Re: Diffraction rings

From: Dan Chaffee (dchaffee_at_blitz-it.net)
Date: 11/30/04 

Date: 29 Nov 2004 16:11:31 -0800

brian@isi.edu (Brian Tung) wrote in message news:<cofkff$a42$1@zot.isi.edu>...

> > 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.

Actually, the rille in the Alpine Valley is not high contrast; much
less
so than Cassini and even Enke. Look at Andrea T's shots:
http://www.geocities.com/andreatax/moon.htm
The context of my point is that the seeing IS
good enough in some instances to allow the full resolving power of an
8" telescope to be realized.

> 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.
>

I meant on one side of the image of the disk, not either side of
focus.
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Dan C.