Is this what I think it is?

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Tony Gondola avatar

This is from a test stack of the SN in NGC-7331 in IR (>685nm). The aperture is 150mm, the imaging focal length is 1800mm. Processing is 2x drizzled so the effective focal length is 3600mm. At this level I’m seeing what looks to me to be a clear diffraction rings around the stars. The white bar just above the SN is 8 arc/sec. long so at that scale, it makes sense. Comments?

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CraigT82 avatar

You’ll never image the airy disc and diffraction rings in a long exposure (from earth anyway). You would need lucky imaging e.g. stacks of very short exposures. The stars in your image look like halos from high cloud or poor transparency?

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Tony Gondola avatar

That’s what I would think too but it was perfectly clear. I have measured it and using a wavelength of 800nm the first minima and first maxima are at about the right radius. I can see that clouds or haze would create halos but it wouldn’t create the dark ring. You’d get a haze of light with a gaussian distribution so I’m still puzzled. I should not that the sub-exposure time was only 15 sec. with the stack made up of 250 out of 1000 subs, culled for lowest FWHM and roundness.

V avatar

I have seen these in IR in several of my images. I can confirm that they are indeed seeing-related, anything below 1.7~ arcseconds and they start to appear.

Tony Gondola avatar

I had at least had that on the night the data was taken.

Tony Gondola avatar

Just to add to this. I’m watching data come in as I write this. I’m imaging the Iris nebula and the seeing is exceptional. I’m watching the red filter data coming in and every time the HFR goes below approximately 2.1 pixels (1.3”) the diffraction pattern that I was seeing with the Ha filter starts to appear around the bright central star when I view it at 1:1. When the seeing bumps the HFR up above that value the pattern breaks up. The subs are 15 sec.

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Tony Gondola avatar

📷 difrac.jpgdifrac.jpg

Aaron H. avatar
The phenomenon of rings around stars when imaging with obstructed optics has been discussed over at CloudyNights quite a few times over the past few years.

See here for the best working theory (courtesy of Mark Shelley):
https://www.cloudynights.com/topic/731780-obstructed-optics-and-rings-around-stars/

Essentially, the central obstruction causes a periodic modulation to the amplitude of the diffraction rings. The brighter rings are spaced widely enough apart to be picked up by a camera sensor.

The larger the obstruction, the more widely spaced the modulation, and the more apparent the rings. I see them on most bright stars with my 6" f/4 Newt. They're rarely visible on my 8" f/5 Newt with its proportionally smaller CO.

Here's an example from a single Ha subframe of Eta Carinae with my 150mm Newt. The large CO makes the rings very apparent.



Zoomed to 400%:
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Tony Gondola avatar

I think sharkmelly nailed it, it is a diffraction pattern. I just would have never guessed that the amplitude of the modulation would die out so slowly. I guess that’s finally a good argument for smaller COs, something that a lot f people tend to dismiss. I have to say it won’t bother me so much as the Hubble has the same issue!

Tommi Liinalampi avatar
You’ll never image the airy disc and diffraction rings in a long exposure (from earth anyway). You would need lucky imaging e.g. stacks of very short exposures. The stars in your image look like halos from high cloud or poor transparency?

Actually, it’s possible to get diffraction rings in images even with longer exposures if the seeing is particularly good and the focal ratio is large, in which case they appear larger. I’ve managed to capture diffraction rings several times when imaging with the EdgeHD 800, but with no other telescope.

But you are right that in that particular image they are probably halos after all. Diffraction rings would look different. I’ve always had clearly more than one diffraction ring when they’ve been visible. Here, only one distinct ring can be seen.
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CraigT82 avatar

Tommi Liinalampi · Aug 24, 2025 at 08:06 AM

You’ll never image the airy disc and diffraction rings in a long exposure (from earth anyway). You would need lucky imaging e.g. stacks of very short exposures. The stars in your image look like halos from high cloud or poor transparency?


Actually, it’s possible to get diffraction rings in images even with longer exposures if the seeing is particularly good and the focal ratio is large, in which case they appear larger. I’ve managed to capture diffraction rings several times when imaging with the EdgeHD 800, but with no other telescope.

But you are right that in that particular image they are probably halos after all. Diffraction rings would look different. I’ve always had clearly more than one diffraction ring when they’ve been visible. Here, only one distinct ring can be seen.

I think you’ve probably experienced the phenomenon described above, rather than resolving the airy disc and diffraction rings.

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Tony Gondola avatar

Here’s a better look at what I was seeing last night when HFR dropped low enough:

📷 result_RED_300s.jpgresult_RED_300s.jpgYou can clearly see it’s evenly spaced multiple rings. I haven’t looked for it with the L filter or no filter but I suspect it won’t be there because of the spread of frequencies but the more monochromatic the light is, the stronger the pattern.

