Dan Chaffee wrote:
On Sun, 28 Nov 2004 18:25:37 -0500, "matt"
wrote:

#1 - size matters. The Orion is diffraction limited at 80mm aperture most of
the time and therefore you see rings. It has a theoretical resolution of
1.73 arcesc , and average seeing is around that value.
The lx90 has a resolution of .69 arcsec and your seeing is never that good ,
even if you're in one of the best places on Earth .

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

I'll break in here to point out that visual detection of long, thin
features complicates interpretations based on the diffraction limit.
The eye is really good at picking up lines at low contrast (that is,
it can be narrower than the "resolution" of the optical system if
its contrast is high enough). Lowell did some experiments on this,
and found that the naked eye could spot black lines only a few arcseconds
wide. One guess where he was going with that, but it didn't
help his case...

Bill Keel