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 mirror is centered in the secondary mirror, but the small reflection of the secondary mirror (with the “dot” of the collimation cap) is
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 larger thumbscrews. The other three smaller thumbscrews lock the mirror’s position in place; these thumbscrews must be loos- ened 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
Figure 17. The three small thumbscrews that lock the primary mirror in place must first be loosened before any adjustments can be made.
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 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 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 14e. Re-tighten the locking thumbscrews.
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
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 |
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Figure 19. 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 scope needs collimation.
Figure 18. The tilt of the primary mirror is adjusted by turning one or more of the three larger thumbscrews.
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