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A telescope's confusion limit refers to a limit on its effective angular resolution due to many sources angularly-near each other making it difficult to separate one from another, a condition termed confusion (or source confusion). A confusion-limited image is an image "bumping on" this limit. This is analogous to the limits imposed by diffraction, such as for a diffraction limited telescope. Confusion limiting is a bigger issue for wavelengths longer than visible light, e.g., far infrared, millimeter astronomy and radio astronomy. Portions of the spectrum with stronger cosmic background radiation (CBR) relative to the brightness of sources will have the problem. The term is often used when extended sources crowd a field. The confusion limit is a major factor influencing the required integration time to produce useful results.
Some globular clusters may create the issue in optical imaging.
Some explanations use term confusion limited to include the confusion of Airy disks, i.e., the effects of diffraction and aperture size, but the term diffraction limited is generally used for observations limited by that effect.