Why does a telescope with a larger aperture show fainter objects?

A telescope's ability to show fainter objects is primarily determined by its light-gathering power, which is a direct function of the diameter of its aperture—the clear objective lens or mirror that collects incoming light.12

Because a larger aperture provides a greater surface area, it can collect significantly more light from a celestial source than a smaller opening, such as the human pupil. This increased light collection allows the instrument to detect objects that are too dim to be seen by the naked eye or by smaller telescopes.31

The light-gathering power of different apertures is calculated by the ratio of their diameters squared. For example, a telescope with a 25-cm aperture collects four times as much light as one with a 12.5-cm aperture, as the light-gathering capacity scales with the square of the diameter ([25 × 25] ÷ [12.5 × 12.5] = 4). By capturing more light, astronomers can observe fainter stars, nebulae, and distant galaxies that would otherwise remain invisible.1