Refractors: Coma
 
The effect of coma derives its name from the comet-like appearance that an image can have when a point object is off the axis of a lens. Its cause can be considered by treating a lens as being made of a series of annuli. Each annular zone gives rise to an annular image in the focal plane. The total aberrated image results from the combination of each component image. The effect of coma is illustrated in a simplified way in figure 1.

Figure 1. When coma is present, any annulus of the lens produces an annular image; the total aberrated image can be thought of as being made up of a series of such annular images, the sizes increasing as the outer zones of the lens are considered.
For any single lens, the size of the comatic image depends on the shape factor. In contrast with spherical aberration, it is possible for an image to be free of coma for a particular value of the shape factor. For objects at infinity, the coma-free condition is obtained when the shape factor is close to the value given by
(1)
and for a lens made of glass with refractive index n = 1·5, q = +0·8. This value is close to the value which results in a lens having minimum spherical aberration. When an achromatic doublet is designed, it is possible to correct coma and spherical aberration at the same time. A telescope objective which is free from these defects is known as an aplanatic lens.