Hack 33. 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.
Some astronomers use a felt-tip marker to apply an actual spot to the center of their mirrors. That's a workable solution, but not ideal. It's much better to apply a notebook paper reinforcing ring as a center spot because that little white donut is much more visible when you collimate the scope. The trick is to get the ring applied as closely as possible to the exact center of the mirror. How close to exact center the spot needs to be depends on the focal ratio of the scope. For slow scopes, anything within a couple millimeters is close enough. For f/5 and faster scopes, you need to be extremely precise, within no more than 0.5mm of true center, and 0.25mm is better. To center-spot your mirror, take the following steps:
Figure 3-3. Carefully apply the ring, making sure to center it against the markSince you have the primary mirror out of the scope anyway, check it to see if it needs to be washed [Hack #34]. After washing the mirror, if necessary, reinstall the mirror in its cell [Hack #35] and the assembled mirror cell into the scope. |