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.