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Halo in my images

Sky-Watcher Explorer 150PDS Canon EOS 6D Acquisition Deep Sky Artifacts
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Menelaos avatar
Menelaos avatar
Hello people and happy new year.
So i am suffering from a big halo in my images. I did a topic on flats recently and it was pointed out to me that i have internal reflections. So i covered the entire inside of my telescope with velour including the inside of the focuser tube. I still get that big halo in my images and i don’t know where it’s coming from. The only part i didn’t cover with velour is the sides of the secondary mirror but it seems unlikely to me that thats what’s causing that huge halo. It doesn’t seem right to cover the secondary with velour. Do you guys have any ideas what might be causing that halo? 
My gear is:
Skywatcher newt 150PDS
TS Maxfield coma corrector 
Canon 6D not modified 
No Filters whatsoever
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Dave Rust avatar
Dave Rust avatar
Hmmm. I don't see a halo. What I see is the light falloff to the corners that every scope has and that Flats eliminate during post-processing. Is there something else I'm missing? Is this falloff AFTER flats have been applied?
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Oscar avatar
Oscar avatar
The black part on the outside of the image is called vignetting and it happens when some light is blocked or is untransmissible in some way, to the outer edges of the sensor; you don't want that (and can be caused if your coma corrector is for APS-C and your camera is Full Frame and it can also be caused if your secondary is too small).

You don't have a halo, the light part is the good part. You need good flats, but I would suggest also using a good processing software such as Pixinsight, or Siril, to do a background extraction.

I had this problem myself even after applying flats, however, I think my flats didn't work because they were all white across the entire image (overexposed), and therefore did not correct anything.
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Quinn Groessl avatar
Quinn Groessl avatar
I would edit in a couple arrows pointing to the halo. I see what you're talking about, but other people don't seem to.
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Sean Mc avatar
Sean Mc avatar
I see it, but sadly I don’t have an answer. I would have said condensation on a cooled camera, but you’re using an unmodded dslr.

maybe take a shot without the coma corrector and see if it makes a difference?
Menelaos avatar
Menelaos avatar
Sean Mc:
I see it, but sadly I don’t have an answer. I would have said condensation on a cooled camera, but you’re using an unmodded dslr.

maybe take a shot without the coma corrector and see if it makes a difference?

Actually before that i was using a Baader MPCC and i didn’t have that halo. However i tried taking a flat with the Maxfield cc removed from the focus tube and the flat was halo free but when i put it in the telescope the halo appears. So I assume its not the coma corrector. Or is it? No idea. 😩 i could continue with the baader MPCC but the maxfield is far more superior in many ways other than the halo.
daywalker avatar
daywalker avatar
A "halo" is an altogether different thing, a term usually used in relation to a large reflection glares around bright stars
I think you mean the dark corner drop off?! ,vignetting as messierman3000and dave rust intimated..
Rüdiger Patommel avatar
Rüdiger Patommel avatar
The cause of vignetting is that a round image is transferred from the optics to a rectangular sensor.
So roughly the squaring of a circle.
That's why the lens hoods on purchased lenses also have this strange shape to compensate for this.
This can only be remedied in post-processing with the “flats” already mentioned.
Menelaos avatar
Menelaos avatar
Quinn Groessl:
I would edit in a couple arrows pointing to the halo. I see what you're talking about, but other people don't seem to.

The question is can this be removed with flats?
Jon Main avatar
Jon Main avatar
Quinn Groessl:
I would edit in a couple arrows pointing to the halo. I see what you're talking about, but other people don't seem to.

The question is can this be removed with flats?

Yes. This is the purpose of flats.

I don't see any halos here. The 'rings' appear to be artifacts of stretching and/or bit depth.
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Jack Groves avatar
Jack Groves avatar
I’ve had this exact issue myself with my Newtonian and I believe the issue is a reflection from your coma corrector protruding close to the main tube. If you are shooting from light pollution, the moon is bright or have any stray light entering the tube this will make it worse. 

I had to adjust the position of my primary to allow me to move my coma corrector outward and that resolved the issue. Unfortunately even flats can’t resolve this issue properly. I think most people here are incorrectly assuming you are talking about the vignetting in your example image. 

I saw your previous comment that you also used the Baader MPCC and there was no halo, that’s most likely because that corrector is shorter than your Maxfield and doesn’t go as near to the tube and catch any stray light. If you are able to adjust your primary to move the corrector out I think that will resolve your issue.
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