【分享讨论】AT 10" RC 评测

原帖：http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4346679/page/0/view/collapsed/sb/9/o/all/fpart/1

这个帖子比较新，感觉有一定的讨论、参考意义，

关于AstroTech 10" Ritchey-Chretien

所以转过来

偶翻译不好，大家一起帮忙吧

One thing you should know about Ritcheys is that they really need almost perfect collimation to perform at their best. I used a site tube to make sure the primary was aligned with the secondary, then used reflections from the front of the scope to make sure the secondary was aligned with the primary. This was an iterative process since adjusting the secondary would slightly throw off the views through the site tube/primary. It's not very difficult to collimate, though, once you get the hang of it. You just have to nail it if you want nice, tight stellar images. If the primary is off even a little bit you will see astigmatism at higher powers.
关于RC系统，光轴要求几乎要完美。 我先用site tube(？？？)确保主镜、副镜同轴，然后从前面镜子的反射光确认副镜与主镜
也同轴。 这是一个反复的步骤。但掌握了技巧就不难吧。 如果星点要圆润、精准，要调一下调整螺丝。 即使主镜有一点点的
偏离，你将会察觉到很明显的相差。
Based on early results, the scope seems to hold collimation quite well. I have transported it three times in the car now without anything shifting or needing to be tweaked. This scope seems to be right at the carrying capacity of my Mach1 GTO. I actually had to start using my anti-vibration pads for optimum results! I've never needed them before with this mount.
前几次的测试，在车里来回载了3次，问题不大，说明光轴比较稳定。  AT 10RC似乎挺配Mach1 GTO赤道仪 。
现在要开始使用抗震板，来获得最佳效果.

I have been using an AstroDon Moster MOAG for guiding and am very happy with it. Only downside is that I needed a new autoguider since the Orion Starshoot wouldn't reach focus even with the shorter focus tube. I wasn't too upset about that, though, since guiding at this focal length really limits the guide star options, so I probably would have needed a lower noise/higher sensitivity guide camera in any event. The Starlight XPress Lodestar fit the bill perfectly, and it has worked without a glitch.

So... How does it perform? Well, it took some real trial and error to get where I wanted to be. My goal was to get 2 arc second stars (on nights of good seeing) across the entire field of view. That's actually a pretty tall order and required a fair amount of work.

First problem was that I didn't have a way to connect the camera to the off axis guider except using a visual back and compression. That works fine for a lighter camera, but I wanted something threaded so the system would stay square to the optical axis. I noticed in my first light images that one corner was noticeably worse than the others, so I knew I didn't have things quite right. Precise Parts quickly provided the solution. If you haven't looked through their website, it's worth taking a peak. They will custom make whatever connector you need by choosing your components and spacing requirements. First problem solved.

The next issue was that I wanted to use my Astro-Physics 0.75x reducer to get the spatial resolution to more closely match my 5.4 micron pixel size. I got it running O.K. (spaced to provide 0.72 pixels per arc second--exactly what I was looking for), but not everything was perfect. I still seemed to have problems with stars in two corners. What was causing the issue? Ritcheys don't quite have flat fields so I was expecting a little elongation in the corners, but it should have been consistent. Also, even in the center of the field things weren't quite as sharp as I thought they ought to be.

I decided to eliminate a variable and took out the reducer. That improved things a bit, but there were still some problems. I was struggling to get the resolution down below 2 arc seconds even in the center of the field, and the corners still weren't identical.

I decided to see if my AP130 field flattener would remove enough of the field curvature to allow me to see what was really going on in the corners. Only problem was that the field flattener requires a ton of back focus, and I couldn't quite reach focus with it in place and all spacers removed. Oh, well.

