Hack33.Center-Spot Your Mirror


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.

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.


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:

  1. Remove the mirror cell from your telescope and the primary mirror from its cell.

  2. Place the mirror flat, shiny side down, on a sheet of paper large enough to extend beyond the edges of the mirror

  3. Use a sharp pencil to trace all the way around the circumference of the mirror, as shown in Figure 3-1. Make sure that the line you trace exactly matches the edge of the mirror.

  4. Remove the mirror carefully, and place it face up in a safe location.

  5. Carefully cut out the circle you traced, keeping exactly on the pencil line.

  6. Fold the paper circle once across its diameter to make a half-circle and then a second time to make a quarter-circle that looks like an ice cream cone.

  7. Cut a tiny piece from the pointy end of the ice cream cone, as shown in Figure 3-2, and then unfold the paper circle.

    Figure 3-1. Trace the circumference of the mirror onto the paper


    Figure 3-2. Cut a center hole just large enough to accept the tip of the pen


  8. With the mirror face-up, carefully lay the paper circle on top of it, matching the edge of the circle exactly to the edge of the mirror.

  9. Hold a felt-tip pen exactly vertical. Insert the tip through the hole you cut in the paper circle, and make a mark on the surface of the mirror.

  10. Remove the paper circle, and apply a notebook reinforcing ring to the mirror, as shown in Figure 3-3. Make sure that the center of its hole is precisely centered on the spot you made with the felt-tip pen.

Figure 3-3. Carefully apply the ring, making sure to center it against the mark


Since 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.



    Astronomy Hacks
    Astronomy Hacks: Tips and Tools for Observing the Night Sky
    ISBN: 0596100604
    EAN: 2147483647
    Year: 2005
    Pages: 112

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