Rs Rt Rp

When Astm 3 is the only third-order aberration, the beam converges on two perpendicular segments which, on the principal ray, are separated by the so-called astigmatism length, i.e. the distance between the tangential and sagittal focal surfaces.

If Sni = 0 and if Sphe 3 and Coma 3 are small, then the least confusion images are obtained on the mean curvature focal surface whose curvature is

as represented by the fourth righthand term of (1.41).

If Astm 3 vanishes (SIII = 0), then Rm = Rs = Rt = RP: the four surfaces merge into one, the Petzval surface.

In the general case, depending on the sign of Astm 3, an off-axis ray intersects these four surfaces by following equal path intervals between them, and in the subscript order (t, m, s, P) or (P, s, m, t) of their radius (Fig. 1.21).

Similarly as Wj4] in (1.41) for the third-order aberrations, other functions may be considered for the analysis of aberrations at any order. The general form of t

0 0

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