Optical aberrations are deviations from geometric optics that are mainly caused by faults in optical lenses. The main types of aberrations are spherical, comatic, distortion, astigmatism, field curvature and chromatic aberration.
Coma or comatic aberration is similar to spherical aberration, caused by off-axis light fluxes. Comatic aberration is more pronounced when the microscope is out of alignment. Optical coma aberration is regarded as the worst among aberrations due to the asymmetry of the image that it causes. The resulting shape of the image distorted by comatic aberration resembles that of a comet with its tail, hence the name coma.
Occurrence and Causes of Comatic Aberration
Coma is caused by the curvature of the principal planes of the optical system. Coma can occur even at short distances from the principal axis. The comet like shape in comatic aberration is due to the refraction differences when light rays pass through the various zones of the lens.
The extent of distortion depends on the shape of the thin lens. In the absence of any aberrations rays passing through a lens will converge at the focal point behind the lens. Rays that are traversing in off axis regions produce varying transverse magnification. Coma becomes apparent when the rays from the object enter the lens at an oblique angle causing the image to be off axis.
There are two types of coma aberrations - negative and positive. The alignment of the tail of the comet shape depends on the type of comatic aberration.The shape of the lens causes the meridional rays passing through its periphery to arrive at the image plane that is closer to the axis when compared to the rays near the axis.
When the peripheral rays produce the smallest image, the comatic aberration is termed negative. In cases wherethe peripheral rays are focused further down the axis, causing greater magnification, the comatic aberration is termed positive.
In negative coma, the tail of the comet is pointing away from the center of the field of view, while in positive coma it points towards the field of view. A strongly concave positive meniscus lens will cause negative comatic aberration, while a bi-convex or plano convex lens will cause negative to zero coma. Positive coma occurs when objects are viewed through the convex side of a plano-convex or a convex meniscus lens.
The comatic image is referred to as a comatic circle, which is divided into different regions, namely, tangential coma, sagittal coma and coma flare (the comet’s tail).
Ways to Minimize Coma Aberration
Concluding from the above,coma can be minimized by choosing the right kind of lenses. Coma of a single lens system can be minimized (even eliminated in some cases) by choosing the apt curvature for the lens to match the application. Lenses that are capable of eliminating both spherical and comatic aberration are called aplanatic or bestform lenses. By using the right combination of lenses that are positioned symmetrically around a central stop, comatic aberration can be corrected.
Sources and Further Reading
- Aberrations – Texas A&M University – Kingsville
- Optical Aberration – Princeton University
- An Introduction to the Aberrations of Optical Systems – Montana University
- Optical Aberrations – Florida State University