> The often
> quoted example is a 6 inch f/10 sphere has just less than 1/4 wave (peak to
> peak wavefront error) of spherical abberation.
Only at focal plane where transverse aberration is minimal. You can easily pick another plane (refocus) where 6" f/10 sphere gets much better (almost 1/8 wave at the front). Use any of the available wavefront calculation programs (mirr5zone or Tex), both will come with similar result (1/7.3 and 1/7.5 wave). Suiter's Aperture will give the same result, but with heaps of additional information, including OTF and the _real_ look of diffraction disk at focus, and extrafocal images. Great program !
Bratislav