Two most popular types of telescopes are refracting and reflecting telescopes. Galileo used a refracting telescope. In the simplest refractors, the light passes through the objective lens and the eyepiece lens. The diameter of the objective is known as the aperture. The larger the aperture, the more light gathering power the telescope will have.
Eyepieces let observers select the magnification of their telescopes.
The magnification is determined by dividing the focal length of the objective by the focal length of the eyepiece. The stability of the telescope mounting is important when using high magnification. Resolving power of the telescope is its ability to distinguish between two points whose angular separation is less than the smallest angle that the observer's eye can resolve. As with the light gathering power, the larger the objective, the better resolving power of the telescope. Reflecting telescopes have a concave spherical or parabolic shape mirror located at the lower end of the telescope tube that reflects the light back to a focus instead of refracting it.