Light-gathering Power
The light-gathering power (or light grasp) of an optical telescope is directly related to the square of the diameter (or aperture) of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.
A bigger telescope can have an advantage over a smaller one, because their sensitivity increases as the square of the entrance aperture. For example, a 7 meter telescope would be about ten times more sensitive then a 2.4 meter telescope.
For a survey of a given area, the field of view is just as important as raw light gathering power. Survey telescopes such as Large Synoptic Survey Telescope therefore try to maximize the product of mirror area and field of view (or etendue) rather than raw light gathering ability alone.
Read more about this topic: Optical Telescope
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