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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 14e.
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 (XT8)
The 2" focuser of the SkyQuest XT8 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 never 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 correctly
collimated, the expanding disk should be a perfect circle
(Figure 19). 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 tel-
escope 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 tele-
scope’s position in order to account for the sky’s apparent
motion.
5. Astronomical Observing
For many users, the SkyQuest XT10 telescope will be a major
leap into the world of amateur astronomy. This section is intend-
ed to get you ready for your voyages through the night sky.
Observing Tips
A. 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.
B. 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 3000 feet. Altitude helps because it decreases the
amount of distortion causing atmosphere you are looking
through.
A good way to judge if the seeing is good or not is to look at
bright stars about 40° above the horizon. If the stars appear
to “twinkle”, the atmosphere is significantly distorting the
incoming light, and views at high magnifications will not
appear sharp. If the stars appear steady and do not twinkle,
seeing conditions are probably good, and higher magnifica-
tions will be possible.
Also, seeing conditions are typically poor during the day. This
is because the heat from the Sun warms the air and causes
turbulence.
Good “transparency” is especially important for observing
faint objects. It simply means the air is free of moisture,
smoke, and dust. All tend to scatter light, which reduces an
object’s brightness.
C. Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um” to achieve maximum stability of the lenses and mirrors,
which is essential for peak performance. When moved from a
warm indoor location outside to cooler air (or vice-versa), a
telescope needs time to cool to the outdoor temperature. The
bigger the instrument and the larger the temperature change,
the more time will be needed.
Allow at least 30 minutes for your SkyQuest XT10 to equilibrate.
If the scope has more than a 40° temperature adjustment, allow
an hour or more. In the winter, storing the telescope outdoors in
a shed or garage greatly reduces the amount of time needed for
the optics to stabilize. It also is a good idea to keep the scope
Figure 19.
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
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