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Fig. 23a & 23b: Jupiter: Examples
of the right amount of
magnification and too much
magnification.
Fig. 22: Meade Super
Plössl eyepieces.
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 observer is to “overpower” a telescope by using high magnifications
which the telescope 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. 23a and 23b). Powers
above 300X should be employed only under the steadiest atmospheric
conditions.
Autostar can calculate the best eyepiece for you to use. Try out the “Eyepiece
Calc” feature in the Utilities menu.
Most observers should have three or four additional eyepieces to achieve the full
range of reasonable magnifications possible with the ETX telescopes. See
OPTIONALACCESSORIES, page 43.
ETX TIPS
Choosing an Eyepiece
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.
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, the focal length is
compressed by the telescope’s secondary mirror, so that a long focal length is housed
in the short ETX optical tube. For example, the ETX-90AT’s focal length is 1250mm
or about 49”. This means that if the ETX–90AT were a classical refracting-type of
telescope, the optical tube would be more than four feet long instead of the
ETX–90AT'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 example,
26mm. Low-power eyepieces are typically available in 26mm, 32mm and 40mm sizes
(see OPTIONAL ACCESSORIES, page 43). The low-power eyepiece provided with your
ETX is designed to provide a wide, comfortable field of view with high image resolution.
Calculating Magnification: On a telescope, such as the ETX, different eyepiece
focal lengths are used to achieve different magnifications, from low to high. A 26mm
eyepiece yields 48X (“48-power”) on the ETX-90AT, 57X on the ETX-105AT and 73X
on the ETX-125AT. 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 eye-
piece (see
OPTIONAL ACCESSORIES, page 43). To calculate magnification, divide the
focal length of the telescope’s optical tube by the focal length of the eyepiece. For
example:
Telescope Focal Length divided by Eyepiece Focal Length = Power
1250mm divided by 26mm = 73X
With Barlow: 73 x 2 = 146X
The eyepiece power or magnification is therefore 73X and 146X with the Barlow.
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.
Tip: If you plan on
viewing using higher
magnification eyepieces,
first locate, center and
focus the object using a
low-power eyepiece
(e.g., 26mm eyepiece).
Then remove the low-
power eyepiece and
replace it with a higher-
power eyepiece; the
object should still be
centered in the field of
view. Objects are much
easier to locate and
center at lower powers;
higher powers are
employed simply by
changing eyepieces.