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Concentric rings around bright stars in fast newt?

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#1 stefannebula

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Posted 09 May 2020 - 04:17 AM

Hello, hopefully this is the right subforum for this question! 

 

I recently bought a new fast Newtonian, Sharpstar Hyperbolic Newtonian 150 F2.8 (6", 420mm FL, F2.8)  and  I've noticed that some of the brightest stars in my image show faint concentric rings. I was wondering if anyone may be able to shed some light on what may be causing this. I'm thinking perhaps internal reflections from my Ha filter?

 

The following image is a 4:1 crop of Eta Carina. It is in the center of the frame but I noticed the same effect with another star near the edge. 

 

EQUIPMENT IN OPTICAL PATH

Sharpstar 150 Hyperbolic Newtonian (6", 420mm FL, F2.8)

Optolong 36mm, 6.5nm Ha filter

ZWO ASI183MM-Pro

 

EXPOSURE PARAMETERS

single raw 180s exposure, stretched

Gain 100

Offset 10 

Temp. -20C

 

 

aNbVemU.jpg

 

 

 

Any advice welcomed, thanks smile.gif



#2 imtl

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Posted 09 May 2020 - 04:26 AM

Maybe you are getting reflections from your filter? It could be some interference with your camera. You are using quite a fast scope. Are you sure your filter is good with that speed? If not it will give some artifacts.

Thats my opinion anyways



#3 stefannebula

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Posted 09 May 2020 - 04:41 AM

Maybe you are getting reflections from your filter? It could be some interference with your camera. You are using quite a fast scope. Are you sure your filter is good with that speed? If not it will give some artifacts.

Thats my opinion anyways

Thanks for the reply, 

The filter I'm using (Optolong) is mid-range, it's not designed for faster systems (like the Baader F2 filters). I thought this would just lead to less light transmission because of wavelength-shift, but I agree it could be the cause. I will try using my other filters and see if that removes it.. 

Thanks



#4 Tapio

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Posted 09 May 2020 - 04:43 AM

This looks like zoomed pretty much so doubt it will look too bad without zoom.

Anyway, I agree with previous writers that it could be the filter.

Why not test without also.



#5 TOMDEY

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Posted 09 May 2020 - 04:48 AM

At F/2.8 the lenticular array will generally start to ~ring~ Nother thing is that the circles are quite uniform in spatial frequency and phase... which could be posterizing artifact of your digital file format and stored bitdepth inadequate to capture sufficient shades of gray (8 vs 12 vs 16)    Tom

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  • 39 rings around star.jpg


#6 stefannebula

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Posted 09 May 2020 - 04:59 AM

At F/2.8 the lenticular array will generally start to ~ring~ Nother thing is that the circles are quite uniform in spatial frequency and phase... which could be posterizing artifact of your digital file format and stored bitdepth inadequate to capture sufficient shades of gray (8 vs 12 vs 16)    Tom

Thanks for the detail Tom, 

 

I'm not very experienced with optics, what is the lenticular array? 

The image file is a raw FITS but the ASI183 camera captures with a 14bit depth so I guess I understand that that could cause the artifact.

 

I did some goolgling and found this image from the Hubble Space Telescope 

 

https://i.stack.imgur.com/Sour1.png

 

This visually seems very similar to what I have observed, but apparently this is Airy-like diffraction, which I doubt is the case in my situation? 


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#7 stefannebula

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Posted 09 May 2020 - 05:00 AM

This looks like zoomed pretty much so doubt it will look too bad without zoom.

Anyway, I agree with previous writers that it could be the filter.

Why not test without also.

You're right it's extremely zoomed, and I don't mind it too much lol.gif  Just wondering if anyone is familiar with the cause



#8 TOMDEY

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Posted 09 May 2020 - 05:15 AM

Thanks for the detail Tom, 

I'm not very experienced with optics, what is the lenticular array? 

The image file is a raw FITS but the ASI183 camera captures with a 14bit depth so I guess I understand that that could cause the artifact.

I did some goolgling and found this image from the Hubble Space Telescope 

https://i.stack.imgur.com/Sour1.png

This visually seems very similar to what I have observed, but apparently this is Airy-like diffraction, which I doubt is the case in my situation? 

I have become convinced that it is the filter internal stacked reflections. You could further process to smooth that out, but just understanding its genesis is probably sufficient to happily live with it. Here is a closely-related illustration from one of my ancient white-papers regarding such affectives. How that then creates (artifactual) uniformly-spaced rings around bright stars is an easy computation. I can derive the equations and confirm that is consistent with your filter thickness, substrate index of refraction, F#, and pixel pitch... if you like. Let me know and I'll do that.   Tom

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  • 41 filter front back ringing Tom.jpg

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#9 stefannebula

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Posted 09 May 2020 - 05:31 AM

I have become convinced that it is the filter internal stacked reflections. You could further process to smooth that out, but just understanding its genesis is probably sufficient to happily live with it. Here is a closely-related illustration from one of my ancient white-papers regarding such affectives. How that then creates (artifactual) uniformly-spaced rings around bright stars is an easy computation. I can derive the equations and confirm that is consistent with your filter thickness, substrate index of refraction, F#, and pixel pitch... if you like. Let me know and I'll do that.   Tom

Tom, thanks again for the detailed explanation and thanks for the diagram

That sounds like the most probable answer to me! Thanks for offering to verify it theoretically, but I think first I'll just repeat the exposure with no filter to verify empirically! 

