Choosing An Eyepiece; The Barlow Lens - Meade DS-2000 Instruction Manual

5.

Choosing an Eyepiece

A telescope's eyepiece magnifies the image formed by the telescope's main optics. Each eye-
piece 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 low-powered 25mm eyepiece which gives a wide, com-
fortable field of view with high image resolution.
Low power eyepieces offer a wide field of view, bright, high-contrast images, and eye relief dur-
Fig. 15: 25mm and
ing long observing sessions. To find an object with a telescope, always start with a lower power
9mm eyepieces.
eyepiece such as a 25mm. 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 pre-
vailing seeing conditions.
DEFINITION:
The power, or magnification of a telescope is determined by the focal length of the telescope
A capital "X" is used
and the focal length of the eyepiece being used. To calculate eyepiece power, divide the tele-
to denote an eye-
scope's focal length by the eyepiece's focal length. For example, you may wish to use a 25mm
piece's (or a
eyepiece with the DS-2000 model DS-2114S. Look up the focal length of the DS-2114S under
Barlow's) power or
"Specifications: DS-2114S," page 33. The focal length is listed as 1000mm.
magnification. For
example, "40X" is
read as "40-power."
The eyepiece power, or magnification is therefore 40X (approximately).

The Barlow Lens

Some Meade telescopes include a power multiplier called a Barlow lens. Consisting of a lens
mounted in a 4"-long (10cm) tube, the Barlow doubles or triples the power obtained when an
eyepiece is used alone. In the example above, a 25mm eyepiece results in 40X magnification
with the DS-2114S telescope; when this same eyepiece is used in conjunction with a 2x Barlow
lens, power is doubled to 80X. To use the Barlow, insert it into the diagonal prism (refractor
models only) or eyepiece holder, followed by the eyepiece.
image) has been aligned to the main telescope. Fig. 13B shows these same images after
the viewfinder and main telescope are aligned.
Check this alignment on a celestial object, such as the Moon or a bright star, and make
any necessary refinements.
Viewing conditions vary widely from night-to-night and site-to-site.
NOTE:
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.
Telescope focal length divided by Eyepiece focal length = Eyepiece power
1000 Ö 25 = 40
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