the changes in technology that have occurred in 40 YEARS! How about the PC, the Internet, Antilock brakes, and GIANT FREAKING 2” WIDEFIELD EYEPIECES and 50Bigabit CCD Cameras! Oh my, the world really as changed.

But though that all, the SCT remained exactly what it was. Yes, there were minor tweaks and at even some special improvements, but mostly, the design stayed unchanged. Only in recent years has the combination of ultra-wide eyepieces and giant megapixel cameras exposed the underlying weakness of the old design. It was a 1.25” telescope living in a 2” world. And folk were starting to notice.

Meade stepped up to the bat first with its ACF design. The ACF design addressed the issue of off axis coma. Now the coma in the SCT design is not severe, however it is not presented as the only aberration. In the SCT, the coma is MIXED with field curvature. If you have an raw comatic blur, it is not always easy to see. MOST of the light from a comatic blur is concentrated in the head of the “Comet” and only on bright stars will the comatic tail have enough energy to really become obnoxious. The problem however is that the coma in the SCT design was NOT presented as a pure, stand alone aberration. The comatic blur was also presented to the observer in a defocused blur, which expanded the head of the comet to the point that the brighter parts of the “V” shaped tail very close to the central part of the comatic blur suddenly became enlarged angularly to the point that it could be EASILY RESOLVED. Throw in your Uber-wide field 2” eyepiece and OH, things could get ugly fast. A bright star at the edge of a 31mm T5 Nagler when views though an SCT could be a Comet Halley experience!

The Meade ACF addressed a BIG part of this problem. In the ACF design, the scope was far better corrected for coma. The result was that now, when you put the bright star at the edge of the field of that same 31mm Nagler, even though the star may not be in perfect focus due to the field curvature of the scope, the unfocused blur remains ROUND. Even though it might actually be resolved as a tiny slightly out of focus image on a brighter star, the absence of the comatic tail gives a MUCH more pleasing round blur rather than the spikey blur of a comatic tail! Still, while the ACF did improve the design, the field curvature remained.

To be fair, younger observers could also apply their visual accommodation to minimize the effects of the curved focal plane of the ACF design. To accommodate the field visually, the observer focuses the telescope on a star near the outside edge of the field. Now, when they look back at a star in the center of the field, their eye will attempt to bring the center into perfect focus. If the field curvature is minor and the observers accommodation is good, an ACF scope can indeed provide a very sharp across the field performance. Still, with the proliferation of 82 degree apparent field eyepieces and their much higher magnification as compared to narrower field eyepieces, even young observers were often at the limits of what they could COMFORTABLY accommodate. Accommodation introduces eye strain if you push yourself, and eyestrain is the LAST thing that a dedicated observer wants to deal with in a long observing session.

The CCD chip is even less forgiving. It cannot perform ANY accommodation. If the field is curved, stars become progressively defocused the further you go off axis, and the effect is to limit the size of the CCD chip that can be used unless one chooses to employ field flatteners. And hey, this is a GREAT way to get a flat field for someone that likes to image. In addition, the flattener could be left on the telescope for visual observers that could not fully accommodate the curvature of the native focal length.

The marketing challenge though is finding a solution for the growing number of aging visual observers using high power wide field eyepieces and becoming more aware of both the off axis coma and field curvature, and also finding a solution for the imagers that wanted to image small targets where a long native focal length and serious aperture become your friends.

Celestron’s response was the EdgeHD. The EdgeHD design incorporates a sub aperture field corrector that both reduces coma and flattens the field. With the EdgeHD, both the needs of the high power ultra-wide field eyepiece observers and the imagers wanting to go after small targets with long focal lengths have telescope that presents a field that is well enough corrected for both coma and field curvature that even the most demanding visual observers using the widest field eyepieces and the dedicated imager using the now common larger format CCD cameras to be happy with the SCT.

楼主英语强人

四级还没过的路过

回复 3# qjy08


    二级没过的路过。。。。

非常好的文章！期待英语高手补充能吧翻译补充完整！ 对楼主辛苦劳动表示感谢！HD还没机会看呢。论坛好像有人买了，期待他的实际评测。

高中毕业，没上大学的飘过

回复 5# 长春目视派


    其间有一段描述慧差形成和影响的英文确实不知道怎么翻译才好（就是那段没翻译的英文），哎，技艺疏漏 ，希望大家补充。

说是评测，其是这篇文章多为分析美德的ACF，对星特朗HD的描述实在有限，感觉就像是在星特朗官方网站copy的。我开始认为，这篇文章还没完，但是确实就是这样。不能叫评测。