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Focus Knob (5, Fig. 1): Turning this knob causes a finely controlled internal motion of the telescope's primary
mirror to achieve precise focus of the image. The ETX can be focused on objects from a distance of about 11.5 ft
(3.5m) to infinity. Rotate the focus knob clockwise to focus on distant objects; rotate the focus knob
counterclockwise to focus on near objects.
Flip-Mirror Control (7, Fig. 1): The ETX includes an internal optically-flat mirror. The Flip-Mirror Control is in the
"up" position when the control is vertical (perpendicular to the main telescope tube). It is "down" when the outer
edge of the control is horizontal (parallel with the main tube of the telescope).
With the Flip-Mirror Control in the "up" position, light is diverted, at a 90° angle, to the eyepiece. Alternately, with
the Flip-Mirror Control in its "down" position, light proceeds straight through the telescope and out the Photo Port
(6, Fig. 1), for photography using the #64 T-Adapter, or for observing with the #932 45° Erecting Prism.
3. 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: Generally speaking, telescope focal length is the distance that light travels inside the
telescope before reaching a focus. In the mirror-lens design of the ETX, however, this focal length is, in effect,
compressed by the telescope's secondary mirror, so that a long effective telescope focal length is housed in the
short ETX optical tube. The ETX's focal length is 1250mm, or about 49". If the ETX were a classical refracting-type
of telescope, its optical tube would thus be more than four feet long instead of the ETX's compact 11" tube length.
Eyepiece Focal Length: The 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. The Meade ETX is supplied with one eyepiece as
standard equipment, a Super Plössl (SP) 26mm eyepiece. Thus, the focal length of the provided eyepiece is
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 Meade ETX
telescopes is a special low-profile version of the standard Meade SP 26mm eyepiece and is about 114" (6mm)
shorter than the standard eyepiece. This low-profile SP 26mm is designed to harmonize with the ultracompact
scale of the ETX telescope and utilizes the exact same optics as the standard SP 26mm eyepiece. The SP 26mm
low-profile eyepiece is not parfocal, however, with other eyepieces in the SP series (i.e., the eyepiece requires
refocusing when it is interchanged with other SP eyepieces).
Calculating Magnification: On a given 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, as stated above,
yields 48X. Optional eyepieces and the #126 2X Barlow Lens are available for powers from 31X to over 300X. To
calculate the magnification obtained with a given eyepiece, use this formula:
Telescope Focal Length
Power
=
Eyepiece Focal Length
Example: The power obtained with the ETX with the SP 26mm eyepiece is:
1250mm
Power =
= 48X
26mm
The most common mistake of the beginning observer is to "overpower" the telescope by using high magnifications
which the telescope's aperture and typical atmospheric conditions can not reasonably support. Keep in mind that a
smaller, but bright and well-resolved image is superior to one that is larger, but dim and poorly resolved. Powers
above 300X should be employed with the ETX only under the steadiest atmospheric conditions.
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