Choosing an Eyepiece
A telescope’s eyepiece magnifies the image formed by the telescope’s
main optics. Each eyepiece has a focal length, expressed in millimeters,
or “mm.” The smaller the focal length, the higher the magnification. For
example, an eyepiece with a focal length of 9mm has a higher
magnification than an eyepiece with a focal length of 25mm.
Your telescope comes supplied with a Plössl 25mm eyepiece which gives
a wide, comfortable field of view with high image resolution.
Low power eyepieces offer a wide field of view, bright, high-contrast ima-
ges, and eye relief during long observing sessions. To find an object with a
telescope, always start with a lower power eyepiece such as the Super
Plössl 26mm. When the object is located and centered in the eyepiece,
you may wish to switch to a higher power eyepiece to enlarge the image
as much as practical for prevailing seeing conditions.
The power, or magnification of a telescope is determined by the focal
length of the telescope and the focal length of the eyepiece being used. To
calculate eyepiece power, divide the telescope's focal length by the
eyepiece's focal length. For example, a 25mm eyepiece is supplied with
the Messier-Series. The focal length of the 8" Messier series model is
900mm (see Specifications, page 20).
Telescope Focal Length ÷ Eyepiece Focal Length = Eyepiece Power
Telescope Focal Length = 1000mm
Eyepiece Focal Length = 25mm
Telescope Focal Length 1000 mm
Magnification = = = 40
Eyepiece Focal Length 25 mm
The magnification is therefore 40X (approximately).
Looking at or near the Sun will cause instant and irreversible damage to your eye!
13
FIRST STEPS
Fig. 15a+b: Jupiter; examples of
the right amount of magnification
and too much magnification
Messier Tips
Too Much Power?
Can you ever have too much power? If the type of power you’re referring to is
eyepiece magnification, yes, you can! The most common mistake of the beginning obser-
ver is to “overpower” a telescope by using high magnifications which the telescope’s
aperture and atmospheric conditions cannot reasonably support.
Keep in mind that a smaller, but bright and well-resolved image is far superior to one that
is larger, but dim and poorly resolved (see Figs. 15a and 15b). Powers above 200X
should be employed only under the steadiest atmospheric conditions.
NNoottee::
Seeing conditions vary widely
from night-tonight and site-to-site.
Turbulence in the air, even on an
apparently clear night, can distort
images. If an image appears fuzzy and
ill-defined, back off to a lower power
eyepiece for a more well-resolved
image.
(see Fig. 15a and 15b below).
