Collimating a Newtonian

Collimating a Newtonian is more complicated, but many times, you can skip the complex part and treat it like a Schmidt-Cassegrain - just adjust the three screws on the back of the main mirror until out-of-focus star images look like concentric doughnuts. Even if you don't get perfect collimation, you can often get a substantial improvement.

Complete collimation of a Newtonian is done in the daytime rather than on a star at night. Aim the telescope at a bright surface, such as the daytime sky, and look into the eyepiece tube. To ensure that your eye is centered, punch a hole in the bottom of a 35-mm film can and use it in place of an eyepiece.

Figure 5.11 shows what you should see. You're looking straight at the secondary mirror, which is not necessarily concentric with the eyepiece tube (Figure 5.12 shows why).

In the secondary mirror you see a reflection of the primary mirror. Your first job is to adjust the secondary, if it's adjustable, so that the reflection of the primary is centered in the eyepiece tube. Then adjust the primary mirror until the central obstruction is centered in it. There - your telescope is collimated. You can and should still check it on a star and make fine adjustments to the primary mirror if any small errors remain.

All this assumes your telescope was fairly close to correct collimation in the first place. If you're building a Newtonian or repairing one that has been assembled incorrectly, you may need more guidance. The definitive handbook on collimating Newtonians is New Perspectives on Newtonian Collimation, by

Telescope aimed at daytime sky or bright wall

Telescope aimed at daytime sky or bright wall

To main mirror

Figure 5.11. Looking down the eyepiece tube of a Newtonian, everything should be concentric except the secondary mirror. Often, the main mirror mount is all you need to adjust.

To main mirror

Figure 5.11. Looking down the eyepiece tube of a Newtonian, everything should be concentric except the secondary mirror. Often, the main mirror mount is all you need to adjust.

Second mirra

Second mirra

This end of secondary is closer to main mirror,

Main mirror

This end of secondary is closer to main mirror,

Main mirror so must intercept rays farther from central axis

Figure 5.12. Why the secondary mirror is not concentric with the eyepiece tube. In high- f -ratio telescopes (f /8 and up) it may be concentric after all.

Vic Menard and Tippy D'Auria (privately printed, but available from Sky Publishing Co., Cambridge, Massachusetts 01238, U.S.A., http://www.skypub.com). This book also discusses Schmidt-Cassegrains, conventional Cassegrains, and mirror star diagonals.

If you collimate a Newtonian frequently, you may want to use one of the laser collimators that have recently appeared on the market. The idea is that a laser beam, aimed at the primary mirror from the eyepiece position, will bounce back along its path if the collimation is correct. This can be a real time-saver.

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