Aligning the Primary Mirror

Figure 9. To center the secondary mirror under the focuser, hold the secondary mirror holder in place with one hand while adjusting the center screw with a Phillips screwdriver. Do not touch the mirror's surface

Figure 10. Adjust the tilt of the secondary mirror by loosening or tightening the three alignment set screws with a 2mm Allen wrench.

If the entire primary mirror reflection is not visible in the sec- ondary mirror, as in Figure 8c, you will need to adjust the tilt of the secondary mirror. This is done by alternately loosening one of the three alignment setscrews while tightening the other two, as depicted in Figure 10. The goal is to center the primary mirror reflection in the secondary mirror, as in Figure 8d. Don’t worry that the reflection of the secondary mirror (the smallest circle, with the collimation cap “dot” in the cen- ter) is off-center. You will fix that in the next step.

The final adjustment is made to the primary mirror. It will need adjustment if, as in Figure 8d, the secondary mirror is centered under the focuser and the reflection of the pri- mary mirror is centered in the secondary mirror, but the small reflection of the secondary mirror (with the “dot” of the col- limation cap) is off-center.

To access the primary mirror collimation screws, remove the cover plate on the rear end of the optical by unthreading the three Phillips-head screws with a screwdriver. The tilt of the mirror is adjusted with three pairs of collimation screws (Figure 11). The collimation screws can be turned with a Phillips head screwdriver and a 2.5mm Allen wrench.

Each pair of collimation screws work together to adjust the tilt of the primary mirror. The set screw pushes the mirror for- ward while the Phillips head screw pulls the mirror cell back. One must be loosened and the other tightened by the same amount in order to adjust the tilt. Try tightening and loosening one of the pairs of collimation screws one turn. Look into the focuser and see if the secondary mirror reflection has moved closer to the center of the primary. You can tell this easily with the collimation cap and mirror center mark by simply watch- ing to see of the “dot” of the collimation cap is moving closer or farther away from the ring on the center of the primary mirror. Repeat this process on the other two pairs of collima- tion screws, if necessary. It will take a little trial and error to get a feel for how to adjust the mirror to center the “dot” of the collimation cap in the ring of the mirror mark.

Set screw

Phillips-head screw

Figure 11. The back end of the optical tube (bottom of the primary mirror cell). The three pairs of set screws and Phillips-head screws adjust the tilt of the primary mirror.

When you have the dot centered as much as possible in the ring, your primary mirror is collimated. The view through the collimation cap should resemble Figure 8e. Make sure all the collimation screws are tight (but do not overtighten), to secure the mirror tilt.

A simple star test will tell you whether the optics are accu- rately collimated.

Star-Testing the Telescope

When it is dark, point the telescope at a bright star and accu- rately center it in the eyepiece’s field of view. Slowly de-focus the image with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle (Figure 12). If the image is unsymmetrical, the scope is out of collimation. The dark shadow cast by the secondary mirror should appear in the very center of the out-of-focus circle, like the hole in a donut. If the “hole” appears off-center, the telescope is out of collimation.

If you try the star test and the bright star you have select- ed is not accurately centered in the eyepiece, the optics will always appear out of collimation, even though they