An astronomical telescope consists of two lenses. The first lens, the objective lens, focuses parallel rays from a distant object to the focal point of the first lens. The focal points of the objective and eyepiece lenses coincide – this is called normal adjustment. The image formed by the objective lens serves as the object for the eyepiece lens. Since this object is at the focal point of the objective lens, the image formed by the eyepiece lens is at infinity.
The angular magnification is given by
The length of the telescope is
A typical value for f-o is 1000 mm and for f-e 25 mm. For these values
Magnifications of several hundred are often quoited but these can be misleading because a major factor in the quality of an image is the amount of light collected.