Uploaded June 2025 | Updated September 2026, 2 hours ago
Coma is a form of optical aberration getting its name from the comet like tail it's best identified by. Coma is often highlighted in astronomical images because stars make ideal point sources to reveal this telltale sign. Objects near the center of the image will be unaffected, while objects near the edges of the image form comet like distortions that point towards the system's optical axis.
Coma is caused by the asymmetry of a beam's rays through an optical element, with off-axis rays or a tilted lens being the most common cause. When a beam is parallel to the lens' axis, rays above and below the axis have symmetric paths and focus to circular spots. Beams that deviate from the optical axis create asymmetry, causing both larger and irregularly shaped spots at focus.
This is caused by rays at the top of the beam having different angles of incidence than rays at the bottom of the beam, which influences the amount of coma present. As a result, the amount of coma is also impacted by the beam’s position in the lens, and the beam’s pupil height can be adjusted to minimize coma for a given configuration of off-axis angle.
Along those lines, rays that are parallel to the optical axis, but offset from the center of the lens can also create coma if the lens has spherical aberration. In this case, the coma is a direct result of spherical aberration, but the lack of symmetry about the optical axis results in coma. The best way to minimize coma is to center both the lens and the beam along the optical axis.
Coma is a form of optical aberration getting its name from the comet like tail it's best identified by. Coma is often highlighted in astronomical images because stars make ideal point sources to reveal this telltale sign. Objects near the center of the image will be unaffected, while objects near the edges of the image form comet like distortions that point towards the system's optical axis.
Coma is caused by the asymmetry of a beam's rays through an optical element, with off-axis rays or a tilted lens being the most common cause. When a beam is parallel to the lens' axis, rays above and below the axis have symmetric paths and focus to circular spots. Beams that deviate from the optical axis create asymmetry, causing both larger and irregularly shaped spots at focus.
This is caused by rays at the top of the beam having different angles of incidence than rays at the bottom of the beam, which influences the amount of coma present. As a result, the amount of coma is also impacted by the beam’s position in the lens, and the beam’s pupil height can be adjusted to minimize coma for a given configuration of off-axis angle.
Along those lines, rays that are parallel to the optical axis, but offset from the center of the lens can also create coma if the lens has spherical aberration. In this case, the coma is a direct result of spherical aberration, but the lack of symmetry about the optical axis results in coma. The best way to minimize coma is to center both the lens and the beam along the optical axis.
