Orion SKYQUEST XX12 - Manual (Page 18)

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The secondary mirror should now be centered in the focuser
drawtube. Now we will shift our attention to the reflections
within the secondary mirror in order to properly adjust the tilt
of the secondary mirror. Adjusting the tilt of the secondary
mirror and the tilt of the primary mirror are the two collimation
adjustments that will be done most often.
If the entire primary mirror reflection is not visible in the sec-
ondary mirror, as in Figure 31c, you will need to adjust the tilt
of the secondary mirror. This is done by alternately loosening
one of the three secondary mirror alignment set screws while
tightening the other two, as depicted in Figure 35. Do not
make excessive turns of these set screws or force them past
their normal travel. A simple 1/2 turn of the screw can dra-
matically change the tilt of the secondary mirror. The goal is
to center the primary mirror reflection in the secondary mirror,
as in Figure 31d. Don’t worry that the reflection of the second-
ary mirror (the smallest circle, with the collimation cap “dot” in
the center) is off-center. You will fix that in the next step.
aligning the Primary Mirror
The final adjustment is made to the tilt of the primary mir-
ror. It will need adjustment if, as in Figure 31d, 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 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 rear end of the opti-
cal tube (bottom of the primary mirror cell). The three smaller
thumb screws 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 thumb screws counterclockwise a
few turns each. Use a screwdriver in the slots, if necessary.
Now, try tightening or loosening one of the collimation knobs
(Figure 36). Look into the focuser and see if the secondary
mirror reflection has moved closer to the center of the primary
mirror reflection. You can easily determine this with the col-
limation 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 31e. Re-tighten the
locking thumb screws in the bottom of the mirror cell.
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 defo-
cus the image with the focusing knob. If the telescope is cor-
Figure 35. Adjust the tilt of the secondary mirror by loosening or
tightening the three alignment set screws with a 2mm hex key.
Figure 36. The tilt of the primary mirror is adjusted by turning
one or more of the three collimation knobs.
Figure 37. 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