Orion 9874 instruction manual 15.16, Adjusting the Primary Mirror

15.16.

Figure 15. 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 screwdriver. Do not touch the mirror’s surface!

Figure 16. Adjust the tilt of the secondary mirror by loosening or tightening the three alignment screws with a 2mm Allen wrench.

loosen the three small alignment set screws in the center hub of the 4-vaned spider several turns. Now keep the mirror’s holder stationary (be careful not to touch the surface of the mirrors), while turning the center screw with a Phillips head screwdriver (See Figure 15). Turning the screw clockwise will move the secondary mirror toward the front opening of the optical tube, while turning the screw counter-clockwise will move the secondary mirror toward the primary mirror.

Note: When making these adjustments, be careful not to stress the spider vanes or they may bend.

When the secondary mirror is centered in the focuser draw‑ tube, rotate the secondary mirror holder until the reflection of the primary mirror is as centered in the secondary mirror as possible. It may not be perfectly centered, but that is OK. Now tighten the three small alignment screws equally to secure the secondary mirror in that position. This adjustment will rarely, if ever need to be done.

If the entire primary mirror reflection is not visible in the sec‑ ondary mirror, as in Figure 14c; you will need to adjust the tilt of the secondary mirror. This is done by alternately loosen‑ ing one of the three alignment set screws while tightening the other two, as depicted in Figure 16. The goal is to center the primary mirror reflection in the secondary mirror, as in Figure 14d. 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.

Adjusting the Primary Mirror

The final adjustment is made to the primary mirror. It will need adjustment if, as in Figure 14d, the secondary mirror is cen‑ tered under the focuser and the reflection of the primary mir‑ ror is centered in the secondary mirror, but the small reflec‑ tion of the secondary mirror (with the “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 opti‑ cal tube (bottom of the primary mirror cell); these are the

Figure 17. The three thumb screws that lock the primary mirror in place must first be loosened before any adjustments can be made.

Figure 18. The tilt of the primary mirror is adjusted by turning one or more of the three spring-loaded collimation thumb screws.

larger thumbscrews. The other three smaller thumbscrews lock the mirror’s position in place; these thumbscrews must be loosened before any collimation adjustments can be made to the primary mirror.

To start, turn the smaller thumbscrews that lock the primary mirror in place a few turns each. (Figure 17) Use a screw‑ driver in the slots, if necessary.

Now, try tightening or loosening one of the larger collima‑ tion thumbscrews with your fingers (Figure 18). 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 if the “dot” of the collimation cap is moving closer or further away from the “ring” on the center of the primary mirror mark. When you have the dot centered as much as is possible in the ring, your primary mirror is collimated. The view through the collimation cap should resemble Figure 15e. Re-tighten the locking thumbscrews.

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

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