TheTelescope:

An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors.

There are three primary types of optical telescope: Refractors (Dioptrics) which use lenses,

Reflectors (Catoptrics) which use mirrors, and Combined Lens-Mirror Systems (Catadioptrics) which use lenses

and mirrors in combination; for example, the Maksutov telescope and the Schmidt camera.

A telescope's power is directly related to the diameter (or aperture) of the objective lens or mirror.

The larger the lens is, the more light the telescope can collect.