12
necessary. It will take a little trial and error to get a feel for how
to tilt the mirror in this way to center the reflection. (It helps to
have two people for primary mirror collimation, one to look in
the focuser while the other adjusts the collimation thumb
screws.) Do not loosen (i.e., rotate counter-clockwise) each
collimation thumb screw too much, or the thumb screw will
completely unthread from the mirror cell. Rather, try tighten-
ing the other two collimation thumb screws.
Once the secondary mirror reflection is centered in the pri-
mary mirror reflection, retighten the thumb screws that lock
the primary mirror’s position in place.
The view through the focuser should now resemble Figure
14a; the reflection of the primary mirror is centered in the sec-
ondary mirror, and the reflection of the secondary mirror is
centered in the reflection of the primary mirror.
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 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 select-
ed 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 correc-
tions to the telescope’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 intended 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 lighting.
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 important-
ly, 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 view-
ing. In conditions of good “seeing”, star twinkling is minimal
and objects appear steady in the eyepiece. Seeing is best
overhead, 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 equili-
brate. 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 covered until the Sun sets so
the tube does not heat greatly above the temperature of the
outside air.
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
