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Equipment Discussions >> Reflectors

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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Diffraction rings new
      #5924565 - 06/16/13 10:42 PM

When you have your telescope in focus, do you see one (or more?) of these around the star and is your star a dot?

Does it matter what magnification you are using to be able to see them?

I ask because I don't think I've ever seen these like I see pictures of in texts or on websites. If stars are faint, they just appear as dots and if they are bright they are more like fireballs with some flame coming off of them in multiple directions (aside from the diffraction spikes I get with bright stars).

Anyway, I'm just wondering. I can't remember seeing an in focus diffraction ring before with any scope I have used and wondered if reflectors don't show these and they are only seen in refractors.

Thanks,

Rob


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Diffraction rings new [Re: robininni]
      #5924578 - 06/16/13 10:46 PM

I rarely see them on stars due to reliably poor seeing. But pinholes in foil placed over a flashlight and located down the other end of my building's hallway will show *lovely* Fresnel patterns in my small refractor when the exit pupil is about 1mm and smaller--especially smaller.

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Jason D
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Re: Diffraction rings new [Re: robininni]
      #5924765 - 06/17/13 01:30 AM

With my 10" reflector, I see the airy disc and the first diffraction ring at high magnification, say above 500X, but on nights with good seeing -- not perfect but good. Still, it will not be a “stable” image like the photos but it tends to dance around a little. I select stars with magnitudes between 3 and 5. If the star is too faint then I can't see the diffraction ring. If it is too bright then seeing condition will show a ball of fire.
Here is the thing. With my premium optics, I see only the first diffraction ring and it is fainter than the airy disc. For my other reflectors with good but not premium optics, I still see one ring but it is brighter compared to the premium optics. For my other reflector with what I consider to be lower than average optics, I see multiple rings.
As optics quality degrades, energy is shifted from the airy disc to the diffraction rings. The worse the optics, the more rings you would see.
Bottom line: If you see more than 1 ring at focus then optics are more on the lower side of the quality scale.
Jason


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Diffraction rings new [Re: Jason D]
      #5924880 - 06/17/13 05:37 AM Attachment (65 downloads)

Here's mine...Arcturus at 300x on a good night.

Often the 2nd and 3rd ring are not solid but in motion and appear as very thin arcs. It takes a very calm moment to get them to settle down and then only briefly. At some moments, I can actually tell one is slightly brighter than the other, but I cannot remember which. My notes say the 2nd ring is a bit brighter than the 3rd.

The 4th and 5th rings are fairly visible on the brightest stars, and the 5th outermost ring is noticeably fainter. It comes and goes with seeing while the 4th is more persistent.

On dimmer stars down to about mag 8, only the first bright ring is visible and noticeably dimmer than the Airy disc. However, on the brightest stars, like Arcturus and Capella, it's really hard to tell a difference in brightness between the Airy disc and the first ring.

Edited by Asbytec (06/17/13 05:51 AM)


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Jon Isaacs
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Reged: 06/16/04

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Re: Diffraction rings new [Re: robininni]
      #5924929 - 06/17/13 06:49 AM

Quote:

Anyway, I'm just wondering. I can't remember seeing an in focus diffraction ring before with any scope I have used and wondered if reflectors don't show these and they are only seen in refractors.




Rob:

The size of airy disk and diffraction ring structure is inversely proportional to the aperture. Larger scopes produce smaller Airy disks with correspondingly smaller diffraction rings.

With a 4 inch refractor, it might take 200x or more to see the rings, in a 10 inch, as Jason said, it generally takes 500x or more, this is a 1/2mm exit pupil. Larger scopes require better seeing because to see the ring, the star must be a point source, clean and round.

With you 25 inch, a 1/2mm exit pupil would be about 1300x, it would be rare to have seeing stable enough to see the rings.

Jon


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GlennLeDrew
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Reged: 06/18/08

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Re: Diffraction rings new [Re: Jon Isaacs]
      #5924959 - 06/17/13 07:35 AM

Jon,
The corollary to the inverse proportionality with aperture is the inverse proportionality with exit pupil. No matter the aperture, the apparent size of the Fresnel pattern is the same at given exit pupil diameter. This is one reason why the exit pupil is a fundamentally important aspect of optics used afocally.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Diffraction rings new [Re: GlennLeDrew]
      #5925101 - 06/17/13 09:38 AM

Interesting point, Glenn. At what exit pupil would a diffraction pattern subtend 120" arc and resolved by the human eye?

