Cooling The Telescope; Astronomical Observing - Orion SKYQUEST XT10 CLASSIC Instruction Manual

Out of collimation
Figure 18.
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 scope needs collimation.
the image with the focusing knob. If the telescope is correctly
collimated, the expanding disk should be a perfect circle
(Figure 18). 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 donut. If the "hole" appears off-center, the tele-
scope is out of collimation.
If you try the star test and the bright star you have selected is
not accurately centered in the eyepiece, 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 collimatable 2" Focuser
(Xt8 and Xt10)
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.

5. astronomical observing

For many users, the SkyQuest XT telescope will be a major
leap into the world of amateur astronomy. This section is
intended to get you ready for your first voyage through the
night sky.
site selection
Pick a location away from streetlights 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 importantly, make sure that any chosen site
has a clear view of a large portion of the sky.
12

cooling the telescope

All optical instruments need time to reach "thermal equilibrium"
to achieve maximum stability of the lenses and mirrors, which
is essential for peak performance. When moved from a warm
indoor location to cooler air outside(or vice-versa), a telescope
needs time to cool (or warm) 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 XT to equilibrate.
Collimated
If the scope experiences more than a 40° temperature change,
allow an hour or more. In the winter, storing the telescope 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.
The XT8 and XT10 have the ability to mount a small fan to
make cooling the tube faster. On the bottom of the mirror cell
there are four holes where a fan can be screwed on.
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 incom-
ing 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 magnifications 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.
One good way to tell if conditions are good is by how many
stars you can see with your naked eye. If you cannot see
stars of magnitude 3.5 or dimmer then conditions are poor.
Magnitude is a measure of how bright a star is, the brighter a
star is, the lower its magnitude will be. A good star to remem-
ber for this is Megrez (mag. 3.4), which is the star in the Big
Dipper connecting the handle to the "dipper." If you cannot see
Megrez, then you have fog, haze, clouds, smog, light pollu-
tion, or other conditions that are hindering your viewing. (See
Figure 19)
let Your Eyes Dark-adapt
Do not expect to go from a lighted house into the darkness
of the outdoors at night and immediately see faint nebulas,
galaxies, and star clusters — or even very many stars, for that