Dan Chaffee wrote:
Even if you're in one of the best places on Earth???
Well, I'm sure not and:

I agree with you here. Pic du Midi is surely sub-arcsecond much of the
year. Since seeing is a time-varying process, the seeing will surely
better 0.7 arcseconds often enough to justify putting an observatory up
there.

The rille in the lunar Alpine Valley is considerably less than .69
arcsec wide along its narrower expanses and its full length is
visible several nights a year here in the Midwest in my 9.6 inch
newtonian. Enke is even less and shows up once or twice a year here in
that scope, although technically we cannot call it truely resolved. I
see the first diffraction ring many nights a year in it as well. In an
8 in. scope with a big obstruction it should be even easier to see the
diffraction pattern.

Detection of reasonably high-contrast features is not the issue here,
especially linear high-contrast features. The Cassini division, after
all, was discovered in a scope that was "too small" to resolve it.

Besides seeing and warm telescopes, a big culprit for not seeing
distinct rings is rough optics. Poorly corrected, but relatively
smooth systems show in-focus diffraction rings brighter and greater
in number in stable conditons than well corrected ones. A badly
collimated scope will show rings on one side of the disk and not the
other.

In my experience, what produces that effect is not miscollimation
(since the caustic is fairly symmetrical) but something like spherical
aberration. That subdues the rings outside focus, assuming the SA is
positive.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt