10
reflection is not visible in the secondary mirror, as in
Figure
14b, you will need to adjust the tilt of the secondary mirror.
This is done by alternately loosening one of the three
secondary mirror alignment screws with a Phillips screwdriver
then tightening the other two
(Figure 15). The goal is to
center the primary mirror reflection in the secondary mirror, as
in
Figure 14c. Don’t worry that the reflection of the secondary
mirror (the smallest circle) is off-center. You will fix that in the
next step. It will take some trial and error to determine which
screws to loosen and tighten to move the reflection of the
primary mirror to the center of the secondary mirror. But be
patient and you’ll get it.
Aligning the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in
Figure 14c, the reflection of the primary
mirror is centered in the secondary mirror, but the small reflec-
tion of the secondary mirror is off-center. The tilt of the pri-
mary mirror is adjusted using the three pairs of collimation
screws on the back end of the optical tube
(Figure 16).
Adjusting the tilt of the mirror requires a “push-pull” tech-
nique involving adjustment of one or more pairs of collimation
screws. With a Phillips screwdriver, loosen one of the screws
one full turn, and then tighten the adjacent screw (the one
right next to it) until it is tight. 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 by simply watching to
see if the “dot” of the Cheshire eyepiece or collimation cap is
moving closer or farther away from the ring on the center of
the primary mirror. Repeat this process on the other two sets
of collimation screws, if necessary. Again, it will take a little
trial and error to get a feel for how to tilt the mirror in this way.
When you have the dot centered as much as possible in the
ring, your primary mirror is collimated. The view through the
collimation cap should resemble
Figure 14d. Make sure all
the collimation screws are tight (but do not overtighten), to
secure the mirror tilt. A simple star test will tell you whether the
optics are accurately 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 de-focus
the image with the focusing knob. If the telescope is correct-
ly collimated, the expanding disk should be a perfect circle
(Figure 17). If the image is unsymmetrical, the scope is out
of collimation. The dark shadow cast by the secondary mir-
ror should appear in the very center of the out-of-focus circle,
like the hole in a donut. 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, 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.
6. Telescope Care and
Maintenance
If you give your telescope reasonable care, it will last a lifetime.
Store it in a clean, dry, dust-free place, safe from rapid changes
Figure 15.
Adjust the tilt of the secondary mirror by adjusting the
three alignment screws with a Phillips screwdriver.
Figure 17
. A star test will determine if the telescope's optics are
properly collimated.
Out
of
collimation
Collimated
Figure 16.
Align the primary mirror by adjusting the three pairs
of “push/pull” screws on the back end of the optical tube.