The Lake County Astronomical Society

Center-Spotting a Secondary

In order to properly collimate a Newtonian telescope, the primary mirror should have a mark of some sort placed at its optical center. The preferred marker is a donut-shaped paper hole reinforcer, which is easy to see and works very well with a laser collimator. But there are a variety of opinions as to whether or not it's worthwhile to center-spot the secondary mirror as well. With a secondary mirror spot, many people wonder - how can you not see a dot that is smack dab in the center of your eyepiece? Here are two differing views.

Geoff Gaherty of the RASC Toronto Center:

You can't see it if it's out of focus. Your eyepiece is focused on the focal plane, several inches outside the tube. The spot will be several inches inside the tube, so there's absolutely no way you'll be able to see it, as a focused image, in the eyepiece.

Should you center spot the secondary? No. First of all, there's absolutely no reason to. Proper collimation involves the edge of the secondary, not its center. Secondly, even if the center spot isn't visible in the eyepiece, it will affect the image, by introducing a small amount of diffraction. The argument about the primary's spot being in the shadow of the secondary doesn't apply here. At the secondary, you've got light coming from all parts of the primary, so it is fully illuminated, and any spot is going to introduce some diffraction and loss of contrast in the final image. Don't do it!

I center spotted the secondary on my Meade 10" f/4.5 Dob, because I hadn't really thought it through, and didn't fully understand proper collimation procedure at the time. It never caused me any detectable problems at the eyepiece, but it didn't help collimation; in fact it added to my confusion while doing collimation, by giving me an extra irrelevant dot to try to center. I'll never do it again.

Astronomy author Phil Harrington:

No question in my mind whatsoever. Offcenter-spotting the secondary mirror is critically important. (By offcenter, I mean taking the offset into account, which is more important in larger scopes than smaller apertures.) Using the template in Kriege and Berry's "The Dobsonian Telescope" book, I offset and spotted the secondary in my 18" f/4.5 and it made a world of difference. I found that the optics, which I had always suspected as having a turned edge, were very good, but that the manufacturer had mislocated the secondary mirror's mount. Once I disassembled and repositioned the spider, and then re-collimated everything, all of a sudden, the images were sharp and clean. I've subsequently had some of the best planetary views ever through that scope, whereas before, everything was very mushy.

So, like I say, there's no question in my mind. And as Geoff said, you're not focusing on the mirror's surface, so you will never see any adverse effect. But again, only make the smallest dot. You certainly don't want to use a paper hole reinforcer, like on the primary!