I think that's comfortably done around 26x per inch, so the exit pupil should be about 1mm, give or take? So, in his 25", it could be done at 650x provided seeing permitted it.

Rob, rest assured refractors are not the only scope to show them, in fact that's the basis for a lot of discussion on contrast between types.


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Mirzam
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Reged: 04/01/08

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Re: Diffraction rings new [Re: Asbytec]
      #5925172 - 06/17/13 10:27 AM

The reason is that refractors are smaller apertures (generally) and so have larger airy disks.

I see beautiful airy disks with my 60 mm refractor. That does not mean that it is a better optic than my 10-inch, where the disks are extremely tiny and hard to see clearly on most nights.

JimC


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Jon Isaacs
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Reged: 06/16/04

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Re: Diffraction rings new [Re: GlennLeDrew]
      #5925451 - 06/17/13 12:43 PM

Quote:

Jon,
The corollary to the inverse proportionality with aperture is the inverse proportionality with exit pupil. No matter the aperture, the apparent size of the Fresnel pattern is the same at given exit pupil diameter. This is one reason why the exit pupil is a fundamentally important aspect of optics used afocally.




Glenn

Thanks for that, it is a simpler way to say what I was trying to say, I will remember that. I still would want to provide some support since its not so intuitive.

In response to Norme's suggestion that a 1mm exit pupil would be sufficient, my experience suggests that it would not be. In a 4 inch that would be 100x, 250x in a 10 inch. My experience is that a 0.5 mm exit pupil is where I can begin to see the rings and more is better. With a scope with brighter rings (larger CO) , less magnification is necessary but with larger Newts and refractors, this has been my experience.

Jon

Edited by Jon Isaacs (06/17/13 02:08 PM)


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GlennLeDrew
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Reged: 06/18/08

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Re: Diffraction rings new [Re: Jon Isaacs]
      #5925697 - 06/17/13 02:50 PM

Jon,
The fact that it's recognized that scopes (of equal quality, and neglecting atmospheric seeing) are all limited to the same magnification per unit aperture is the direct result of the Fresnel pattern subtending the same apparent angle at given exit pupil.

And the apparent angle of the Fresnel pattern defines the perceived image sharpness. Say you have a small and big scope trained on Jupiter, with both working at, say, a 0.5mm exit pupil. The bigger scope naturally provides a bigger image with increased detail commensurate with the aperture.

But qualitatively, both images will suffer the same degree of perceived 'softness' because the Fresnell pattern for each subtends the same angle on the retina. Let me elaborate with this...

Suppose both scopes have the same f/ratio, but one has twice the aperture. Combining optics put each image through a beam splitter so that both can be viewed simultaneously, side by side, with a single eyepiece. One image will be twice as large, and reveal linear detail twice as fine. The view would look exactly like just one scope viewing two Jupiters, one planet being twice the diameter of the other. The diffraction-induced blur--let's say it's of width 2 arcminutes on the retina--is identical for both images.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Diffraction rings new [Re: GlennLeDrew]
      #5926328 - 06/17/13 08:49 PM

Quote:


Jon,
The fact that it's recognized that scopes (of equal quality, and neglecting atmospheric seeing) are all limited to the same magnification per unit aperture is the direct result of the Fresnel pattern subtending the same apparent angle at given exit pupil.






Glenn.. indeed, that's the derivation of the exit pupil and aperture/inch guidelines, the linear scaling with aperture, the resolution of the telescope versus the resolution of the eye...

But while it's recognized, it's not necessarily widely known or understood...

Jon


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Nils Olof Carlin
Carpal Tunnel
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Reged: 07/26/04

Re: Diffraction rings new [Re: GlennLeDrew]
      #5926842 - 06/18/13 04:18 AM

Quote:

The corollary to the inverse proportionality with aperture is the inverse proportionality with exit pupil. No matter the aperture, the apparent size of the Fresnel pattern is the same at given exit pupil diameter. This is one reason why the exit pupil is a fundamentally important aspect of optics used afocally.




