Meade ETX-125EC Understanding Magnification, Terrestrial Observing, 1900mm Power = = 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

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).

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

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).

astronomers to help locate stars in the night sky.

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.

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