嘿嘿，拙作，大家多批评：）
Mikhail译
经过数年的发展，Tele Vue 和Al Nagler已经成为高质量广角目镜的同义词了。Type 5系列现在有了20mm Type 5 Nagler来继承这一传统，它是六片结构两英寸接口的目镜，虽然非常贵，但是有着非常可靠地表现。
产品描述：
20mm Type 5 Nagler在目镜中是少数派，尽管它比有老式手榴弹之称的我的Meade 14mm Ultrawide目镜（最初版本）还是稍微小一些。它大约3.5英寸长(8.89cm)，最宽处2.125英寸(5.3975cm)，约471克。目镜是由黑色的金属制的类似火山额锥状顶端部分和装有为了便于握持的带突触的橡胶环的中段组成。中段靠下的部分用大号的亮绿色字体标记有目镜的型号，但是在暗弱的红光下这型号相当难看清楚。尽管如此，但是一旦你拥有了它，你就会明白它在你所拥有的广角目镜中的地位。
目镜有一个可调整的橡胶眼罩，经使用证明这个眼罩能够有效的确定眼睛的位置以防止散射光线的干扰。接目端的镜片大约有18mm，而接物端的巨大镜片有36mm。它们的表面都有着美妙无暇的镀膜。目镜的两端都有盖子，尽管橡胶眼罩一端有时没有必要盖上。坚硬的镀铬的O.D.桶大约1.3英寸长（3.302cm），上半部分上面有宽约10mm的安全凹槽，当然还有标准的48mm滤镜螺纹。
下面是目镜的主要光学指标：
可见观测角度：81度（经测量的）
目镜视场截止直径（不知道术语是什么――译者）：27.4mm
出眼点：10mm（经过测量，但是不精确）
以上和其所标称的82度视场和12mm的出眼点虽然有些出入，但是并不是很大。
具体表现：
我用我的100mmF/6折射镜和Celestron NexStar 9.25 inch f/10施密特·卡塞格林对20mm Type 5 Nagler做了测试。总体来说，我对目镜的表现非常满意，因为它提供的宽阔视场让人感觉相当美妙，犹如太空漫步一般，并且视场中大部分的成像都相当锐利。（和NexStar相配成像视场有40弧分，和100mmF/6折射镜相配成像视场有2.62度。）事实上，星点的成像在由中心开始的75％的视场中都非常锐利，在此之外的区域成像有些偏软但也不会太令人感到失望。仔细观测你会发现一些离轴场曲（pin-cushion distortion―――准确是哪种畸变没有查到――译者）和很少的边缘色差，但是总体来说，这个目镜对大部分像差矫正的很棒。作为一个广角目镜，它的各种场曲变形并不十分严重，那情况大概就像我用我的一个相当不错的20mm Plossl目镜观测时一样。
在用100mm f/6折射镜观测时，在非常靠近视场边缘的亮星的成像会带有很少的散光，亮星的周围还会有一点非常微弱的蓝色色差，当然，这种情况仍然是很轻微的。在一个黑暗的夜晚，金牛座的昴星团和猎户座的腰带地区通过我的UHC滤光镜后在这个20mm目镜里的成像足以使你感到震惊。虽然最终真正成像后的视场并不比我通过Tele Vue 24mm Panoptic目镜观测时的大，但是稍稍大一些的放大倍数和通过具有大视场成像能力的折射镜之后得到的明显大一些的视场使得最终的成像相当令人着迷。
在我的NexStar 9.25英寸施卡镜上目镜仍然表现的十分出色。土星的位置很棒接近天顶处，成像相当的漂亮和锐利。当我将土星移至视场的边缘时，成像开始有些微的变形，但是即使是土星由于在视场中运动而导致成像模糊时，光环和行星的盘面仍然是清晰可辨的。当我以117倍观测满月的中心位置后，我根本没有对月面边缘进行重新对焦的情况下，只有在视场的最边缘处存在极轻微的场曲的痕迹。
这个20mm目镜的反差非常高，甚至当满月已经从东方的低空升起时，我通过20mm Nagler目镜和20mm Plossl目镜都能够看到距离土星光环不远处它的一颗暗弱的卫星Enceladus。虽然确切来说这个20mm Nagler目镜不是一个行星目镜，但是它能够以很出色的表现来应用于这一方面。在观测深空天体时，它仍然有很出色的表现，它分解出了许多疏散星团中暗弱的星点。当我带着眼镜观测时出眼点的位置并不足以使我看到完整的视场，当带着眼镜时，大约仅仅有二分之一到三分之二的视场是可见的。但是我仍然认为它的出眼点是相当合适的，因为当我用裸眼观测时根本不需要将眼睛紧紧贴在眼罩上以获得完整的视场。
我注意到橡胶眼罩没有做成适当的菜豆形状或者其他合适的形状来达到最好的屏蔽光线的作用，除非我将眼罩折叠变形。去除眼罩后，眼睛的位置变得略微更加灵活，尽管我在寻找合适的位置以便看到完整的视场时遇到了一点麻烦。总的说来，在使用20mm Nagler目镜后，我不得不向你们推荐它。但是就我个人而言，我认为我的Tele Vue 24mm Panoptic目镜是一个更超值的选择，在花费更少的情况下，24mm目镜在略低的放大倍数下提供了近似与20mm目镜的最终成像视场大小（当然这仅仅在和1.25英寸接口的目镜比较情况下成立）。
20mm Nagler目镜的优点：
1.矫正很棒的广角视场（81度的犹如太空漫步的感觉）；
2.优秀的反差；
3.锐利的成像。
