The size of the objective lens in a refracting telescope, or the primary mirror in a reflecting telescope, and the distance the light travels through the telescope’s tube are the most important elements in determining the power of the telescope.
The diameter of a lens or mirror is called the aperture; the larger the aperture, the more light can be gathered and focused.
The distance between the center of the lens or mirror and the point where the object’s light rays are bent to a common point is called the focal distance or focal length.
In both kinds of telescopes, the size of the image grows larger when focal distance is longer.
Larger telescopes are more difficult and more expensive to build because of the complexities involved.