A laser collimator is often used for Step 3, by centering the returning beam on the laser's faceplate. However, this method has problems that belie the laser's presumed accuracy. Why? Suppose that in Step 2 the laser beam has missed the true center of the primary mirror by a small distance, for example, 2 mm. Even if the primary mirror happens to be exactly collimated (its center precisely aligned with the center of the eyepiece), the returning laser beam will be parallel to the main mirror's axis but will miss the center of the laser faceplate by 2 mm. If you then tilt the main mirror to center the beam returning to the laser, the collimation will be 1 mm off! Unintentionally, you will have caused a miscollimation great enough to affect the performance of a short-focal-length telescope. This extreme sensitivity to a small and otherwise unimportant error in Step 2 is the Achilles' heel of the laser collimator. So even if you use one for rough alignment in Step 3, it is better to use a Cheshire eyepiece for the final adjustment.
激光器也常在第3步中使用,把激光的反射光调整到中央。但是,该方法与激光的精确性之间有差距。为什么?假定第2步中激光错过了主镜的中央点,误差只有一点点,例如2毫米。即使主镜完全调整好了,返回的激光束将平行于主镜的光轴,但将错过激光器的面板2毫米。如果你又调整了主镜,把光束居中的话,调整后的镜子将有1mm的偏差。无意之中,你已经把短焦比的镜子的性能大打折扣了。这是在第2步中使用激光器的局限性。所以即使你第3步调整的粗一些,我也推荐你使用Cheshire 目镜做最后的调节。
Star-Testing Your Collimation 星点实验
Once your telescope has cooled down and is well collimated, it should be ready to perform at its best. At high magnification (25× to 50× per inch of aperture, or 1× to 2× per mm of aperture) and in good seeing conditions, stars at focus should appear in the eyepiece as tight, symmetric diffraction disks. However, if stars at the center of the field show the telltale asymmetry of coma, double-check your collimation with the Cheshire eyepiece. If the center spot still looks centered, then it isn't located at the primary's true optical center.
一旦你的望远镜已经冷却,并调整好,就可以观测了。在高倍下(25× to 50× per inch of aperture, or 1× to 2× per mm of aperture),大气宁静,星光汇聚应该呈现出致密的,对称的光衍射环。然而,如果星光呈现拖尾巴的慧差,一定要用Cheshire目镜检查。如果中央点还是居中的,说明它并不是真正的主镜的中央点。
If this is the case with your mirror's center spot, ignore it for now and try tweaking the primary's collimation, in small steps, until you have centered the best image in the field of view. (This method was described in detail in the June 2001 issue of Sky & Telescope, page 125, and is illustrated below.) The Cheshire will now indicate the location of the primary mirror's true optical center. If necessary, move the spot to the correct position or put another, larger piece of tape on top of it.
如果是镜子的中点出了问题,眼下不必管它,继续调整主镜,经过几个步骤,直到你看到最佳的图象。(该法在2001年的skyandtelescope中,125页有介绍。)使用Cheshire目镜将提示出此刻主镜的真正光学中央点。如果需要的话,把黑胶布的位置贴过去。
If you know that your primary mirror spot is okay (and in most cases it will be, if carefully centered), there is no need to routinely fine-tune your collimation with a star test — the Cheshire eyepiece is not only easier to use, but it is more accurate if the seeing is less than ideal, which it is most nights.
如果你知道你的主镜点是调好的,就不必经常做星点实验了——Cheshire目镜不仅容易使用,而且更精确。
Now your telescope is in perfect tune, and the improvement in performance should be obvious. If not, try to deliberately miscollimate the primary, and see what it does to a high-magnification view of a planet. After this demonstration, you'll never let your scope go out of collimation again.
现在你的望远镜就调好了,性能的改进应该是很明显的。如果没有达到的目的,仔细地调整主镜,看行星的表面在高倍下有什么变化。经过讲解,你已经掌握调整过程了吧。 |