Tommi Liinalampi avatar
Tommi Liinalampi · Aug 24, 2025 at 08:06 AM
You’ll never image the airy disc and diffraction rings in a long exposure (from earth anyway). You would need lucky imaging e.g. stacks of very short exposures. The stars in your image look like halos from high cloud or poor transparency?

Actually, it’s possible to get diffraction rings in images even with longer exposures if the seeing is particularly good and the focal ratio is large, in which case they appear larger. I’ve managed to capture diffraction rings several times when imaging with the EdgeHD 800, but with no other telescope.

But you are right that in that particular image they are probably halos after all. Diffraction rings would look different. I’ve always had clearly more than one diffraction ring when they’ve been visible. Here, only one distinct ring can be seen.


I think you’ve probably experienced the phenomenon described above, rather than resolving the airy disc and diffraction rings.

These rings are fully seeing related, but if there is another explanation for these, I can't be 100% sure what they are. When I have seen these rings, it's the situation when FWHM is less than 2.0 arcsec.
Aaron H. avatar
Tommi Liinalampi:
Tommi Liinalampi · Aug 24, 2025 at 08:06 AM
You’ll never image the airy disc and diffraction rings in a long exposure (from earth anyway). You would need lucky imaging e.g. stacks of very short exposures. The stars in your image look like halos from high cloud or poor transparency?

Actually, it’s possible to get diffraction rings in images even with longer exposures if the seeing is particularly good and the focal ratio is large, in which case they appear larger. I’ve managed to capture diffraction rings several times when imaging with the EdgeHD 800, but with no other telescope.

But you are right that in that particular image they are probably halos after all. Diffraction rings would look different. I’ve always had clearly more than one diffraction ring when they’ve been visible. Here, only one distinct ring can be seen.


I think you’ve probably experienced the phenomenon described above, rather than resolving the airy disc and diffraction rings.

These rings are fully seeing related, but if there is another explanation for these, I can't be 100% sure what they are. When I have seen these rings, it's the situation when FWHM is less than 2.0 arcsec.

They are rings caused by diffraction, but not the individual diffraction rings.

Effectively, you are seeing a pattern around every nth diffraction ring, where "n" depends on the wavelength of the narrowband filter, the focal ratio and the size of the central obstruction.
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Tony Gondola avatar

This might explain something I would often see in my high resolution, lucky lunar imaging work. Sometimes there would be a light echo from small brightly lit areas that are on a dark background, circles of light inside small craters and around protruding peaks as you would find in Mare Imbrium. At first I just thought it was an artifact of sharpening but it wasn’t the cause as I could see it before sharpening was applied. The sharpening just made it easier to see. This would show up with greater intensity when the data was from an SCT. I’ve never seen it with a refractor.

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John Hayes avatar

Mark is a very sharp guy and as usual, he got this right! Inspired by his post on CN, I did my own quick analysis of this phenomenon. Here’s what the Air rings look like for an unobscured aperture showing the rings on a log scale (in db).

📷 image.pngimage.pngYou can clearly see that they monotonically decrease in amplitude into the quantization noise of the calculation in the extreme wings. If I then add a 47% obscuration (following Mark’s example), you get the following PSF.

📷 image.pngimage.pngYou can clearly see the periodic structure in the ring pattern that Mark identified and that makes sense. When you have an obscured aperture, the Airy pattern becomes the difference of the the diffraction pattern for the entire aperture minus the diffraction pattern for the obscuration. Because the two diffraction patterns have slightly different spatial frequencies, the difference produces a Moire pattern with a beat frequency that is lower than the fundamental frequency of the unobscured main aperture. That’s why the pattern becomes more easily visible—even when the seeing isn’t all that good.

The spatial frequency of the Moire pattern will of course depend on the diameter of the outer aperture and the size of the obscuration so it will be lower for small apertures and higher for larger apertures. This is probably why I’ve never observed this pattern with my 24” scope even when the seeing is pretty good. It will always be more visible for smaller scopes with a moderately large obscuration under pretty good seeing.

Finally I have to say, “Good catch Tony!” You pointed out something that’s actually an interesting and relevant optical property of obscured apertures. I’d never really thought much about this and it’s an interesting effect. It’s really just another “optical signature” for these types of optical systems.

John

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Tony Gondola avatar

Thanks John. It’s great to know that this hobby can still throw a surprise now and then. The learning continues…

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John Hayes avatar

There is one thing that I forgot to mention. The scale of the Airy pattern depends on the wavelength. That means that the contrast of the rings is pretty low in broadband so this effect should be tough to see with a Lum filter. It will get more visible with RGB filters and it will be most prominent with narrow band filters. It will also be easier to see with filters that pass longer wavelengths (red/NIR).

John

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Tony Gondola avatar

That’s exactly what I’ve observed.