This is why the quantity (2S m + Siv) was calculated in Table 3.3.

Consider first the astigmatism term. As with coma, the total wavefront aberration is

2 ym because cos0 = +1 and cos n = — 1. The 'positive focus change at azimuth 0 is accompanied by a negative focus change at azimuth n/2 because of the term cos 2^. Figure 3.19 shows the wavefront aberration (w'ii)gf for the sections ^ = 0 and ^ = n/2.

The practical consequence is shown in Fig. 3.20. The principal ray E'Ipr leaves the axis at the exit pupil at E' and cuts the image plane at I . Rays in the tangential section (plane of the paper) focus at the point It on the tangential astigmatic surface, whereas rays in the sagittal section (at right angles to the plane of the paper) focus at Is on the sagittal astigmatic surface.

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