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[经验交流] 巴洛激光法校准牛反主镜光轴

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发表于 2015-9-15 21:05:06 | 显示全部楼层 |阅读模式
本帖最后由 grass_root 于 2015-9-15 21:05 编辑

    牛反光轴校准,尤其是短焦牛反的校准历来是令人头疼的事情。国内外的论坛有不少研究和讨论。国内论坛讨论的最多是十字目镜、激光器、星点三种校准方法。十字目镜法,我总觉得不够方便,而且无法夜间开展;激光器校准则广受诟病,认为其自身的准直度太差或不稳定,无法可靠的校准光轴;星点法最为高大上,是公认最准确的方法,但需要较多技巧,而且也要在目镜和主镜调节螺丝间往返,多有不便。国外论坛比国内多了两种方法,一是Autocollimator;另外一种这是巴洛激光(Barlowed laser)。Autocollimator貌似cn上的Jason D提出并深入的讨论了原理
http://www.cloudynights.com/topi ... -tool/#entry2590708
http://www.cloudynights.com/topi ... ments/#entry3189737;)
并且有成熟产品出售(http://www.catseyecollimation.com/)。这种方法的精度和可靠性在CN上被广泛认可,被很多人认为是最优方法,但产品的价格太高。巴洛激光法由Nils Olof Carlin提出,相关文章在S&T上发表,同时他的个人网站也对这个方法以及牛反的校准的各种问题进行了深入的阐述(http://w1.411.telia.com/~u41105032/)。巴洛激光法的精度和稳定性都不错,而且简便易行,但国内没看到有人讨论。这种方法只是在传统激光校准主镜光轴时,加入了巴洛(如图1)。调整的时候使得返回的光斑形成同心圆即可(如图2)。

equipment1.jpg
图1 巴洛+激光器 我把paracorr 当成巴洛使用了
B.jpg
图2 左上角是激光点照到主镜中心标记,大图是返回激光器斜面的同心圆
另外,AP130EDF 在 neaf 2015 的帖子里(http://www.astroview.com.cn/foru ... p;extra=&page=1),拍了两张绿色激光器的效果,比我拍的清晰多了。巴洛激光校准起来非常方便,你可以在主镜后一边拧螺丝,一边观察返回光斑的模式,夜间操作也极为便利。更重要的是,巴洛激光的校准精度受激光器自身准直度的影响极小,几乎可以忽略不计。我跟Nils Olof Carlin本人请教过这个问题,他给出了计算的过程。


 楼主| 发表于 2015-9-15 21:07:00 | 显示全部楼层
Nils Olof Carlin给我邮件回复了 采用巴洛激光法时,激光器自身误差与校准误差的关系讨论。
Let us look at a numerical example, to get an idea of what effects to expect: Take a telescope with 1600 mm focal length, and a 2x Barlow with the focal length of the (thin) lens = 80 mm. Also, the edge of the focuser is at the focal plane - as it often is during viewing. For a 2x Barlow, the length of the Barlow tube from lens to outer edge equals its focal length (not true for 3x etc, and not exact considering the thickness of the negative lens, but a reasonable approximation), and it is inserted with the original focal plane midway between the opening and lens (and target for the barlowed laser). Thus the Barlow is inserted with its outer opening 1/2 focal length or 40 mm outside the original focal plane, and the lens element and the target 40 mm inside.
What I have considered is that the laser is exactly aligned, but there is a misalignment, tilt, of the focuser axis. This means that the spot will miss the center mark on the primary. The light leaving the barlow appears as coming from the focal point of the lens, in this case near the outer opening of the Barlow and exactly centered on its axis. Assume it is tilted to the left around its middle: the virtual point source is moved to the left, and the reflected shadow of the mirror spot is thus moved to the right. But the target is also moved as much to the right, so the shadow and target are still perfectly aligned and the primary's optical axis is centered in the focuser at the focal plane - despite the tilt. So, in practice, a small tilt of the focuser axis is rather well balanced out.

But I realise the problem you pose is a bit different, and I had not considered it before: the laser itself is misaligned, but you collimate to make the collimations of the focuser axis, and also the primary, appear to be exact.

We use the constants as above, but assume a misalignment of 1/100 radian (0.57 deg). If the collimation is right, and you insert the laser, it will then point 1600/100=16 mm off the primary's center. But you don't know that it is the laser that is wrong, so you adjust the focuser axis (via the secondary mirror) to center the spot. Now use the Barlow, and insert the laser in the same rotation: the virtual point source will be 80/100 mm=0.8 mm from the optical axis (40 mm outside the focal plane) while the target is centered (40 mm inside the focal plane). After collimation when the target and shadows coincide, you get half the miscollimation =0.4 mm at the focal plane. I hope this is clear, the central ray passes through the center of the lens and is tilted 1/100. In practice, you would not accept such a poorly aligned laser.
But what if you now turn the laser 180 deg in the Barlow? Then the virtual point source is moved 2*0.8=1.6 mm toward the opposite side, and the return shadow is moved also as much away from the center of the target but in the opposite direction. So if you re-collimate and center the shadow, the optical axis will move 0.8 mm and end up 0.4 mm off at the focal plane, as in the first example but in the opposite direction.

Thus, if I am right here (I hope!), a misaligned laser will indeed cause an error in the collimation of the primary, even if the error is small, very much smaller than that of the laser when pointed at the primary. You can try rotate the laser in the focuser and see if it makes a circle on the primary - in my example a 32 mm circle, this should be very obvious and you should re-align it. Also, if you have the Barlow fixed but rotate the laser only, and you find the shadow moves against the target, you should suspect a misaligned laser.
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 楼主| 发表于 2015-9-15 21:10:13 | 显示全部楼层
过程没怎么看明白,结论大致清楚,就是这个误差取决于两个因素,一个是激光器歪斜的角度,另外一个是激光器光源到巴洛镜片的距离,因为因素2非常小,所有整体的误差也很小(哪怕歪斜角度很大也没关系)。
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发表于 2015-9-16 14:21:25 | 显示全部楼层
学习!有时间试试
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发表于 2015-9-16 17:57:44 来自手机 | 显示全部楼层
Autocollimator似乎有点问题,我有一个美国朋友送的。送给我的原因是他觉得不准。所以我也没用过……最终还是用星点调的。摄影的话,还得星点校准,而且最终精调必须是实拍星点,人工的都不行,其他的都不准
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发表于 2015-9-30 12:48:23 | 显示全部楼层
同意星点调的最准,高倍目视的话 还是星点调一下 不要用太亮的星 反而没那么准了
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发表于 2015-10-10 02:00:52 | 显示全部楼层
自己把激光校准器先校准好不是更省事?
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发表于 2017-11-28 19:26:11 | 显示全部楼层
请教一下楼主,巴洛的倍数对精度的影响是如何的?
我看上面的文章,好像巴洛的倍数没有任何影响?
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