Figure 5.1 shows the essential parts of a telescope: an objective and an eyepiece (ocular). The objective forms an image, and you use the eyepiece as a magnifier to look at it. The objective and eyepiece each normally have more than one lens element.
The aperture of the telescope is the diameter of the objective, which determines the amount of light it collects. This is the most important optical parameter. An 8-inch or 20-cm telescope, for instance, is one with an aperture of 8 inches (20 cm).
The focal length of the telescope determines the image size. If the objective is a simple lens, then the focal length is the distance from lens to image. With more complicated telescopes, the focal length may be appreciably more than the physical length of the telescope; it is defined in terms of the simple lens that would give an image of the same size.
The magnification (power) of the telescope depends on the eyepiece:
Focal length of telescope
Focal length of eyepiece
For example, a telescope with 2000 mm focal length and a 25-mm eyepiece magnifies 80x because 2000 ^ 25 = 80.
That is why astronomical telescopes are not rated as "10 x" or "100 x" the way binoculars and microscopes are. By changing eyepieces, you can get any power you want, but not all powers work equally well.
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