Diffraction-limited

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The resolution of an optical imaging system like a telescope or camera can be limited by multiple factors like imperfections in the lenses or misalignment. There is however a fundamental maximum to the resolution of any optical system which is due to diffraction. An optical system with the ability to produce images with angular separations as small as the instrument's theoretical limit is said to be diffraction limited.

In astronomy, a diffraction-limited observation is one that is only limited by the optical power of the instrument used, as opposed to seeing-limited, where the main limit is given by atmospheric effects.

The resolution of a given instrument is proportional to the diameter of its objective, and inversely proportional to the wavelength of the light being observed. The size of the smallest feature in an image that is diffraction limited is the size of the Airy disc.

Most telescopes on the Earth at ground level work at a much lower resolution than diffraction-limited, because of the distortion introduced by the passage of light through several kilometres of turbulent atmosphere. Some advanced observatories have recently started using adaptive optics technology, resulting in greater resolution as the technology improves, but it is still difficult to reach the diffraction limit. Radiotelescopes are better in this regard, because the wavelengths they use (from millimeters to meters) are so long that the atmospheric distortion is negligible.

Space-based telescopes (such as the HST, or a number of non-optical telescopes) always work at their diffraction limit, if their design is free of optical aberration.