This is fundamental.
A simple experiment to give you an idea of what exit pupils allow you to see:
Take some thin aluminium (I have used soda/beer cans) and drill/poke several holes of different diameters (as circular as you can make them!) - from 2 mm down to 0.5 mm or less.
For stars, you can use the same material with holes (also in pairs to simulate double stars) placed in front of a fluorescent lamp (or any small light source).

This simulates a telescope with perfect, unobstructed optics and in practice no seeing degradation, with a magnification of 1 and the exit pupil = hole diameter.

Nils Olof


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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Diffraction rings new [Re: Nils Olof Carlin]
      #5926938 - 06/18/13 07:43 AM

Nils Olof,
I essentially did this very experiment many years ago. Placing such a variety of hole diameters before the unaided eye and noting the effects on a daytime scene is most instructive!


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Diffraction rings new [Re: GlennLeDrew]
      #5927286 - 06/18/13 11:14 AM

Interesting.
In my 12.5" newt. I cannot see diffraction rings on bright stars because the diffraction spikes are too bright.
I cannot see diffraction rings on faint stars because the points are too small and the rings too faint.
I can sometimes see them on magnitude 6-8 stars if I look carefully. It seems to take 230-305X in my 12.5" to make them show up.

I used to see them all the time in my 8" SCT, showing how much easier it is to see them when there are no diffraction spikes. Seeing was the factor that determined how many rings could be seen. In the best possible seeing, I could see one ring. In mediocre seeing, several rings.


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Darren Drake
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Reged: 10/09/02

Loc: Chicagoland
Re: Diffraction rings new [Re: Starman1]
      #5927474 - 06/18/13 12:47 PM

I've seen them many times through my 18 inch. Its sometimes easier to look at close doubles near the limit of resolution to confirm that the airy disks are indeed being seen. 2 nights ago I observed the 0.49 arcsecond double zeta bootis to see them as well as a few other close doubles.

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Jason D
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Reged: 10/21/06

Loc: California
Re: Diffraction rings new [Re: Darren Drake]
      #5927519 - 06/18/13 01:19 PM

Quote:

I observed the 0.49 arcsecond double zeta bootis



With my 10" scope, I see clearly both airy discs touching at 600X but the discs are clearer at 1200X. Of course seeing has to be good and the image at 1200X is bouncy but discernible.
Jason


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Diffraction rings new [Re: Darren Drake]
      #5928308 - 06/18/13 09:40 PM

Quote:

I've seen them many times through my 18 inch. Its sometimes easier to look at close doubles near the limit of resolution to confirm that the airy disks are indeed being seen. 2 nights ago I observed the 0.49 arcsecond double zeta bootis to see them as well as a few other close doubles.




Darren:

According to Sky Tools 3 which computes the orbits of short period binaries like Zeta Bootes, the separation is currently at 0.43 arc-seconds, with in the reach of an 18 inch but slightly beyond the Dawes limit of a 10 inch/250mm, at 0.46 arc-seconds.

A few nights ago I took a look at it with my 250mm GSO Dob, nothing, not even a hint, just a nice round star even at 800x. I had split it a couple of years ago. Later in the evening I realized I was confused and I had been looking at Eta Bootes rather than Zeta Bootes..



Looking at Zeta Bootes, like Jason, I saw a peanut with a ring but couldn't make out a Dawes split. That was with my favorite combination, the 10 inch F/5, the Paracorr, a 2x Celestron Shorty Barlow and a 3.5mm Nagler.

Jon


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bunyon
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Reged: 10/23/10

Loc: Winston-Salem, NC
Re: Diffraction rings [Re: Jon Isaacs]
      #5928333 - 06/18/13 09:58 PM Attachment (35 downloads)

I feel compelled to share this image of Zeta Bootis. It includes a diffraction ring. For what it's worth, I rarely notice diffraction rings in my 15" unless I'm imaging. I'm not sure what that suggests.

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Diffraction rings new [Re: bunyon]
      #5928445 - 06/18/13 11:36 PM Attachment (21 downloads)

I like diffraction rings, actually...you can "read them" and they are striking on doubles.

Edited by Asbytec (06/18/13 11:38 PM)


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Mirzam
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Re: Diffraction rings new [Re: bunyon]
      #5928738 - 06/19/13 07:19 AM

Nice job on the image of the close double! Was your effective focal length just 4800 mm? Wow.

JimC


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