The aperture of a telescope is the diameter of the light collecting region, assuming that the light collecting region has a circular geometry . For an optical instrument, the aperture is the diameter of the objective lens (refracting telescope) or the primary mirror (reflecting telescope). The larger the aperture, the more light the telescope can gather, and the fainter the limiting magnitude of the instrument. The field of view of the telescope decreases as the aperture increases, but the resolving power increases.

For ground-based telescopes, increasing the aperture is often the easiest way to improve observations of faint objects. However, larger telescopes become more susceptible to the small-scale fluctuations (turbulence or seeing) in the Earth’s atmosphere.

The largest aperture refracting telescope is the 40-inch telescope at the Yerkes Observatory, University of Chicago. Larger objective lenses are subject to physical constraints that reduce their usefulness:

• The high mass of the lens causes it to sag;
• It is difficult to make very large defect-free lenses; and
• Chromatic aberration must be corrected.

Larger aperture reflecting telescopes are more common as astronomical instruments, such as the 8-metre Gemini telescopes (Mauna Kea, Hawaii and Cerro Pachón, Chile) and the 10-metre Keck telescopes (Mauna Kea, Hawaii). Large reflectors like these use either a single large mirror (which can be deformed mechanically from behind) or a segmented mirror.

Apertures are sometimes expressed in terms of the dimensionless f-number:

f/# = f/D

where f is the focal length of the telescope, and D is the diameter of the aperture. Both f and D are measured in the same units.