13
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 18)
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 perhaps
80% of their full dark-adapted sensitivity. Many observers
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. 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 light
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 flashlight with red cellophane or paper. Beware, too,
that nearby porch and streetlights and automobile headlights
will spoil your night vision.
Tracking Celestial Objects
The Earth is constantly rotating about its polar axis, complet-
ing one full rotation every 24 hours; this is what defines a
“day.” We do not feel the Earth rotating, but we can tell that it
is at night by seeing the apparent movement of stars from
east to west. This movement translates into a rate of approxi-
mately .25° per minute, or 15 arc-seconds per second. (There
are 60 arc-minutes in 1°, and 60 arc-seconds in one arc-
minute.) This is called the sidereal rate.
When you observe any astronomical object, you are watching
a moving target. This means the telescope’s position must be
continuously updated over time to keep an object in the field
of view. This is easy to do with the SkyQuest XT because of
its smooth motions on both axes. As the object moves off
toward the edge of the field of view, you just lightly nudge the
telescope to bring it back to the center.
You will notice that it is more difficult to “track” objects when
the telescope tube is aimed nearly straight up. This is inherent
to the basic design of the Dobsonian, and stems from the fact
that there is very little mechanical leverage to move in azimuth
when the tube is in a near vertical position. To gain more lever-
age, try grasping the tube close to the altitude side bearings
with both hands.
Remember that objects appear to move across the field of
view faster at higher magnifications. This is because the field
of view becomes narrower.
Eyepiece Selection
By using eyepieces of varying focal lengths, it is possible to
attain many magnifications with the SkyQuest XT. The tele-
scope comes with two high-quality Sirius Plössl eyepieces: a
25mm, which gives a magnification of 48x, and a 10mm,
which gives a magnification of 120x. 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. 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!
2.4
1.7
3.4
2.4
4.9
1.9
1.9
2.5
Figure 18.
Megrez connects the Big Dipper’s handle to it's “pan”.
It is a good guide to how conditions are. If you can not see Megrez
(a 3.4 mag star) then conditions are poor.
