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Out of collimation
Figure 36.
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.
tube moves by itself under the weight of the eyepiece), you
can adjust it by tightening or loosening the drawtube tension-
ing set screw on the focuser, which is located just below the
focus lock thumb screw. Adjust this set screw with the includ-
ed 2.5mm hex key. Do not loosen the set screw too much
as there must be some tension to keep the drawtube secure
within the focuser. The other set screw below the drawtube
tensioning set screw does not affect drawtube tension and
should not be adjusted.
Viewing with Eyeglasses
If you wear eyeglasses, you may be able to keep them on while
you observe, if your eyepieces have enough eye relief to allow
you to see the whole field of view. You can try this by looking
through the eyepiece first with your glasses on and then with
them off, and see if the glasses restrict the view to only a
portion of the full field. If they do, you can easily observe with
your glasses off by just refocusing the telescope as needed.
If you suffer from severe astigmatism, however, you may find
images noticeably sharper with your glasses on.
Aiming/Pointing the Telescope
Finding objects in GoTo mode is covered in sections 7 and 8.
If you're using AutoTracking mode, you'll need to use the EZ
Finder II aiming device to help in putting objects in the field
of view of the telescope's eyepiece for viewing. When the EZ
Finder II is properly aligned, the telescope can be pointed at
anything you wish to observe and it will then be centered, or
nearly so, in the telescope's field of view as well.
Start by moving the telescope by hand or using the hand
controller's directional buttons 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 EZ Finder II. If your general aim is accurate,
the object should appear somewhere in the EZ Finder II's field
of view. Make small adjustments to the telescope's position
until the EZ Finder II's red dot is centered on the object. Now,
look in the telescope's eyepiece and enjoy the view!
Magnification
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:
The XX12g, for example, has a focal length of 1500mm. So,
the magnification with the supplied 35mm 2" eyepiece is:
Collimated
The magnification provided by the 12.5mm illuminated eye-
piece is:
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 50x per inch of aperture. This
translates to about 600x for the XX12g. Of course, such high
magnification will only yield acceptable images if atmospheric
conditions are extremely favorable.
More typically, useful magnifications will be limited to 200x or
less, regardless of aperture. This is because the Earth's atmo-
sphere 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 rap-
idly mixing. This causes significant distortion of the incoming
light, which prevents sharp views at high magnifications.
Keep in mind that as magnification is increased, the bright-
ness of the object being viewed will decrease; this is an inher-
ent principle of the physics of optics and cannot be avoided.
If magnification is doubled, an image appears four times dim-
mer. If magnification is tripled, image brightness is reduced by
a factor of nine!
The SkyQuest XXg Dobs are designed to accept eyepieces
with a barrel diameter of either 1.25" or 2". At low magnifica-
tions, 2" eyepieces can provide a wider field of view than stan-
dard 1.25" eyepieces. A wider field can be desirable for view-
ing extended deep-sky objects that are too large to fit within a
narrower field of view.
11. Care and Maintenance
If you give your telescope reasonable care, it will last a life-
time. Store it in a clean, dry, dust-free place, safe from rapid
changes in temperature and humidity. Do not store the tele-
scope outdoors, although storage in a garage or shed is OK.
Be sure to keep the dust cover on the telescope when not in
use.
Your SkyQuest XXg requires very little mechanical mainte-
nance. The optical tube is steel and has painted finish that is
fairly scratch-resistant. If a scratch does appear on the tube, it
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
1500 mm
35mm
1500 mm
12.5mm
= Magnification
= 43x
= 120x
27
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