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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 eyepiece employed.
Telescope Focal Length is the distance that light travels inside the telescope before reaching a focus. In
the mirror-lens design of the 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, the 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. For instance, the Super Plössl (SP) 26mm eyepiece supplied
with the three ETX models 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 resolution.
Technical note to the advanced amateur astronomer: The SP 26mm eyepiece supplied with
the Meade ETX-90EC, ETX-105EC, and 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 work in harmony with the ultracompact scale
of the 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, 57X on the ETX-105EC, and 73X on the ETX-125EC. A variety of powers
may be obtained with the addition of optional eyepieces as well as the #126 2X Barlow Lens which doubles
the power of an eyepiece (see OPTIONAL ACCESSORIES, page 20).
Use this formula to calculate the magnification obtained with a given eyepiece:
Most observers should have 3 or 4 eyepieces plus the #126 2X Barlow to achieve the full range of
reasonable magnifications possible with ETX models.
Terrestrial Observing
All three of the ETX models make excellent, high-resolution terrestrial (land) telescopes. When you set the
telescope on its drive base, as shown below in Fig. 9, you may use it for an extremely wide range of
observations. When you look through the eyepiece at a terrestrial object, you will notice that the image is
right-side-up, but reversed left-for-right. Normally, such an image orientation is not bothersome, unless you
are trying to read a distant sign, for example. If the telescope is to be used for extensive terrestrial
observations, a fully correctly oriented image is provided with the #932 45° Erecting Prism (see OPTIONAL
ACCESSORIES
, page 20).
Viewing terrestrial objects requires
looking along the Earth's surface through
heat waves. Heat waves often degrade
image quality. Low-power eyepieces, like
the SP 26mm eyepiece, magnify these
heat waves less than higher-power
eyepieces. Therefore, low-power
eyepieces provide a steadier, higher-
quality image. If the image in an
eyepiece is fuzzy or ill-defined, reduce to
a lower power. Observing in the early
Azimuth
Altitude
Fig. 9: Alt/Az mounting moves the telescope in vertical and
horizontal directions.
Telescope Focal Length
Power =
____________________
Eyepiece Focal Length
Example: To obtain the power of an ETX-125EC (which has a 1900mm focal length) using a
SP 26mm eyepiece:
1900mm
Power =
________
= 73X
26mm
Note:
Throughout this manual,
you will notice the term
"Alt/Az." Alt/Az is
frequently used to refer
to altitude or vertical
and azimuth or
horizontal. Alt/Az is just
one of many methods
used by amateur
astronomers to help
locate stars in the night
sky.