Aligning the Primary Mirror

The final adjustment is made to the primary mirror. It will need adjustment if, as in Figure 21d, the secondary mirror is cen- tered 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 off-center.

The tilt of the primary mirror is adjusted with the three large spring-loaded collimation knobs on the back end of the opti- cal tube (bottom of the primary mirror cell). The three smaller thumbscrews lock the mirror’s position in place. These thumb- screws must be loosened before any collimation adjustments can be made to the primary mirror.

To start, turn the smaller thumbscrews counterclockwise a few turns each (Figure 25). Use a screwdriver in the slots, if necessary.

Figure 25. The three small 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 larger collimation knobs (Figure 26). Look into the focuser and see if the sec- ondary mirror reflection has moved closer to the center of the primary mirror. You can easily determine this with the collima- tion cap and mirror center mark by simply watching 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. If turning the one knob does not seem to bring the dot closer to the ring, try using one of the other collimation knobs. It will take some trial and error using all three knobs 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 is possible in the ring, your primary mirror is collimated. The view through the collimation cap should resemble Figure 21e. Re-tighten the locking thumbscrews.

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

Figure 26. The tilt of the primary mirror is adjusted by turning one or more of the three collimation knobs.

Star-Testing the Telescope

When it is dark, point the telescope at a bright star high in the sky and center it in the eyepiece’s field of view. Slowly defocus the image with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle (Figure 27). If the image is unsymmetrical, the telescope 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 doughnut. If the “hole” appears off- center, the telescope is out of collimation.

If you try the star test and the bright star you have selected is not accurately centered in the eyepiece, then the optics will always appear out of collimation, even though they may be perfectly aligned. It is critical to keep the star centered, so over time you will need to make slight corrections to the telescope’s position in order to account for the sky’s apparent motion.

Out of collimation

Collimated

Figure 27. A star test will determine if a telescope’s optics are properly collimated. An unfocused view of a bright star through the eyepiece should appear as illustrated on the right if the optics are perfectly collimated. If the circle is unsymmetrical, as in the illustration on the left, the telescope needs collimation.

Note About the 2"s Focuser

The 2" focuser of the SkyQuest XT12 IntelliScope can be col- limated using three pairs of push-pull screws located at the base of the focuser (Figure 20). The focuser was collimated at the factory, however, and should not need to be adjusted. Focuser collimating is only required under very rare circum- stances, but has been made available for this telescope should such a need arise.