By fun I meant that I *like* calculating things by hand. Having a formula rather than some numbers can also be more enlightening, although in this case knowing that the coma increases linearly in the third-order expansion is not much more enlightening than doing the ray-tracing for a few different displacements from the axis and noticing that it increases.
Also, I have an Amiga, not a PC. I'm sure there's a ray-tracing program available for the Amiga somewhere. Although I like thinking about optical designs (I've computed the formulas for a two-mirror Schiefspiegler), I don't really need to do ray-tracing yet. For quite a while I'll be struggling to produce a good Newtonian. (I did the Schiefspiegler partly for fun and partly because I couldn't find the formulas published anywhere.)
Clay cspence@sarnoff.com