Orion 9854 instruction manual Eyepiece Selection, 700 ÷ 25 =, Magnification Limits

Eyepiece Selection

By using eyepieces of different focal lengths, it is possible to attain many magnifications or powers with the Observer

60.Your telescope comes with two Kellner eyepieces (Figure 9): a 25mm, which gives a magnification of 28x, and a 10mm, which gives a magnification of 70x. Other eyepieces can be used to achieve higher or lower powers. It is quite common for an observer to own five or more eye- pieces to access a wide range of magnifications.

To calculate the magnification, or power, of a telescope- eyepiece combination, simply divide the focal length of the telescope by the focal length of the eyepiece.

Telescope Focal Length (mm) ÷ Eyepiece Focal

Length(mm) = Magnification

For example, the Observer 60, which has a focal length of 700mm, used in combination with the 25mm eyepiece, yields a magnification of

700 ÷ 25 = 28x

Whatever you choose to view, always start by inserting your lowest-power (longest focal length) eyepiece to locate

25mm 10mm

Figure 9. The 25mm and 10mm Kellner eyepieces.

and center the object. Low magnification yields a wide field of view, which shows a larger area of sky in the eyepiece. This makes finding and centering an object much easier. Trying to find and center objects with a high power (narrow field of view) eyepiece is like trying to find a needle in a haystack!

Once you’ve centered the object in the eyepiece, you can switch to a higher magnification (shorter focal length) eye- piece, if you wish. This is recommended for small and bright objects, like planets and double stars. The Moon also takes higher magnifications well.

The best rule of thumb with eyepiece selection is to start with a low power, wide-field eyepiece, and then work your way up in magnification. If the object looks better, try an even higher magnification eyepiece. If the object looks worse, then back off the magnification a little by using a lower-power eyepiece.

Magnification Limits

Every telescope has a useful magnification limit of about 2X per millimeter of aperture. This comes to 120X for the Observer 60. Some telescope manufacturers will use misleading claims of excess magnification, such as “See distant galaxies at 640X!”. While such magnifications are technically possible, the actual image at that magnifica- tion would be an indistinct blur.

Moderate magnifications are what give the best views. It is better to view a small, but bright and detailed image than a dim, unclear, oversized image.

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