From: Richard Schwartz <richas@idt.net>

For a single parabolic reflector, the coma is given by 

k = (3/16) * u / F^2

k is the spot size due to coma
u is the distance from the center of the focal plane
F is the aperture ratio of the mirror.

I got that formula from Frank Ross, "Lens Systems for Correcting Coma of
Mirrors", in SPIE MS-73, p. 241.

I wish somebody with a web page would post this formula!