Re: Diffraction rings

From: Brian Tung (brian_at_isi.edu)
Date: 11/29/04 

Date: Mon, 29 Nov 2004 16:58:23 +0000 (UTC)

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 <brian@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
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