The light-gathering power of a telescope is determined by the diameter of its aperture, or opening—that is, by the area of its largest or primary lens or mirror. The primary optical element in a telescope is either a convex lens (in a refracting telescope) or a concave mirror (in a reflector) that brings the light to a focus.

A telescope collects the faint light from astronomical sources and brings it to a focus, where an instrument can sort the light according to wavelength. The resolution of a visible-light or infrared telescope is degraded by turbulence in Earth’s atmosphere.

light-gathering power of a telescope — determined by the diameter of its aperture, or opening—that is, by the area of its largest or primary lens or mirror. seeing — unsteadiness of Earth’s atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady. refracting telescope — telescope in which the principal light collector is a lens or system of lenses. resolution — detail in an image; specifically, the smallest angular (or linear) features that can be distinguished.