Page 95 - Modern Optical Engineering The Design of Optical Systems

78 Chapter Five

Figure 5.5 The coma patch. The
image of a point source is spread
out into a comet-shaped flare.

point image formed by a comatic lens is indicated in Fig. 5.5. Obviously
the aberration is named after the comet shape of the figure.
Figure 5.6 indicates the relationship between the position at which
the ray passes through the lens aperture and the location which it
occupies in the coma patch. Figure 5.6a represents a head-on view of
the lens aperture, with ray positions indicated by the letters A through
H and A′ through D′, with the primed rays in the inner circle. The
resultant coma patch is shown in Fig. 5.6b with the ray locations
marked with corresponding letters. Notice that the rays which formed
a circle on the aperture also form a circle in the coma patch, but as the
rays go around the aperture circle once, they go around the image

Figure 5.6 The relationship between the position of a ray
in the lens aperture and its position in the coma patch.
(a) View of the lens aperture with rays indicated by letters.
(b) The letters indicate the positions of the corresponding
rays in the image figure. Note that the diameters of the
circles in the image are proportional to the square of the
diameters in the aperture.