Re: Diffraction rings

From: matt (mariusrf_at_bellsouth.net)
Date: 11/28/04 

Date: Sun, 28 Nov 2004 18:25:37 -0500

Steve Maddison wrote in message ...
>Hi all,
>
>I've been reading up on optical theory, but can't seem to find any
>answers to a query or two I have about diffraction rings.
>
>I've had a Meade LX90 for several months, but to this day never seen any
>hint of diffraction rings around the Airy disk of any stars, regardless
>of brightness. I never thought anything of it, but I recently got my
>hands on an Orion 80mm ED refractor, and in this scope the rings are
>obvious, epecially at powers above 100x.
>
>I'm trying to get the reason for this straight. From what I've read, I'd
>expect the rings to be more obvious in the LX90, due to the central
>obstruction, whilst the opposite is true. I'm curious as to which
>factors affect the diffraction pattern of a scope. I can imagine things
>like the quality of the optics and collimation have something to do with
>it, but can't seem to work out how this all fits together.
>
>Whether or not such rings are visibile doesn't bother me too much - I
>find a nice, sharp, ringless disk pleasing enough, although a disk with
>rings also has a certain beauty to it. Is there an ideal or prefered
>situation?
>
>Any, even partial, explanation would be most appreciated, as would a
>link to a web site which covers such topics in detail.
>
>Thanks in advance, and clear skies to you,
>
>
> Steve
>
>
>--
>Steve Maddison
>Den Haag, The Netherlands
>http://www.cosam.org/

#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 . The lx90 Airy disk is
always smudged by atmospheric turbulence . If you want to compare apples to
apples, cut a 3" hole in a piece of cardboard and cover your lx90 aperture
with it, then look for a clean Airy disk .
#2- lx90 is reaching thermal balance slower due to its larger mass and
thermally insulated primary mirror (placed inside OTA with no cooling) . You
can't reach the instrument's limits as long as there are tube currents , and
it takes 1-2 hours to stabilize.

best regards,
matt tudor

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