CHAPTER
2
z
9
Chapter 2:TELESCOPE FUNDAMENTALS
INTRODUCTION
Both ETX model telescopes can be used for immediate
observation right out of the box. However, becoming familiar
with the fundamentals of a telescope makes subsequent
viewing easier and more rewarding.
Choosing an Eyepiece
The function of a telescope’s eyepiece is to magnify 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. Low power eyepieces
offer a wide field of view, bright high-contrast images, and eye
relief during long observing sessions. To find an object with a
telescope it is always best to start with a low power eyepiece
such as the SP 26mm supplied with both ETX models. When
the object is located and centered in the eyepiece, switch to a
higher power eyepiece to enlarge the image as much as
practical for prevailing seeing conditions.
Lower power eyepieces are recommended for terrestrial
viewing. Haze, heat waves, and particulate matter in the air
distort images when using higher powers.
For astronomical observing a selection of several different
eyepieces is recommended. For general observing of the Moon
and planets, low to medium powers are preferred. For deep-sky
objects such as nebulae and galaxies, higher powers may be
needed for the best view if conditions permit.
NOTE: Viewing conditions vary widely from night to night.
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 better resolved image (see
Fig.9).
Understanding Magnification
The magnification, or power, at which a telescope is operating is
determined by two factors: the focal length of the telescope and
the focal length of the e yepiece employed.
Telescope Focal Length is the distance that light travels inside
the telescope before reaching a focus. In the mirror-lens design
of both ETX models, however, this focal length is, in effect,
compressed by the telescope’s secondary mirror, so that a long
effective focal length is housed in the short ETX optical tube. For
example, the ETX-90EC’s focal length is 1250mm, or about 49”.
This means that if the ETX–90EC were a classical refracting-
type of telescope, its optical tube would be more than four feet
long instead of the ETX–90EC's compact 11” tube length.
Eyepiece Focal Length is the distance light travels inside the
eyepiece before reaching focus. Focal length is usually printed on
the side of the eyepiece. Both ETX models are supplied with at
least one eyepiece as standard-equipment. For instance, the
Super Plössl (SP) 26mm eyepiece supplied with the ETX-90EC
and ETX-125EC has a focal length of 26mm. “Super Plössl”
refers to the optical design of the eyepiece, a design specifically
intended for high-performance telescopes and one which yields a
wide, comfortable field of view with extremely high image
r e s o l u t i o n .
Technical note to the advanced amateur astronomer: The
SP 26mm eyepiece supplied with the Meade ETX-90ECand
ETX-125EC is a special low-profile version of the standard
Meade SP 26mm eyepiece which is about 1/4” (6mm) shorter
than the standard eyepiece. This low-profile SP 26mm is
designed to harmonize with the ultracompact scale of both ETX
models and utilizes the exact same optics as the standard SP
26mm eyepiece. The SP 26mm low-profile eyepiece is not
parfocal with other eyepieces in the SP series (i.e., the eyepiece
requires re-focusing when it is interchanged with other SP
eyepieces).
Calculating Magnification: On a telescope, such as the ETX,
different eyepiece focal lengths are used to achieve different
magnifications, from low to high. The standard-equipment
SP 26mm eyepiece yields 48X (“48-power”) on the ETX-90EC
and 73X on the ETX-125EC. A variety of powers are obtainable
with the addition of optional eyepieces as well as the #126
2X Barlow Lens which double the power of the eyepiece
employed (see OPTIONAL ACCESSORIES, page 16).
Use this formula to calculate the magnification obtained with a
given eyepiece:
Too Much Power: The most common mistake of the beginning
observer is to overpower the telescope by using high
magnifications, which typical atmospheric conditions and the
telescope’s aperture cannot reasonably support. A smaller but
bright and well-resolved image is far superior to one that is
larger, but dim and poorly resolved (see Fig. 9). Powers above
300X (ETX–90EC) or 500X (ETX–125EC) should be employed
only under the steadiest atmospheric conditions.
Most observers should have 3 or 4 eyepieces plus the #126 2X
Barlow Lens to achieve the
full range of reasonable
magnifications
possible
with both ETX models.
Telescope Mountings
The telescope mounting or mount is the mechanical means that
causes the telescope’s optical tube to move in various
directions. Telescope mounts are of two basic types.
Altazimuth (Alt/Az) mounts permit motion of the telescope
tube in vertical (altitude) and horizontal (azimuth) directions.
Both ETX models incorporate an altazimuth mount (as shown in
Fig. 10). For all terrestrial applications and for casual
astronomical observing, the telescope operates very well in the
altazimuth configuration. The telescope may be placed on a
rigid tabletop or on the optional Meade #883 Deluxe Field Tripod
(see page 17) to provide a secure, variable-height, altazimuth
observing platform. To track objects, either terrestrial or
astronomical, with the telescope in the altazimuth configuration,
press the arrow keys of the Electronic Controller (see page6).
Fig.9: Example of too much magnification (Galaxy M51).
Telescope Focal Length
Power =
____________________
Eyepiece Focal Length
Example: The power obtained with the ETX-125EC using the
SP 26mm eyepiece is:
1900mm
Power =
________
= 73X
26mm
