
5.8. Aberrations for Decentered Pupil 103
where Ri and 7?2 ^^^ the radii of curvature of the first and second surfaces,
respectively, of the lens and the index n of the lens is positive. Setting R2 = oo
gives a lens whose flat surface faces the image surface and, though this is not
required, we make this choice for convenience. Therefore the Petzval surface is
flat if
nRi R
Because R of
the
mirror is negative, so also is i^j, hence the lens is plano-convex
in cross section (thickest at the center) and has positive power. Though the
combination now has a flat Petzval field, the thin lens may introduce some
astigmatism. We show in Chapter 9 tha ...