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Telescopes extend our view of the cosmos in several ways. They collect more light than the human eye, magnify distant objects with an eyepiece, and—most critically—resolve closely spaced objects. That resolving capability is known as a telescope’s resolving power.
The resolving power is directly linked to the diameter of the telescope’s objective—its light‑gathering aperture. In refractors the objective is the front lens; in reflectors it is the primary mirror; in Schmidt‑Cassegrain designs the primary mirror also serves as the objective. As the aperture grows, so does the ability to distinguish fine detail.
Resolution is bounded by the diffraction limit, which represents the smallest angular separation between two visible points. The limit is expressed in arcseconds and decreases as the aperture’s diameter increases. Larger telescopes can therefore separate objects that appear much closer together.
Because diffraction scales with wavelength, longer wavelengths produce a higher diffraction limit. For instance, a one‑meter telescope achieves a diffraction limit of about 2.5 arcseconds in the near‑infrared, whereas the same aperture reaches roughly 0.1 arcseconds in blue light. Consequently, the same instrument delivers sharper images at shorter wavelengths.
The Earth’s atmosphere introduces refractive turbulence, commonly called “seeing,” that blurs stellar images. To mitigate this, the largest ground‑based observatories are sited on high, dry mountain peaks, and space‑based platforms—such as the Hubble Space Telescope—eliminate atmospheric effects entirely.