20mm Nagler目镜的缺点：
1.价格（大约450美元的建议零售价，但是嘿，他可是Tele Vue的产品！）；
2.重量（大约一磅重）―――（1磅约等于453克－译者）；
3.偏小的出眼点。
总结：
Tele Vue 20mm Type 5 Nagler目镜是一个非常可靠的广角目镜，尽管在价格上有些贵，但是这些钱的付出为你带来了非常棒的体验。
以下是原文：
Tele Vue 20mm Type 5 Nagler
by David Knisely 04/08/06
Over the years, Tele Vue and the word ?i>Nagler? have become synonomous with quality wide-field eyepieces. The Type 5 series has continued this tradition with the 20mm Type 5 Nagler, a six element two-inch barrel occular which, although fairly expensive, offers solid performance.
PRODUCT DESCRIPTION:
The 20mm Type 5 Nagler is definitely a handful as far as eyepieces go, although it still was somewhat smaller than the old class hand grenade? my Meade 14mm Ultrawide (original version). It is about 3.5 inches in length with a maximum width of 2.125 inches and a weight of about 471 grams (1.04 lbs). The eyepiece is one of the volcano-top?variety with a hard black metal conical upper section and a cylindrical mid-body which has a rubberized and knurled central band for better gripping. The lower portion of the central body is marked with the eyepiece label in large bright green lettering, but in dim red light, this label is rather hard to read. Still, once you have grabbed this eyepiece, it is pretty clear that you have something of the wide-field variety in your mitts.
The eyepiece has an extendable rubber eyecup which proved useful for eye positioning as well as for blocking stray light. The eye lens is about 18mm in diameter, while the field lens is a whopping 36mm across, and both lenses showed nice unblemished coatings on their surfaces. Both ends of the eyepiece are provided with dust covers, although the one over the rubber eyecup sometimes did not want to stay on. The solid chrome-plated 2 inch O.D. barrel is about 1.3 inches long and has the usual 10mm wide safety groove in its upper half. It is also threaded for standard 48mm filters.
The main eyepiece optical specifications are as follows:
Apparent Field of View: 81 degrees (measured).
Eyepiece Field Stop Diameter: 27.4mm (internal, but consistent with true field measurements and the Field Stop Formula for true field of view).
