Orion XT12 - Manual (Page 12)

12
Aligning the Primary Mirror
The final adjustment is made to the primary mirror. It will need
adjustment if, as in Figure 21d, 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
off-center.
The tilt of the primary mirror is adjusted with the three large
spring-loaded collimation knobs on the back end of the opti-
cal tube (bottom of the primary mirror cell). The three smaller
thumbscrews lock the mirror’s position in place. These thumb-
screws must be loosened before any collimation adjustments
can be made to the primary mirror.
To start, turn the smaller thumbscrews counterclockwise a
few turns each (Figure 25). Use a screwdriver in the slots, if
necessary.
Now, try tightening or loosening one of the larger collimation
knobs (Figure 26). Look into the focuser and see if the sec-
ondary mirror reflection has moved closer to the center of the
primary mirror. You can easily determine this with the collima-
tion 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. If
turning the one knob does not seem to bring the dot closer
to the ring, try using one of the other collimation knobs. It will
take some trial and error using all three knobs to properly
align the primary mirror. Over time you will get the feel for
which collimation screws to turn to move the image in a given
direction.
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 21e. 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 high in
the sky and 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 27). If the image is unsymmetrical, the telescope
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 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, 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.
Note About the 2"s Focuser
The 2" focuser of the SkyQuest XT12 IntelliScope can be col-
limated using three pairs of push-pull screws located at the
base of the focuser (Figure 20). The focuser was collimated
at the factory, however, and should not need to be adjusted.
Focuser collimating is only required under very rare circum-
stances, but has been made available for this telescope
should such a need arise.
Figure 25. The three small thumbscrews that lock the primary
mirror in place must first be loosened before any adjustments can
be made.
Figure 26. The tilt of the primary mirror is adjusted by turning one
or more of the three collimation knobs.
Figure 27. 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 telescope needs collimation.
Out of collimation Collimated