Orion XT4.5 - Manual (Page 15)

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the secondary mirror (the smallest circle, with the collimation cap
“dot” in the center) is off-center. You will fix that in the next step.
Adjusting the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 16d, the secondary mirror is centered
under the focuser and the reflection of the primary mirror is centered
in the secondary mirror, but the small reflection of the secondary mir-
ror (with the “dot” of the collimation cap) is off-center.
The tilt of the primary mirror is adjusted with three spring-loaded col-
limation thumbscrews on the back end of the optical tube (bottom of
the primary mirror cell); these are the wide thumbscrews. The other
three thin thumbscrews lock the mirror's position in place; these
thin thumbscrews must be loosened before any collimation adjust-
ments can be made to the primary mirror.
To start, turn the thin thumbscrews that lock the primary mirror in
place a few turns each (Figure 19).
Now, try tightening or loosening one of the wide collimation thumb-
screws with your fingers (Figure 20). 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 further away from being centered in the “ring”
of the primary mirror mark. When you have that dot centered as
much as is possible in the ring, your primary mirror is collimated. The
view through the collimation cap should resemble Figure 16e. Re-
tighten the locking thumbscrews.
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 accurately
center it in the eyepiece’s field of view. Slowly de-focus the image
with the focusing knob. If the telescope is correctly collimated, the
expanding disk should be a perfect circle (Figure 21). 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 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.
Appendix B: Cleaning the Optics
Cleaning Lenses
Any quality optical lens cleaning tissue and optical lens cleaning fluid
specifically designed for multi-coated optics can be used to clean the
exposed lenses of your eyepieces or finderscope. Never use regular
glass cleaner or cleaning fluid designed for eyeglasses.
Figure 17. To
center the
secondary mirror
under the focuser,
hold the secondary
mirror holder in
place with your
fingers while
adjusting the
center screw with
the Phillips head
screwdriver. Do not
touch the mirror’s
surface.
Figure 18. Adjust
the tilt of the
secondary mirror
by loosening or
tightening the
three alignment
set screws with a
2mm Allen
wrench.
Figure 19. The
three thin
thumbscrews that
lock the primary
mirror in place
must first be
loosened before
any adjustments
can be made.
Figure 20. The
tilt of the primary
mirror is adjusted
by turning one or
more of the three
wide collimation
thumbscrews.
Alignment setscrews (3)
Figure 21. A star
test will determine
if the telescope's
optics are properly
collimated. An
unfocused view of
a bright star
through the
eyepiece should appear as illustrated on the right if optics are perfectly
collimated. If the circle is unsymmetrical, as illustrated on the left, the
scope needs collimation.
Out of collimation Collimated