dC = 2 [pi]J0 (p[rho]) [rho] d[rho], (12).
For the complete circle,
_ pR
2[pi] /
C = ----- | J0(z) zdz
p^2 _/0
2[pi] /p^2R^2 p^4 R^4 p^6 R^6 \
= ------ ( ------ - ------- + ----------- - ... )
p^2 \ 2 2^2.4^2 2^2.4^2.6^2 /
2J1(pR)
= [pi]R^2 . ------- as before.
pR
In these expressions we are to replace p by k[xi]/f, or rather, since
the diffraction pattern is symmetrical, by kr/f, where r is the
distance of any point in the focal plane from the centre of the
system.