Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 13d, 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 reflection
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 larg-
er 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 16). Use a screwdriv-
er in the slots, if necessary.
Now, try tightening or loosening one of the larger collimation
thumbscrews with your fingers (Figure 17). 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 13e. 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 18). 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
13
Figure 14.
To center the secondary mirror under the focuser,
hold the secondary mirror holder in place with one hand while
adjusting the center bolt with a Phillips screwdriver. Do not touch the
mirror’s surface!
Figure 15.
Adjust the tilt of the secondary mirror by loosening or
tightening the three alignment set screws with a 2mm hex key.
Figure 16.
The three small thumbscrews that lock the primary
mirror in place must first be loosened before any adjustments can
be made.
Figure 17.
The tilt of the primary mirror is adjusted by turning
one or more of the three larger thumbscrews.
