Why do I get this Halo in my exposures?

Acquisition Deep Sky Help Needed Vignetting
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Menelaos avatar
Menelaos avatar
Hi everyone! So I am new to astrophotography and I recently upgraded my gear with a Samyang 135mm f/2 Lens with a canon 250d modified. The Lens seems very sharp but I get a strange halo in all my exposures. It looks like vignetting but I'm not quite sure. The image was taken in a very light polluted area and I tried using a light pollution filter as well but still I get the Halo. Is it light pollution? Does anyone know what this is and how to fix it? The image in the example is a single exposures stretched. I tried stacking and processing all the way with calibration frames and everything and I still get the halo in the image. I'm starting to think there is a problem with my Lens or my sensor. Thank you in advance.

https://astrob.in/deg1b7/0/
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dkamen avatar
dkamen avatar
Lack of flats or incorrect flats/darks/dark flats.

Having the same lens, it *is* very difficult to correct it especially under high light pollution due to stray light. Aim for sky flats, much better than using a tablet or other artificial light source.
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Brian Puhl avatar
Brian Puhl avatar
Bad flats more than likely.     You say this is a single exposure though?  Single exposures wont be calibrated, so we'd need to see the stack to really understand what's going on.   Vignette is pretty common, and one of the main purposes of flats is to get rid of it.  Make sure you're aiming for the middle of the histogram on your flat exposures.   Unlike dkamen, I highly recommend against sky flats, especially at your focal length due to uneven light gradients that the sky will produce.  Amazon has great, cheap light (tracing) panels that you can use and they work great.    Make sure you take your flats in a dark location, this helps eliminate any possibly light leaks.       I can also say most folks I know who use the 135mm typically have issues shooting wide open.   Maybe try stopping down a couple, it may go away.
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Dan H. M. avatar
Dan H. M. avatar
The Rokinon 135 is a full frame lens so these types of issues should not be caused by shooting at f/2.  And judging by the star spikes on the brighter stars it looks like the lens is already stepped down, no?

In either case this just looks like bad flats.  How exactly are you taking them?
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Menelaos avatar
Menelaos avatar
Thank you guys for all the info. I looked into taking better flats and i will improve on that. The lens was also stepped down a couple stops. However, what i thought might be the issue is dust particles in the atmosphere or humidity perhaps? The picture was taken on a night with moonlight and dust (although very subtle) in the atmosphere. Because with similar settings i took pictures from the same location the same object and the frames didn’t have that weird halo in them. I had strong gradient from light pollution but no halo or vignette whatsoever. After that the weather was a bit unstable and i cant seem to take good pictures anymore. So could this actually be the cause?
Menelaos avatar
Menelaos avatar
The Rokinon 135 is a full frame lens so these types of issues should not be caused by shooting at f/2.  And judging by the star spikes on the brighter stars it looks like the lens is already stepped down, no?

In either case this just looks like bad flats.  How exactly are you taking them?

Thank you guys for all the info. I looked into taking better flats and i will improve on that. The lens was also stepped down a couple stops. However, what i thought might be the issue is dust particles in the atmosphere or humidity perhaps? The picture was taken on a night with moonlight and dust (although very subtle) in the atmosphere. Because with similar settings i took pictures from the same location the same object and the frames didn’t have that weird halo in them. I had strong gradient from light pollution but no halo or vignette whatsoever. After that the weather was a bit unstable and i cant seem to take good pictures anymore. So could this actually be the cause?
Dan H. M. avatar
Dan H. M. avatar
The Rokinon 135 is a full frame lens so these types of issues should not be caused by shooting at f/2.  And judging by the star spikes on the brighter stars it looks like the lens is already stepped down, no?

In either case this just looks like bad flats.  How exactly are you taking them?

Thank you guys for all the info. I looked into taking better flats and i will improve on that. The lens was also stepped down a couple stops. However, what i thought might be the issue is dust particles in the atmosphere or humidity perhaps? The picture was taken on a night with moonlight and dust (although very subtle) in the atmosphere. Because with similar settings i took pictures from the same location the same object and the frames didn’t have that weird halo in them. I had strong gradient from light pollution but no halo or vignette whatsoever. After that the weather was a bit unstable and i cant seem to take good pictures anymore. So could this actually be the cause?

Yeah, a bright moon near the target can really mess things up, particularly when combined with poor transparency (whether caused by moisture in the air or dust or haze). 

Do you use the lens hood?  I am thinking of putting black felt on the inside of my lens hood to block more stray light.
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dkamen avatar
dkamen avatar
Bad flats more than likely.     You say this is a single exposure though?  Single exposures wont be calibrated, so we'd need to see the stack to really understand what's going on.   Vignette is pretty common, and one of the main purposes of flats is to get rid of it.  Make sure you're aiming for the middle of the histogram on your flat exposures.   Unlike dkamen, I highly recommend against sky flats, especially at your focal length due to uneven light gradients that the sky will produce.  Amazon has great, cheap light (tracing) panels that you can use and they work great.    Make sure you take your flats in a dark location, this helps eliminate any possibly light leaks.       I can also say most folks I know who use the 135mm typically have issues shooting wide open.   Maybe try stopping down a couple, it may go away.

Thing is with that lens at f/2.8 or more open, a nearby light source is just too directional. For example, I have found it matters a great deal whether you have the hood on (I actually had to remove it to get workable flats with a tablet, probably because things got reflected on the hood walls). The sky has gradients and that is a drawback but at least everything is at infinity.

That said, flats have been my nemesis. In every combination of scope/camera/lens that I have ever used, they just never seem correct well enough.  So I am not exactly the authority on the subject

Cheers,
Dimitris
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