Diffraction rings

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/

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

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 . 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.

Thanks, Matt, for the swift response - I shall sleep better for it! I
suspected seeing would have its say in the situation but hadn't gotten
as far as to figure out the link. The 3" mask sounds like a nice
experiment to try out.

#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.

This one I'd thought of - I probably should have mentioned that I
checked after the scope had a chance to reach thermal eqilibrium.

Thanks again,

Steve

--
Steve Maddison
Den Haag, The Netherlands
http://www.cosam.org/

Hi Steve,

A couple of thoughts:

(1) If you really want to pump them up, tape a piece of cardboard to make
your CO larger for the test g

(2) Cooldown is critical with SCTs.

(3) Collimation helps. Go out of focus until you have several diffraction
rings and collimate until you have them perfectly centered at the center of
the FOV.

(4) Wait until a very good night. With the larger scope you want better
seeing to push it up against the limits of diffraction. Otherwise you will
not see rings in that scope.

(5) If you are concerned about the scope, try star testing it or contact a
local astro club to have someone look at it. Then enjoy!

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

Are you interested in optics?
Try http://groups.yahoo.com/group/ATM_Optics_Software/

************************************


"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/

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


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 .

The resolution on an 8" is 0.56" arc seconds, so if he needs 100x
to see diffraction rings with the 80mm, he'll need at least 250x
on the 8" and it would be better to use around 400x for clarity.
Also, cutting an off axis stop will not allow him to properly evaulate
the airy disc on the 8" since it will represent only a fraction of
the optical surface and won't provide a true picture of what it's
like.
Also, seeing conditions do not always "smudge" out the diffraction
rings on an 8" scope, but you will need a steady night to see them.


#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

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.

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.

Dan C.

The distance you must travel inside or outside of focus to achieve
DR's varies with focal length; differs for the 80mm widefield refractor
vs the longer focal length lx90. Rack the lx90 further in or out to
find the diff rings vs. your 80mm refractor.



Steve Maddison wrote:

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/

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

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

(Brian Tung) wrote in message ...

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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o | | |
/ /
/

Dan C.