Eye Relief: 10mm (measured, but approximate only). The Tele Vue figures for apparent field (82 deg.) and eye relief (12mm) differed from what I measured, although the difference wasn't exactly huge.
PERFORMANCE:
I tested the 20mm Type 5 Nagler using my 100mm f/6 refractor and my Celestron NexStar 9.25 inch f/10 Schmidt-Cassegrain telescope. Overall, I was quite pleased with the performance of the eyepiece, as it gave a wonderful wide 搒spacewalk feel while maintaining fairly sharp images across most of the field (40 arc minutes in the NexStar, 2.62 degrees in the 100mm f/6 refractor). In fact, the star images were quite sharp over the central 75% of the field, and the degredation beyond this area was mild and not extremely annoying. Careful study did reveal some pin-cushion distortion and a little lateral color towards the edge of the field stop, but generally, the eyepiece is well-corrected for most aberrations. As wide-field eyepieces go, the amount of distortion is not terribly significant, and it was about as much as I saw when using a decent 20mm Plossl.
In the 100mm f/6 refractor, a small amount of astigmatism was visible with bright star images very close to the edge of the field, along with a faint very short bluish ray extending inward from bright stars, but again, the degradation overall was slight. Indeed, the views in the 20mm of the Pleiades and the entire Sword of Orion using my UHC filter and that eyepiece on one dark night were simply stunning. The actual field of view wasn't all that much larger than I get with my 24mm Panoptic, but the slightly higher power and wider apparent field with this rich-field refractor still made the view quite pleasing.
In my NexStar 9.25 inch SCT, the eyepiece continued to perform very well. Saturn was well placed nearly overhead in the sky, and looked very nice and sharp. When placed near the outer edge of the field of view, it started to get slightly distorted. Still, the rings and the ball of the planet were quite recognizable even as Saturn passed out of the field during slews. There was only the slightest trace of field curvature right at the very outer edge of the field, as at 117x with the full moon centered in the field of view, I did not have to refocus the scope at all for the edge of the lunar disk.
Contrast with the 20mm eyepiece was fairly high, as even with the full moon low in the eastern sky, I noted Saturn's faint moon Enceladus not far from the rings in both the 20mm Nagler and in my 20mm Plossl. Indeed, while I wouldn't exactly call the 20mm a 損lanetary? eyepiece, it can be successfully used in that manner with fairly good results. On Deep-sky objects, the eyepiece continued to do quite well, showing many faint stars in open clusters. The eye relief was not enough for me to use my glasses for seeing the entire field, as with glasses, only between half to two-thirds of the field was visible. However,I would still judge the eye relief as fairly adequate, as I did not have to press in on the eyecup at all to get the full-field with my bare eye.
I noted no kidney beaning and little in the way of excessive blackout effects unless I folded down the rubber eyecup. Without the eyecup in place, eye position did become somewhat more critical, although I still had little trouble getting my eye in the right spot to fully view the field. All in all, I saw nothing in the performance of the 20mm Nagler that would keep me from recommending it. However, for my own use, I felt that my own Tele Vue 24mm Panoptic might have been a somewhat more cost effective option, as for a little less money, it gives about the same true field of view at a slightly lower magnification (and in only an 1.25 inch barrel eyepiece).
WHAT I LIKED ABOUT THE 20MM NAGLER
1.
Wide well-corrected field of view (81 degree 揝pacewalk?.
2.
Good contrast.
3.
Sharp images.
WHAT I DID NOT LIKE ABOUT THE 20MM NAGLER
1.
Cost (around $450 MSRP, but hey, its from Tele Vue!).
2.
Weight (around a full pound).
3.
Modest eye relief.
SUMMARY:
The Tele Vue 20mm Type 5 Nagler eyepiece is a solid premium grade wide-field eyepiece which, although a bit on the pricy side, still gives very good performance for the money.
[ 本帖最后由 Mikhail 于 2007-8-5 17:20 编辑 ] | 
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