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Focusing
You will find the helical focus knob next to the visual back on
the back end of the optical tube. Rotate the rubber-covered
knob to change the focus. It operates by sliding the primary
mirror inside the telescope incrementally forward or backward.
Point the telescope so the front end is aimed in the general
direction of an object you wish to view. When you first look in
the eyepiece, the image you see may be fuzzy, or out of focus.
If so, gently turn the focus knob with your fingers until the
image becomes sharp. Go a little bit beyond sharp focus until
the image just starts to blur again, then reverse the rotation of
the knob, just to make sure you've hit the exact focus point.
You will have to readjust the focus when aiming at subjects of
varying distances, or after changing eyepieces.
If you have trouble focusing, rotate the focus knob counter-
clockwise as far as it will go. Now look through the eyepiece
while slowly rotating the focus knob clockwise. You should
soon see the point at which focus is reached.
Calculating Magnification
To calculate the magnification, or power, of a telescope with an
eyepiece, simply divide the focal length of the telescope by the
focal length of the eyepiece:
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
For example, the 180mm Maksutov-Cassegrain, which has
a focal length of 2700mm, used in combination with a 25mm
eyepiece, yields a magnification of:
2700 mm
25 mm
2" Visual back
1.25" adapter
Figure 1. Key parts of the 180m Maksutov-Cassegrain.
2
= Magnification
= 108x
Finder
scope base
It is desirable to have a range of eyepieces of different focal
lengths to allow viewing over a range of magnifications. It is
not uncommon for an observer to own five or more eyepieces.
Orion offers many different eyepieces of varying focal lengths,
so check the catalog or www.OrionTelescopes.com for a wide
selection of additional eyepieces to choose from.
Every telescope has a useful limit of power of about 2x per mil-
limeter of aperture (i.e. 360x for the 180mm Mak-Cass). Claims
of higher power by some telescope manufacturers are a mis-
leading advertising gimmick and should be dismissed. Keep in
mind that at higher powers, an image will always be dimmer
and less sharp (this is a fundamental law of optics). The steadi-
ness of the air (the "seeing") can also limit how much magnifi-
cation an image can tolerate.
Always start viewing with your lowest-power (longest focal
length) eyepiece in the telescope. It's best to begin observing
with the lowest-power eyepiece, because it will typically pro-
vide the widest true field of view, which will make finding and
centering objects much easier. After you have located and cen-
tered an object, you can try switching to a higher-power eye-
piece to ferret out more detail, if atmospheric conditions permit.
If the image you see is not crisp and steady, reduce the mag-
nification by switching to a longer focal length eyepiece. As a
general rule, a small but well-resolved image will show more
detail and provide a more enjoyable view than a dim and fuzzy,
over-magnified image.
Photography
For photography with the 180mm Maksutov-Cassegrain, you
can use a DSLR or any of a wide variety of astronomical CCD
or CMOS cameras. For DSLRs you will need a T-ring for your
particular camera as well as a camera adapter with a 2" nose-
piece to couple the camera to the telescope. Larger astronomi-
cal cameras may already be equipped with a 2" nosepiece; if
not you will need to purchase a 2" T-adapter. Smaller "plan-
etary" cameras often have a 1.25" barrel, which you can insert
Mounting bar
Focus knob
Tapped holes for
optional finder
scope base
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