My first Magazine Sky & Telescope - 04.2019

Spider Scratch that. Unless somebody drop-kicked your scope across the Eyepiece observing ﬁeld, your focuser is ﬁne. Focal plane So start by making a peephole about Secondary mirror 1/8″ in diameter that goes in the focuser like an eyepiece. A 35-mm Secondary ﬁlm container with the bottom mirror holder cut off and a hole drilled in the as choosing whether to cut the red cap works great for this if you can wire or the green wire when defus- ﬁnd one. In a pinch, you can tape ing a bomb, but it’s really pretty some paper over the focuser and simple. The hardest part is probably poke a hole in it. The purpose of the learning to say it properly. (It’s coll- Primary mirror cell peephole is to force your eye to stay i-ma-tion, not cul-min-a-tion or Primary mirror centered over the drawtube. coll-im-na-tion.) Check ﬁrst to see if the second- Collimation means lining the ary is too high or too low in the mirrors up in a straight line. The optical tube. It probably won’t be, but it might be if someone primary mirror needs to be pointing more or less straight out ﬁddled with it. You can use the far end of the focuser’s draw- of the tube, the secondary mirror needs to be more or less tube as a reference. The shiny part of the secondary should centered over the primary, and it needs to bounce incoming look centered within that circle. (You might have to put a light down the center of the focuser’s drawtube. I say “more piece of white paper behind the secondary and wave a ﬂash- or less” because the primary’s aim down the tube isn’t all light around to see where the edges of the secondary are.) that critical, and the secondary is seldom truly centered. A Cheshire eyepiece (a collimation tool consisting of a sight In fact, it’s usually deliberately off-center. We’ll get to the tube containing a beveled reﬂective surface and crosshairs) “why” of that in a minute. makes this a little easier by constricting the diameter of the reference circle, but you can do ﬁne with just your centered Begin With the Secondary eyeball. This adjustment doesn’t need to be exact; just get it You start your adjustments at the top of the scope. If you’ve close, both vertically and horizontally. already looked up collimation instructions online, you may The next step is to align the secondary mirror so you have found some whose ﬁrst step is “Square the focuser.” tu ALIGNMENT ANXIETY One of the most frequently noted disadvantages attributed to the Newtonian refl ector is its need for regular collimation of its two mir- rors. But this supposed disadvantage can be reduced to a minor task if approached methodically. Figure 3 Spider vanes and reﬂec- tion of focuser drawtube should be centered. If you look in the open focuser without the peephole, you’ll see your eyeball centered too! Spider vane Center marker Cheshire’s cross-hairs Ignore the extra bulge of the secondary offset Cheshire’s illuminated window reﬂected in primary Cheshire’s cross- hairs reﬂected in primary Cheshire’s peep sight p IN PERFECT TUNE Left: When both mirrors are aligned properly, everything will be centered except possibly the outline of the secondary mirror. Right: A Cheshire eyepiece provides crosshairs and a bright inner reference circle. This view also shows the optics in collimation. sk yandtele scope.com • A PR I L 2 019 69

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