Why do astronomers use telescopes with large apertures?

Astronomers use telescopes with large apertures primarily to enhance two critical capabilities: light-gathering power and resolving power.1

Light-Gathering Power

The most significant function of a telescope's aperture—the diameter of its primary lens or mirror—is its ability to collect light. Because a telescope acts like a large bucket for light, a larger aperture allows it to capture significantly more photons than the human eye or a smaller instrument. This increased capacity is essential for observing faint celestial objects, such as distant galaxies, nebulae, and dim stars.12

Resolving Power

Aperture also determines a telescope's resolving power, which is the ability to distinguish fine detail and separate two points that are very close together in the sky. Due to the nature of light diffraction, a larger aperture reduces the size of the Airy disk—the pattern of light created by a point source—allowing astronomers to see sharper images and resolve closely spaced objects like binary stars.13