Figure 2-64. Astigmatic refraction of light .

TM 9-258

d. Astigmatism is reduced to acceptable quality by

relatively wide portion of a lens. The lens may be

considered to be divided into concentric circular zones or

the use of several lenses in the same manner as

rings of varying thickness. To form a sharply defined

spherical and other aberrations. Lenses are made of

point of light, the rays from each zone must come to a

optical glasses possessing different degrees of

focus at exactly the same place in the focal plane.

refraction, ground to different curvatures, so that the

c. In a lens producing coma, rays of light originating

aberrations of all types cancel each other.

at a point located off the axis and refracted through the

inner zone form a well defined image of the point. Rays

2-41.

Coma.

refracted through the next zone form a larger, less-

defined image of the point which is offset slightly from

a. Coma is due to unequal refracting power of the

the first. The image formed by each successive zone is

various zones or concentric ring surfaces of a lens for

larger, less defined, and farther removed from the initial

rays of light coming from a point which lies a distance off

point of light (fig 2-65). The displacement of the

the axis. It is caused by the rays from the various zones

successive images is in a direction toward or away from

coming to a focus at slightly different points so that they

the center of the field of the lens.

are not exactly superimposed. It appears as blurring of

the images for points off the axis.

b. The image of a point of light is formed by a cone

of useful light rays refracted through a

2-48