Orion 6/6I instruction manual Adjusting the Secondary Mirror’s Tilt, Aligning the Primary Mirror

Figure 31. The collimation and mirror locking thumbscrews

in the secondary mirror holder about a quarter of a turn only (counterclockwise). That should be enough to free up the sec- ondary mirror to rotate slightly in either direction. Look into the collimation cap and rotate the mirror slightly in each direction to get an idea of how it affects the view of the secondary mir- ror. Now rotate the mirror as needed so that it precisely faces the focuser. Hold the mirror holder stationary in that position while turning the center screw clockwise until it is just tight (do not over-tighten). Sometimes the mirror may rotate slightly when tightening the screw, so keep at it until the mirror faces the focuser squarely and is secured in place.

Adjusting the Secondary Mirror’s Tilt

Finally, the tilt of the secondary mirror may occasionally require adjustment. If the entire primary mirror reflection is not visible in the secondary mirror when using the collimation cap, as in Figure 27c, you will need to adjust the tilt of the second- ary mirror. Using a 2mm hex key, first loosen one of the three alignment set screws by, say, one full turn, and then tighten the other two to take up the slack. Do not loosen the center screw during this process. The goal is to center the primary mirror reflection in the secondary mirror, as in Figure 27d. When it is centered, you’re done adjusting the secondary mirror. Don’t worry that the reflection of the secondary mirror (the dark cir- cle with the four spider vanes adjoining it) is off-center, since that adjustment is made when aligning the primary mirror in the next step.

Aligning the Primary Mirror

The final collimation adjustment is made to the primary mir- ror. It will need adjustment if, as in Figure 27d, the second- ary mirror is centered under the focuser and the reflection of the primary mirror is centered in the secondary mirror, but the reflection of the secondary mirror (dark circle containing the light reflective surface and center black “dot” of the collimation cap) is off-center.

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 (Figure 31). The other three thin thumbscrews

Figure 32. A star test will determine if the telescope’s optics are properly collimated. An unfocused view of a bright star through the eyepiece should appear as illustrated on the right if optics are perfectly collimated. If the circle is unsymmetrical, as illustrated on the left, the scope needs collimation.

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 mirror in place counterclockwise a few turns each.

Now, while looking into the focuser through the collimation cap, turn one of the wide collimation thumbscrews a half turn or so in either direction and see if the secondary mirror reflec- tion moves closer to the center of the primary. That is, does the “dot” of the collimation cap appear to move closer to the ring on the center of the primary mirror? If it does, great, keep going until you get it as close as you can. If it doesn’t, try turning the collimation thumbscrew in the opposite direction. If turning the one collimation thumbscrew does not seem to bring the dot closer to the ring, try using one of the other collimation thumbscrews. It will take some trial-and-error using all three collimation thumbscrews to properly align the primary mirror. Over time you will get the feel for which collimation screws to turn to move the image in a given direction.

When you have the dot centered as much as possible in the ring, your primary mirror is collimated. Now lightly tighten the three thin locking thumbscrews to secure the primary mirror in place.

The view through the collimation cap should now resemble Figure 27e. A simple star test will indicate how well the tele- scope optics are 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 correct- ly collimated, the expanding disk should be a perfect circle (Figure 32). 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 tele- scope is out of collimation.

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