11
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 AstroView 102 EQ comes with 25mm
(L) and 10mm (M) Plossl eyepieces, which will suffice nicely to
begin with. You can purchase additional eyepieces later if you
wish to have more magnification options.
Magnification is calculated as follows:
For example, the AstroView 102 EQ Refractor has a focal length of
700mm, which when used with the supplied 25mm eyepiece yields:
The magnification provided by the 10mm eyepiece is:
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 tele-
scopes. Going beyond that will yield simply blurry, unsatisfac-
tory views. The AstroView 102 EQ Refractor has an aperture of
102mm, or 4.0 inches, so the maximum magnification would
be about 200x (4.0 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 magnifica-
tion is tripled, image brightness is reduced by a factor of nine!
Start by centering the object you wish to see in the 25mm
eyepiece. Then you may want to increase the magnification
to get a closer view, by switching to the 10mm eyepiece. If the
object is off-center (i.e., it is near the edge of the field of view)
you will lose it when you increase magnification, since the field
of view will be narrower with the higher-powered eyepiece. So
make sure it is centered in the 25mm eyepiece before switching
to the 10mm eyepiece.
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 initial 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 eyepiece.
If desired you can adjust the amount of focusing friction using
the drawtube tension thumbscrew on the top of the focuser (see
Figure 10
). Rotate it clockwise to increase the tension; counter-
clockwise to decrease it.
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, what is there to look
at in the night sky?
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 phases,
shadows are cast on the surface, which reveal more detail,
especially right along the border between the dark and light
portions 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 hori-
zon 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’ll
find that the Moon filter improves viewing comfort, and helps to
bring out subtle features on the lunar surface.
B. The Planets
The planets don’t stay put like the stars, so to find them you
should refer to the monthly star charts at OrionTelescopes.
com, or to charts published monthly in Astronomy, Sky & Tele-
scope, or other astronomy magazines. Venus, Mars, Jupiter, and
Saturn are the brightest objects in the sky after the Sun and the
Moon. Other planets may be visible but will likely appear star-
like. Because planets are quite small in apparent size, optional
higher-power eyepieces or a Barlow lens are recommended and
often needed for detailed observations.
C. The Sun
You can change your nighttime telescope into a daytime Sun
viewer by installing an optional full-aperture solar filter over
the front opening of the telescope. The primary attraction is
sunspots, which change shape, appearance, and location daily.
= Magnification
Telescope Focal Length (mm)
Eyepiece Focal Length (mm)
= 28x
= 70x
700mm
25mm
700mm
10mm
