[ATM] Schmidt Corrector Calculator

[Richard F.L.R. Snashall]

I searched several "google's worth" of pages, but could not
find a three-term calculator (except maybe JODAS).  So:

    http://users.rcn.com/rflrs/scorr.xls

It basically is modeled after Kingslake's ("Lens Design
Fundamentals") proof.  It does, however, allow for either
front and back side aspherics (or some combination) and some
displacement of the corrector from the exact mirror CoC.