I tried using the scope visually so I could do a star test. I hadn't really checked it visually with a high power eyepiece since early in the process. When I finally got a night of reasonably good seeing, it turned out the scope had quite a bit of spherical aberration--I'd estimate 1/4 to 1/3 wave--but I had successfully removed the astigmatism I had seen on my first few nights, and diffraction patterns were concentric. I did a bit of reading (Herr Rohr's website) and found that the spacing between primaries and secondaries may not be optimized to the design specification! In fact, I found that I could greatly reduce the spherical aberration by bringing the secondary a bit closer to the primary. FYI, this practice is explicitly discouraged in the instruction manual since it should be right from the factory, so you could easily be making things worse rather than better. Plus, if you aren't careful you can drop your secondary onto the primary since the secondary mounting is spring loaded. I managed to get my spherical aberration down to perhaps 1/6 wave or maybe even 1/8 wave. Obviously, I don't have an optical bench so these are my own estimates based on views of diffraction patterns at high power (330x). In any event, there was a noticeable improvement. There was an added benefit as well--I now had enough back focus that I could try out the field flattener again.

Still not there. Stars in the center of the field were now consistently around 2 arc seconds on nights of above average seeing, but the corners still varied. I started to look for a mechanical issue again... I found it two days ago. The mounting plate on my filter wheel covers a screw on one edge that is recessed in a countersunk hole. It turns out the countersink isn't quite deep enough, and the screw was slightly "cocking" the mounting plate. It was a very slight difference--no more than one or two tenths of a millimeter--but it was enough to keep the camera from being square to the optical train. I had never noticed an issue when using my 5" refractor, but at these longer focal lengths/resolutions the tolerances get ridiculously small.

So I tried it out again last night. With the focal reducer (since that gives optimal spatial resolution). The seeing was quite good--I'd say 7/10. Brighter stars still showed some flickering, but nothing too serious. The 'e' and 'f' stars were easily visible in the Trapezium at roughly 100x.

Still no joy. In one corner, stars looked almost like doubles--serious astigmatism. I took out the focal reducer and put in the field flattener. Bingo! Finally, everything looked perfect. I had my first really successful night (meaning higher resolution than I could have gotten out of my refractor--I had already managed a couple of images I liked, but they weren't quite showing the resolution I thought the scope was capable of).

Here is a three minute guided image of M37--the star field I was using to measure FWHM values since it was well placed after dark... 这是M37 三分钟导星结果，用来测试FWHM值

And here is a FWHM measurement from the center of the field. Nothing cherry picked--the 1.7 arc second value was typical of the stars in the center of the frame. 这是中间星像的FWHM值，没有挑选最佳的。 中间星点1.7角秒算正常。

And here are measurements from all four corners. Still under 2 arc seconds! Wahoo!
这是像场四个角的测试值，全都低于2角秒，不错。

Here is my first light image of the Horsehead--I believe raw frames on this showed 2.3 arc seconds in the center of the field and around 2.7 in the corners. Not bad, but, frankly, no better than I could get out of my 5" refractor. This was taken the middle of December.  10年12月拍的马头星云，作为开光照。   原始RAW图像中间区域应该有2.3角秒，周围差不多2.7角秒。 不错，但坦白说，效果没有5寸折射的好。

A quick pic of Orion I took just before New Year's... Still nothing great in terms of resolution
猎户座m42星云，年前草草拍摄。。。始终解析度不理想。

And, finally, the results from last night. This is the first image I have captured with this scope that I simply couldn't have gotten with the 5" refractor!
最后的测试，2011年1月27日拍的M81。这是第一幅我用5寸折射无法轻易拍摄到的图片。
Depth is still pretty mediocre. I was imaging from my back yard in Oakland, after all, and I've got a NELM of 4.0 on a dark, transparent night like last night. Still, the resolution was quite good.
深度还是一般，再自家后院（奥克兰）拍的，NELM of 4.0 (极限星等4.0 ???? )解析度很不错

有太多东西不懂，赫赫，高手快来~

有太多东西不懂，赫赫，高手快来~
steveyzh 发表于 2011-3-26 07:57 PM

                               
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另： 我名字里有一个“赫”字，吓我一跳

大段的英文，有种亲切的感觉，就像高中时做阅读理解。

解析度的确比不上5寸折射，视角又小，不过是真便宜