Astronomy Hacks

Center-Spot Your Mirror

Collimate faster and more precisely.

All telescopes must be collimated if they are to provide the best possible image quality. Collimation consists of aligning the lenses and mirrors in a telescope to share a common optical axis. Because of their physical design, Newtonian reflectors, including Dobsonians, require more frequent collimation than other telescope designs. (Fortunately, most Newts are pretty easy to collimate.)

The importance of accurate collimation varies with the focal ratio of the scope. Slow scopes, those with focal ratios of f/8 [Hack #9] or higher, provide reasonably good image quality even if they are only roughly collimated. An f/6 scope, such as a typical 8” Dob, must be collimated with moderate precision to provide good images. An f/5 scope, such as a typical 10” or 12” Dob, must be collimated with extreme precision.

In order to collimate a Newtonian precisely with standard, inexpensive collimation tools [Hack #37], it’s necessary that the center of the mirror have a visible mark, called a center spot. Unfortunately, many scope manufacturers don’t center-spot their mirrors at the factory. If you have such a mirror, you’ll have to apply a center spot yourself.

Tip

Don’t worry about the center spot affecting the image. The center of the primary mirror, where the center spot is applied, is shaded by the secondary mirror. The shaded center of the primary mirror never “sees” star light, so any marking there has no effect on image quality.