multi coating: Meade: ACF Celestron: Starbright XLT secondary: According to Telescopes, Eyepieces, and Astrographs by Smith et al, page 250, aspheric secondary coma-free SCT's like the Meade ACF have prolate ellipsoidal secondaries. In other words, a stretched sphere. The primary is still spherical. The schmidt corrector still addresses spherical aberration. The secondary addresses coma but not field curvature. Theoretical on-axis performance is not quite as good as in a standard Schmidt Cassegrain, but off axis spot sizes are, of course, much better due to the suppression of coma. An 8" f/10 version (per Smith et al) should be diffraction limited over a 20 arc minute diameter vs. 7 arc minutes for a standard SCT. That means a diffraction-limited field of roughly 12mm for that same 8" f/10 scope. They don't happen to have theoretical values for an f/8 version. For comparison, the aplanatic flat-f...