An optical defect in reflector telescopes in which in-focus star images appear progressively more triangular or comet-like the closer they get to the edge of the field of view. The faster the focal ratio, the more prominent the coma. The visually coma-free field of a telescope in millimeters is roughly equal to the square of the scope's focal ratio - an f/5 focal ratio scope has a 25mm field (5 squared = 25), an f/6 scope has a 36mm field (6 squared = 36), etc. Since a 1.25" eyepiece barrel only about 29mm in internal diameter, and a 35mm film negative or slide measures 44mm across its diagonal, it can be seen that even a 25mm coma-free field is more apparent in photos than it is in most visual observing. Coma can superficially appear similar to a star's image in a poorly collimated telescope. With coma, however, the brightest portion of the comatic wedge (actually the Airy disk) always points toward the center of the field. This differs from an out-of-collimation telescope, where the Airy disks are all offset to the same side of the diffraction rings, no matter where in the field the star image is located.

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