Thanks a lot!

Stefan


Edited by stefannebula, 09 May 2020 - 06:11 AM.


#10 RJF-Astro

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Posted 09 May 2020 - 06:28 AM

Hi stefan, I have the same issue on my 6RC, regardless of filter (Baader, ZWO, Chroma) and bit depth. My RC is at f/6. My other scopes did not have this issue. Usually it is gone when I combine different filters (although this may lead to color artifacts in stars, which I somethimes have). But they always show up on single filter b/w images. This is the Christmas Tree cluster for instance:

 

20200205 NGC2264 Cone.jpg

 

I have learned to live with it, does not bother me much. I suspect there is something in the mirrors which causes this, not sure.



#11 Tapio

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Posted 09 May 2020 - 08:50 AM

That image looks more like ASI1600 diffraction pattern. https://www.cloudyni...ght-star-issue/

#12 RJF-Astro

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Posted 09 May 2020 - 09:07 AM

Nah, I have that too on bright stars, but it looks different. These are perfect circles. The microlensing is more square.

#13 RJF-Astro

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Posted 09 May 2020 - 11:22 AM

Ok so here are a few more examples, to show the difference.

 

The first is definitely microlensing, although circles are also visible. This is η Gem, mag 3.3 (Baader Ha):

 

20200118 IC443.jpg

 

The second is WR 136 in the center of the Crescent nebula. This one does not have visible microlensing, but it does have concentric circles (also Baader Ha):

 

20190622 NGC6888 3h.jpg

 

The same star with the Baader O3, less pronounced but still visible:

 

20190902 NGC6888 4h4 O3.jpg

 

I will have to search for recent Chroma examples, I have not used these filters a lot yet. But I have seen them on test shots I take from bright stars when focussing. The RGB I have found so far do not show the rings, but I am certain I have seen them. Will have to verify that though.



#14 Alex McConahay

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Posted 09 May 2020 - 12:42 PM

Are you dithering? ANd, I assume this is a single shot, not a stack. 

Alex



#15 DrGomer

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Posted 09 May 2020 - 12:48 PM

Any way this can be related to the baffling rings?


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#16 RJF-Astro

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Posted 09 May 2020 - 03:16 PM

Dont know about OP, but I am dithering. These are from stacked images, but subs are the same.

Baffling could be possible with the RC. I could count them and see if it matches, will try that. But I dont know if the f/2.8 newt has baffles?

#17 DrGomer

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Posted 09 May 2020 - 06:39 PM

Just checked some other images. No baffle with the sharpstar, so that is ruled out. 



#18 Alex McConahay

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Posted 09 May 2020 - 09:45 PM

You know. a perfectly diffracted star does in fact have rings. They do not look like that.....they are spaced differently, and decrease as they get away from the center. 

 

The fact that they are present in a "Stacked" image is intriguing. 

 

Alex



#19 stefannebula

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Posted 10 May 2020 - 01:13 AM

Hello all, thanks for the replies and ideas! 

RJF-Astro that looks identical to what I observe! 

 

There are no baffling rings present in my reflector. 

The rings I observe are also present in a stacked image, where very-high dithering was applied for every subframe

This was taken with the ASI183 which as far as I know doesn't suffer from the same microlensing as the ASI1600.

 

Doesn't bother me too much, I'm more just intrigued. Glad to see someone else has observed this before though.

 

This is a zoom of eta-carina the same as before, but this is a stack of 5 * 180s exposures taken back-to-back with very-high dithering applied for every subframe

IVDtX08.png



#20 RJF-Astro

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Posted 16 May 2020 - 04:41 PM

Ok, I have an update. This weekend I exchanged the 6RC for my 130PDS newt. I was focussing the newt on Arcturus with the Chroma 3nm Ha filter. And then I saw the rings again. So I must have missed them before.

 

That rules out a lot of options for me. The only thing in common in the optical train is the camera. Mirrors, correctors and filters are all different. Could it be the ASI? Not just the 1600, maybe the AR window used on these cameras? I think that is the only constant factor here. Could that fall under your theory Tom?

 

Single__Ha_2020-05-16_23-27-13_Bin1x1_7s__-5C.jpg



#21 sharkmelley

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Posted 29 September 2020 - 03:19 AM

Here's my attempt at an explanation:

https://www.cloudyni...s-around-stars/

 

In brief, it's a larger scale pattern imposed on the Airy rings by the central obstruction.

 

Mark




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