Going over the various objective-eyepiece combination spot diagrams in "Telescope Optics" (Rutten & van Venrooij), it seems that the Nagler gets pretty darn good as f-ratio increases. At f/15 it's almost perfect, even very far off-axis. An f/8 APO yeilds extremely tight spots also. It would be interesting to try the Naglers in long-focus Maks & refractors. I've only ever used them in fast Dobs, where the diagrams and my own experience show the performance to be much worse than a good ortho. This makes sense; long-focus instruments tend to have a much larger well-corrected prime focus, a condition which complements the Nagler's wide-field perfectly. A fast Dob, really anything faster than about f/7 or f/8 in this case, is presenting the eyepiece with a horribly curved field off-axis, in comparison. I like the approach of the Rutten & van Venrooij book, that is, that eyepiece properties should be chosen to complement objective properties. Abberations can cancel each other out quite effectively. It also shows that there can be no "perfect" eyepiece for all telescope types and f/ratios. C. __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com