Orion Observer II 70mm Altazimuth Refractor Instruction Manual page 8

To see what you're doing in the darkness, use a red-filtered
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. Beware, too, that nearby porch, streetlights,
and car headlights will ruin your night vision.
Eyepiece Selection
Magnification, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece being used.
Therefore, by using eyepieces of different focal lengths, the
resultant magnification can be varied. 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 and
viewing conditions. Your Observer II 70mm Altazimuth refrac-
tor comes with 25mm and 10mm Kellner eyepieces, which will
suffice nicely to begin with. You can purchase additional eye-
pieces later if you wish to have more magnification options.
Magnification is calculated as follows:
Telescope Focal Length (mm)
Magnification
Eyepiece Focal Length (mm)
For example, the Observer II 70mm Altazimuth has a focal
length of 700mm, which when used with the supplied 25mm
eyepiece yields:
700 mm
25 mm
The magnification provided by the 10mm eyepiece is:
700 mm
10 mm
The maximum attainable magnification for a telescope is directly
related to how much light it can gather. The larger the aperture,
the more magnification is possible. In general, a figure of 50x per
inch of aperture is the maximum attainable for most telescopes.
Going beyond that will yield simply blurry, unsatisfactory views.
Figure 13. The Kellner eyepieces have barrels threaded to
accept optional Orion 1.25" filters. A Moon filter is useful for cut-
ting glare and revealing more detail on the lunar surface.
8
=
= 28x
= 70x
Your Observer II 70mm Altazimuth has an aperture of 70mm,
or 2.8 inches, so the maximum magnification would be about
140x (2.8 x 50). This level of magnification assumes you have
ideal atmospheric conditions for observing (which is seldom the
case).
Keep in mind that as you increase magnification, the brightness
of the object viewed will decrease; this is an inherent principle
of the laws of physics 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!
So start by using the 25mm eyepiece, then try switching to the
10mm eyepiece later if you want to boost the magnification.
Focusing the Telescope
To focus the telescope, turn the focus wheels (Figure 10)
forward or back until you see your target object (e.g., stars,
the Moon, etc.) in the eyepiece. Then make finer adjustments
until the image is sharp. If you're having trouble achieving ini-
tial focus, rack the focuser drawtube all the way in using the
focus wheels, then while looking into the eyepiece slowly turn
the focus wheels so that the drawtube extends outward. Keep
going until you see your target object come into focus. Note
that when you change eyepieces you may have to adjust the
focus a bit to get a sharp image with the newly inserted eye-
piece.
What to Expect
So what will you see with your telescope? You should be able to
see bands on Jupiter, the rings of Saturn, craters on the Moon,
the waxing and waning of Venus, and many bright deep-sky
objects. Do not expect to see color as you do in NASA photos,
since those are taken with long-exposure cameras and have
"false color" added. Our eyes are not sensitive enough to see
color in deep-sky objects except in a few of the brightest ones.
Objects to Observe
Now that you are all set up and ready to go, one critical decision
must be made: what to look at?
A. The Moon
With its rocky surface, the Moon is one of the easiest and most
interesting objects to view with your telescope. Lunar craters,
maria, and even mountain ranges can all be clearly seen from a
distance of 238,000 miles away! With its ever-changing phases,
you'll get a new view of the Moon every night. The best time
to observe our one and only natural satellite is during a partial
phase, that is, when the Moon is not full. During partial phas-
es, shadows are cast on the surface, which reveal more detail,
especially right along the border between the dark and light por-
tions of the disk (called the "terminator"). A full Moon is too bright
and devoid of surface shadows to yield a pleasing view. Make
sure to observe the Moon when it is well above the horizon to
get the sharpest images.
Use an optional Moon filter to dim the Moon when it is very
bright. It simply threads onto the bottom of the eyepieces (you
must first remove the eyepiece from the focuser to attach a fil-
ter). You'll find that the Moon filter improves viewing comfort, and
helps to bring out subtle features on the lunar surface.