Coma (optics)

Coma (Optics)

Illustration of coma in an optical system

Coma is an optical aberration that occurs in optical systems, such as telescopes or camera lenses. It is characterized by off-axis point sources appearing as comet-like shapes instead of sharp points. Coma can significantly degrade the image quality, especially in astrophotography and other applications where off-axis objects need to be captured accurately.

Causes

Coma is primarily caused by the non-uniform refraction of light rays passing through an optical system. This non-uniform refraction occurs due to imperfections in the lens or mirror surfaces, as well as the design of the optical system itself. Coma is more pronounced in systems with larger apertures and shorter focal lengths.

Effects

When coma occurs, point sources of light away from the optical axis appear elongated and distorted, resembling the shape of a comet. The amount of coma increases as the distance from the optical axis increases. This effect is particularly noticeable in astrophotography, where stars near the edges of the field of view exhibit significant coma.

Correction

Several methods can be employed to correct or minimize coma in optical systems. One common approach is to use multiple lens elements or mirrors with carefully designed shapes and curvatures. These corrective elements can help reduce the non-uniform refraction and bring the off-axis light rays closer to the ideal focal point.

Another technique involves using aspherical lens surfaces, which can compensate for the non-uniform refraction caused by coma. Aspherical surfaces are more complex to manufacture but can significantly reduce coma and other aberrations.

Applications

Coma correction is crucial in various fields where accurate imaging of off-axis objects is essential. In astronomy, for example, telescopes with minimal coma are highly sought after for capturing sharp and detailed images of celestial objects. Coma correction is also important in photography, especially for wide-angle lenses used in landscape or architectural photography.

See Also

• Optical Aberration
• Astrophotography
• Telescope
• Camera Lens