Orion 9966 - Manual (Page 13)

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Now, try tightening or loosening one of the larger collima-
tion thumbscrews with your fingers (Figure 23). 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 watch-
ing 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 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 19e.
Re-tighten the locking thumbscrews.
A simple star test will tell you whether the optics are accu-
rately collimated.
Note About the Collimatable 2" Focuser
The 2" focuser of the SkyQuest XT12 can be collimated using
three pairs of push-pull screws located at the base of the
focuser. The focuser was collimated at the factory however,
and should not need to be adjusted. Focuser collimation is
only required under very rare circumstances, but has been
made available for this telescope should such a need arise.
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 correct-
ly collimated, the expanding disk should be a perfect circle
(Figure 24). 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
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.
5. Astronomical Observing
For many users, the SkyQuest XT12 telescope will be a
major leap into the world of amateur astronomy. This section
is intended to get you ready for your voyages through the
night sky.
Site Selection
Pick a location away from street lights and bright yard light-
ing. Avoid viewing over rooftops and chimneys, as they often
have warm air currents rising from them, which distort the
image seen in the eyepiece. Similarly, you should not observe
through an open window from indoors. Better yet, choose
a site out-of-town, away from any “light pollution”. You’ll be
stunned at how many more stars you’ll see! Most importantly,
make sure that any chosen site has a clear view of a large
portion of the sky.
Seeing and Transparency
Atmospheric conditions play a huge part in quality of viewing.
In conditions of good “seeing”, star twinkling is minimal and
objects appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
after midnight, when much of the heat absorbed by the Earth
during the day has radiated off into space. Typically, seeing
conditions will be better at sites that have an altitude over about
Figure 22. The three thumb screws that lock the primary mirror in
place must first be loosened before any adjustments can be made.
Figure 23. The tilt of the primary mirror is adjusted by turning one
or more of the three spring-loaded collimation thumb screws.
Figure 24. 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 right if optics are perfectly
collimated. If circle is unsymmetrical, as in illustration on left, scope
needs collimation.
Out of collimation Collimated