Meade Autostar Manuel d'instructions télécharger pdf (Page 15)

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

OPTIONAL ACCESSORIES, 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.

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Meade Autostar Manuel d'instructions Page 15

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