speckles

Speckles are flaws in an image obtained in a telescope: such flaws often show as spots on the image. Among the causes are noise (e.g., photon noise), seeing issues, diffraction, aberrations, and stray light within the optics (and the analogous issues for other bands such as radio). By definition, speckles affect images in any electromagnetic-radiation band, and the term speckle is commonly used for visible light images as well as ultraviolet, infrared, and radio astronomy, including radio interferometry. The term speckle suppression refers to techniques for eliminating or reducing speckles.

Speckles caused by short-term random phenomena (e.g., seeing) can often be averaged out over the long term, or can be addressed by adaptive optics. Causes of long-term speckles can be flaws in the optics, i.e., systematic errors, which can be analyzed and addressed or compensated for. Speckles that fall between (some are termed quasi-static speckles, QSS), such as those that vary from repointing the telescope, e.g., when the aim is adjusted to follow a source (changing the stray light, or affecting the geometry of the telescope), are sometimes the limiting factor in images produced.