Orion 9966 - Manual (Page 10)

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screws will be needed until the object comes into the finder
scope’s field of view.
The SkyQuest XT12’s finder scope uses a spring-loaded
bracket that makes alignment of the finderscope very easy. As
you turn either of the thumbscrews, the spring in the bracket’s
tensioner moves in and out to keep the finder scope secure
in the bracket.
Note: The image in both the finder scope and the main
telescope will appear upside-down (rotated 180°). This is
normal for finder scopes and reflector telescopes (see
Figure 18).
With the image in the finder scope’s field of view, you now
need to fine-adjust the alignment thumbscrews to center the
object on the intersection of the crosshairs. Adjust the aim of
the finder scope by turning the thumbscrews, one at a time,
until the object is centered.
The finder scope alignment needs to be checked before
every observing session. This can easily be done at night,
before viewing through the telescope. Choose any bright star
or planet, center the object in telescope eyepiece, and then
adjust the finder scope bracket’s alignment thumbscrews until
the star or planet is centered on the finder’s crosshairs.
Focusing the Finder Scope
If, when looking through the finder scope, the images appear
somewhat out of focus, you will need to refocus the finder
scope for your eyes. Loosen the lock ring located behind the
objective lens cell on the body of the finder scope (see Figure
15a). Back the lock ring off by a few turns, for now. Refocus
the finder scope on a distant object by threading the objective
lens cell in or out on the finder scope body. Precise focusing
will be achieved by focusing the finder scope on a bright star.
Once the image appears sharp, retighten the lock ring behind
the objective lens cell. The finder scope’s focus should not
need to be adjusted again.
Aiming/Pointing the Telescope
With the finder scope aligned, the telescope can be quick-
ly and accurately pointed at anything you wish to observe.
The finder scope has a much wider field of view than the
telescope’s eyepiece, and therefore it is much easier to first
center an object in the finder scope. Then, if the finder scope
is accurately aligned, the object will also be centered in the
telescope’s field of view.
Start by once again moving the telescope until it is pointed
in the general direction of the object you want to see. Some
observers find it convenient to sight along the tube to do this.
Now, look in the finder scope. If your general aim is accu-
rate, the object should appear somewhere in the field of view.
Make small adjustments to the telescope’s position until the
object is centered on the finder’s crosshairs. Now, look in the
telescope’s eyepiece and enjoy the view!
Magnification
Once the object you want to view is well centered in the 25mm
eyepiece, you may want to increase the magnification to get
a closer view. Loosen the thumbscrew on the 1.25" eyepiece
adapter and remove the eyepiece. Place it in the eyepiece
rack, if you wish. Insert the 10mm eyepiece into the 1.25"
eyepiece adapter, then retighten the thumbscrew. If you were
careful not to bump the telescope, the object should still be
centered within the field of view. Notice that the object being
viewed is now larger, but somewhat dimmer.
The SkyQuest XT12 is designed to accept any eyepiece
with a barrel diameter of 1.25" or 2". Magnification, or power,
is determined by the focal length of the telescope and the
focal length of the eyepiece. Therefore, by using eyepieces
of different focal lengths, the resultant magnification can be
varied.
Magnification is calculated as follows:
Telescope Focal Length (mm)
= Magnification
Eyepiece Focal Length (mm)
The SkyQuest XT12 Dobsonian has a focal length of 1500mm.
So, the magnification with the supplied 25mm eyepiece is
1500mm
= 60x
25mm
The magnification provided by the 10mm eyepiece is
1500mm
= 150x
10mm
The maximum attainable magnification for a telescope is
directly related to how much light its optics can collect. A tele-
scope 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 60x per inch of aperture. This
translates to about 720x for the SkyQuest XT12.
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