13
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 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 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 screwdriv-
er in the slots, if necessary.
Now, try tightening or loosening one of the larger collimation
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 watching
to see if the “dot” of the collimation cap is moving closer or fur-
ther 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 accurate-
ly 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 defocus
the image with the focusing knob. If the telescope is correctly
collimated, the expanding disk should be a perfect circle
(Figure 19). 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 doughnut. If the “hole” appears off-center, the tel-
escope 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.
Figure 17.
The three small thumbscrews 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 larger thumbscrews.
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.
Out of collimation
Collimated
