The tilt of the primary mirror is adjusted with three spring- loaded collimation thumbscrews on the back end of the optical tube (bottom of the primary mirror cell); these are the wide thumbscrews. The other three thin thumbscrews lock the mirror’s position in place; these thin thumbscrews must be loosened before any collimation adjustments can be made to the primary mirror.
To start, turn the thin thumbscrews that lock the primary mir- ror in place counterclockwise a few turns each (Figure 13).
Figure 11. To center the secondary mirror under the focuser, hold the secondary mirror holder in place with your fingers while adjusting the center screw with a Phillips head screwdriver. Do not touch the mirror’s surface.
Now tighten the three small alignment screws equally to secure the secondary mirror in that position.
If the entire primary mirror reflection is not visible in the sec- ondary mirror, as in Figure 10c; you will need to adjust the tilt of the secondary mirror. This is done by alternately loos- ening one of the three alignment set screws while tightening the other two, as depicted in Figure 12. The goal is to center the primary mirror reflection in the secondary mirror, as in Figure 10d. Don’t worry that the reflection of the secondary mirror (the smallest circle, with the collimation cap “dot” in the center) is
Alignment set screws (3)
Figure 13. The three thin thumbscrews that lock the primary mirror in place must first be loosened before any adjustments can be made.
Now try tightening or loosening one of the wide collimation thumbscrews with your fingers (Figure 14). Look into the fo- cuser 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 watching to see if the “dot” of the collimation cap is moving closer or further away from being centered in the “ring” of the primary mirror mark. When you have the dot centered as much as is possible in the ring, your primary mirror is col- limated. The view through the collimation cap should resem- ble Figure 10e.
A simple star test will tell you whether the optics are accu- rately collimated.
Figure 12. Adjust the tilt of the secondary mirror by loosening or tightening the three alignment
Aligning the Primary Mirror
The final adjustment is made to the primary mirror. It will need adjustment if, as in Figure 10d, the secondary mir- ror is centered under the focuser and the reflection of the primary mirror is centered in the secondary mirror, but the small reflection of the secondary mirror (with the “dot” of the collimation cap) is
Figure 14. The tilt of the primary mirror is adjusted by turning one or more of the three wide collimation thumbscrews.
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