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Large halo and thick lines around a bright star with a refractor telescope

Astrophotography Refractor
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#1 arasha

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Posted 09 May 2021 - 11:13 PM

I'm using a WO Zenithstar 73 APO, which is a doublet with 430mm F/L and a ZWO ASI294MC Pro for imaging. I was imaging the Iris nebula last night. I took 50x3-minute light frames, followed by flats and dark flats. I also used darks when stacking the subs.

 

There is a bright star on top of the image, and it has a large halo around it. There are also thick lines around it, which remind me of diffraction spikes. I think refractors should not cause diffraction spikes, so I am wondering what could be the source of these lines. Is it normal to have them for bright stars? Or could it be caused by something in my optical train?

 

With regard to the halo, can I do something (other than post-processing) to reduce that?

 

There is also a weird artifact at the bottom of the image, which I think should be removed by flats. However, I did not see the same artifact in my flats. If you know what that could be, and why it wasn't present in the flat frames, please share your thoughts.

 

Thank you.

 

Attached Thumbnails

  • iris_nebula_uncropped_q20_2.jpeg


#2 ravenhawk82

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Posted 09 May 2021 - 11:47 PM

The artifact in the bottom looks like stray light getting into your scope. I get similar artifacts when there is a street light at just the right (wrong?) angle relative to where I'm pointing, so if I had to guess I'd say that there is a light source nearby that was bouncing into your objective. The halo is odd though... My first guess is dew on the glass but I'm not sure why it's taking that shape. I sometimes diffraction spikes in my pictures when the target passes behind a power line but never so wide like that 0_o I'll let someone smarter than me take a guess.


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#3 arasha

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Posted 10 May 2021 - 12:02 AM

The artifact in the bottom looks like stray light getting into your scope. I get similar artifacts when there is a street light at just the right (wrong?) angle relative to where I'm pointing, so if I had to guess I'd say that there is a light source nearby that was bouncing into your objective. The halo is odd though... My first guess is dew on the glass but I'm not sure why it's taking that shape. I sometimes diffraction spikes in my pictures when the target passes behind a power line but never so wide like that 0_o I'll let someone smarter than me take a guess.

You're right. The scope was facing toward our street at a relatively low angle.



#4 Tapio

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Posted 10 May 2021 - 12:08 AM

That looks like microlens diffraction effect.

https://www.cloudyni...fect-explained/


Edited by Tapio, 10 May 2021 - 01:06 AM.

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#5 TOMDEY

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Posted 10 May 2021 - 12:13 AM

That's "normal" >>>

 

Alfirk is magnitude 3.2. That's lateral xy photon blooming induced by the lenticular array, which is ~normal~ and expected. Flat-fielding does nothing to remove that kinda thing. A camera without the lenticules would not manifest that, but such arrays are uncommon and have terribly low QE. All the other stars are of course doing that too, but too dim to notice. The electron antiblooming (typically 1000x) characteristic of your array also suppresses the star itself by that much, so it seems a lot dimmer (by 1000x) than it actually is. Roll all that together and you get exactly what you see there.    Tom


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#6 sharkmelley

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Posted 10 May 2021 - 01:13 AM

You are seeing microlens diffraction.  The ASI294MM suffers from it:

https://www.cloudyni...-35nmzwo-294mm/

 

So I've no doubt that the ASI294MC suffers from it.

 

Mark


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