Coma
Above: Spot diagram of a star at the edge of the field affected by coma
Coma is an off-axis aberration. Stars in the center of the field are not affected by coma, but the effect grows stronger toward the edge of the field. Stars affected by pure coma are shaped like little comets (hence the name) pointed toward the center of the field. In the diagram above the center of the field is down. In essence, what occurs to cause coma is that light passing through the center of the field (but at an angle so it focuses off-axis) does not focus at the same distance along the optical path as light from farther off axis.
Above: Diagram of how coma arises in an optical system
Since coma affects the edges of a field, and grows larger with increasing distance from the optical axis, it is a significant aberration with regards to wide-field viewing and imaging. For professional astronomers and advanced amateurs who are interested in scientific study, coma can be very problematic because it is an asymmetrical aberration. This is problem because it makes it impossible to accurately measure the position of stars (astrometry). For this reason, most professional instruments are designed specifically to eliminate coma (although sometimes by introducing another less problematic aberration). For most amateur astronomers, a small amount of coma is tolerable. For wide-field applications, some telescope designs eliminate or minimize coma, while coma-correcting lenses are available for other designs to minimize the effect if desired.
Telescope Designs with Coma
The classic example of coma comes from the Newtonian telescope. Coma is the primary aberration inherent in the Newtonian design and is the limiting factor in this design. Most Newtonian designs show coma at the edge of the field. Coma is a function of both off-axis distance and focal ratio, meaning faster-focal-ratio (smaller f-number) telescopes will have more coma than a similar size but slower telescope. Therefore, an 8" f/4 Newtonian has more coma than an 8" f/6 Newtonian. f/4 is usually considered the fastest a Newtonian can be made without having excessive coma. Coma correctors are available that can minimize the amount of coma in a Newtonian design. These lenses fit into the focuser ahead of the eyepiece of camera. They are typically used on f/5 and faster telescopes.
Most commercial Schmidt-Cassegrain and Maksutov-Cassegrain designs also suffer from coma. Since they typically have long focal ratios, in the range of f/10 to f/15, the coma is less than in a similar-sized Newtonian. The amount of coma is not normally problematic when observing, but can appear at the edges of a large photographic field. Note that coma is not necessarily inherent in the Schmidt- and Maksutov-Cassegrain design, but exists because of the choice of optical parameters chosen to minimize the cost of manufacturing these commercial scopes. See theOptical Design section on Schmidt-Cassegrains for more details.
Telescope Designs Free from Coma
Some telescope designs are free from coma. Classical Cassegrain telescopes, for example, have coma inherent in the design. But by slightly changing the configuration of the mirrors, the very similar Ritchey-Chrétien design eliminates coma but instead suffers from astigmatism. This compromise is made because astigmatism is a symmetrical aberration and allows astronomers to make accurate positional measurements. For amateur astronomers interested in viewing or taking pretty pictures, there is little advantage to one aberration over the other.
Most refractors have little or no coma, contributing to their being well-suited to wide-field viewing and imaging.
Other imaging systems such as hyperbolic astrographs, Schmidt cameras, and other uncommon designs are usually designed to be free from coma.