9
length of the eyepiece. Therefore, by using eyepieces of dif-
ferent focal lengths, the resultant magnification can be varied.
Magnification is calculated as follows:
Magnification =
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
The SkyQuest XT10 Dobsonian has a focal length of
1250mm. So, the magnification with the supplied 25mm eye-
piece is 1250mm ÷ 25mm = 50x. The magnification provided
by the 9mm eyepiece is 1250mm ÷ 9mm = 139x.
The maximum attainable magnification for a telescope is
directly related to how much light its optics can collect. A tel-
escope with more light collecting area, or aperture, can yield
higher magnifications than a smaller aperture telescope. The
maximum practical magnification for any telescope, regard-
less of optical design, is about 50x per inch of aperture. This
translates to about 500x for the SkyQuest XT10.
Keep in mind that as magnification is increased, the bright-
ness of the object being viewed will decrease; this is an
inherent principle of the physics of optics and cannot be
avoided. If magnification is doubled, an image appears four
times dimmer. If magnification is tripled, image brightness is
reduced by a factor of nine!
Note About High Magnifications:
Maximum magnifications are achieved only under the most
ideal viewing conditions at the best observing sites. Most of
the time, magnifications are limited to 200x or less, regard-
less of aperture. This is because the Earth’s atmosphere
distorts light as it passes through. On nights of good “seeing”,
the atmosphere will be still and will yield the least amount of
distortion. On nights of poor seeing, the atmosphere will be
turbulent, which means different densities of air are rapidly
mixing. This causes significant distortion of the incoming light,
which prevents sharp views at high magnifications.
Tube Balance
Dobsonians are designed to balance with standard supplied
accessories, such as an eyepiece and a finder scope. But
what if you want to use other accessories such as a barlow
lens or a heavy 2" eyepiece? Traditional Dobsonian designs
expect the user to compensate for heavier accessories by
adding weight to the opposite end of the telescope tube. Such
counterweighting systems can be expensive and unwieldy.
The CorrecTension Friction Optimization system of the
SkyQuest XT10 Dobsonian, however, solves the finicky bal-
ance problem. The spring coils pull the tube down onto the
base, thereby increasing the friction on the altitude bearing
pads! With CorrecTension, the added weight of small front-
end loads will not adversely affect the balance of the
telescope.
If you install an array of heavier accessories onto your
SkyQuest XT10’s optical tube, you may need at some point to
counterbalance the telescope with a counterweight system.
Carrying the Telescope
Moving the SkyQuest XT10 is easy to do. Remove any eye-
pieces from the telescope and eyepiece rack, and place them
in an eyepiece case. You can also remove the finder scope
and finder scope bracket, if you wish. Disengage the
CorrecTension springs by unhooking them from the posts on
the base, using the pull loops. The springs remain captive on
the telescope side bearings. The base and tube are now dis-
engaged and can be transported separately.
To carry the base, simply grasp the handle on the front of it.
The tube should be carried with two hands. One way to do
this is to grasp the tube with one hand while grasping the mir-
ror cell end with the other (Figure 12). Another way is to grasp
the tube with both hands around its circumference (Figure
13). Be careful when setting the tube down on its end so as
not to bend or damage the primary mirror collimation screws
on the bottom of the primary mirror cell.
Figure 13: Another way to carry the tube is to grasp the tube
with both hands around its circumference.
Figure 12: You can carry the tube by grasping the tube with
one hand while grasping the mirror cell end with the other.
