18
B. seeing and transparency
Atmospheric conditions play a huge part in quality of view-
ing. Light from stars and other celestial objects must travel
through miles of Earth’s atmosphere to reach our eyes. The
air in the atmosphere will refract and bend the light. Atmo-
spheric turbulence will worsen the effects of refraction, which
can cause the image you see in your telescope to be unsta-
ble. The steadiness of the atmosphere is called “seeing.”
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 bet-
ter later in the evening as much of the heat absorbed by the
Earth during the day has radiated off into space. In condi-
tions of bad seeing, stars will twinkle and objects will appear
unsteady and blurry in the telescope.
“Transparency” is the clarity of the atmosphere, which can
be adversely affected by the presence of moisture, smoke,
and dust. All tend to scatter light, which reduces an object’s
brightness. Good transparency is desirable for astronomical
observing, especially for viewing faint objects.
One good measure of transparency is by how many stars you
can see with your unaided eyes. If you cannot see stars of
magnitude 3.5 or dimmer then transparency is poor. Magni-
tude is a measure of how bright a star is. The brighter a star,
the lower its magnitude. A good star to remember for this is
Megrez (magnitude 3.4), which is the star in the Big Dipper
that connects 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 25).
c. cooling the telescope
All optical instruments need time to reach “thermal equilib-
rium” to achieve maximum stability of the lenses and mir-
rors, which is essential for peak performance. Images will be
unstable if the optics are not in equilibrium with the outdoor
temperature. When moved from a warm indoor location out-
side 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 IntelliScope to
equilibrate. If the temperature difference between indoors and
outdoors is more than 40°, allow an hour or more. In the win-
ter, 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.
SkyQuest XT8i, XT10i and XT12i models are designed to
accept an optional Orion Cooling Accelerator Fan. When
attached to the rear cell of the telescope, the fan hastens the
equilibration of the primary mirror to ambient temperature.
D. 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 matter. Your eyes take about 30 minutes to reach per-
haps 80% of their full dark-adapted sensitivity. Many observ-
ers notice improvements after several hours of total darkness.
As your eyes become dark-adapted, more stars will glimmer
into view and you will be able to see fainter details in objects
you view in your telescope. Exposing your eyes to very bright
daylight for extended periods of time can adversely affect your
night vision for days. So give yourself at least a little while to
get used to the dark before you begin observing.
To see what you are doing in the darkness, use a red-filtered
flashlight rather than a white light. Red light does not spoil
your eyes’ dark adaptation like white light does. A flashlight
with a red LED light is ideal, or you can cover the front of a
regular incandescent flashlight with red cellophane or paper.
Dim light is preferable to bright light. Beware, too, that nearby
porch and streetlights and automobile headlights will spoil
your night vision.
Eyepiece selection
By using eyepieces of different focal lengths, it is possible
to attain many different magnifications with your telescope.
The telescope comes with two high-quality Sirius Plössl eye-
pieces: a 25mm focal length, and a 10mm focal length, which
provides higher magnification. Other eyepieces can be used
to achieve higher or lower powers. It is quite common for an
observer to own five or more eyepieces to access a wide
range of magnifications. This allows the observer to choose
the best eyepiece to use depending on the object being
viewed and the sky conditions. At least to begin with, the two
supplied eyepieces will suffice nicely.
Whatever you choose to view, always start by inserting your
lowest-power (longest focal length) eyepiece to locate and
center the object. Low magnification yields a wide field of
view, which shows a larger area of sky in the eyepiece. This
makes acquiring and centering an object much easier. If you
try to find and center objects with high power (narrow field of
view), it’s like trying to find a needle in a haystack!
Once you’ve centered the object in the eyepiece, you can
switch to higher magnification (shorter focal length eyepiece),
1.9
1.9
1.7
4.9
2.4 3.4
2.5
2.4
Figure 25. Megrez connects the Big Dipper’s handle to its “pan”.
It is a good guide to judging the “seeing” conditions. If you cannot
see Megrez (a 3.4-magnitude star) then seeing is poor.