Bright band at bottom of images with Canon 6D

Pleiades Astrophoto PixInsight DSLR Help Needed Troubleshooting
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Cameron Stout avatar
Cameron Stout avatar

  • Rosette Nebula

  • Camera: Canon EOS 6D

  • Scope: Askar FRA300.

  • Guide Camera ASI120mm

  • Guide Scope SVBONY SV165

  • Dithered 30 pixels every 5 frames

  • Captured using ASIAir.

  • 73 thirty second exposures at ISO 1600.

  • Stacked and processed the fits files from the ASIAir in PixInsight.

  • My location is a bortle 6/7 neighborhood.

I am trying to figure out what is causing this bright band at the bottom of my image. I can’t seem to process it out, except if I crop it, but I’d rather not have to crop out that much of my image every time I use this camera and scope set up.

This also happens if I use a camera lens with this camera, and it does not happen if I use my ASI585MC with my FRA300, so I know it is specific to the 6D camera. The camera lens I used was an 85mm lens set at f2.3.

The image I attached has been calibrated with darks, flats, and dark flats, but this band shows up even if I don’t use calibration frames.

Have any of you experienced this before? If so how did you deal with it?

If I am missing any information please let me know. Thank you for reading and clear skies!

📷 rosette with light band under 5mb.jpgrosette with light band under 5mb.jpg

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Tony Gondola avatar
Tony Gondola avatar

Probably a stacking artifact, not uncommon at all. You are moving the frame around with drizzling and doing it by 30 pixels is a lot. A much smaller movement, 5 to 10 pixels, would suffice.

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Cameron Stout avatar
Cameron Stout avatar

Tony Gondola · Mar 8, 2026, 05:19 PM

Probably a stacking artifact, not uncommon at all. You are moving the frame around with drizzling and doing it by 30 pixels is a lot. A much smaller movement, 5 to 10 pixels, would suffice.

This happens even if I do not dither. When I used my 85mm lens I didn’t guide or dither and the band was still there.

Also, it happens regardless of how many sub exposures I take. I only did a short session this time, but if I have 3-4 hours worth of data the artifact is still just as bright.

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Miesilmannimea avatar
Miesilmannimea avatar

The issue is well known – it is the shadow of the mirror mechanism, or even the mirror itself, cast onto the converter. The shadow is very subtle – it is not visible in normal photography, but in astrophotography, where you are pulling out a weak signal, it is already visible. Check the mirror mechanism – does the mirror definitely rise all the way to the top, or could it go a fraction of a millimeter higher?

If the high of this strip is perfectly repeatable, a flat will fix it :)

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Cameron Stout avatar
Cameron Stout avatar

Miesilmannimea · Mar 8, 2026, 05:53 PM

The issue is well known – it is the shadow of the mirror mechanism, or even the mirror itself, cast onto the converter. The shadow is very subtle – it is not visible in normal photography, but in astrophotography, where you are pulling out a weak signal, it is already visible. Check the mirror mechanism – does the mirror definitely rise all the way to the top, or could it go a fraction of a millimeter higher?

If the high of this strip is perfectly repeatable, a flat will fix it :)

Here are some pictures of the mirror box and sensor. The mirror snaps up to those small foam pads and I don’t really see how it could go any higher.

Also I do use flats. I use 50. The band doesn’t calibrate out. Unless my flat taking method has a flaw. I put clean white cloth tight over the objective, and I put my ipad on top of that and use the auto exposure feature in the ASIAir. That method works with my ASI585MC, so not sure why it would not also work with my Canon 6D.

Canon eos 6D sensor - Imgur.jpgCanon eos 6D sensor - Imgur(3).jpgCanon eos 6D sensor - Imgur(2).jpgCanon eos 6D sensor - Imgur(1).jpg

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andrea tasselli avatar
andrea tasselli avatar
It is as Tony said. If there is no or little signal this sort of things happen. Especially if you use scripts.
Tony Gondola avatar
Tony Gondola avatar

Tony Gondola · Mar 8, 2026, 05:19 PM

Probably a stacking artifact, not uncommon at all. You are moving the frame around with drizzling and doing it by 30 pixels is a lot. A much smaller movement, 5 to 10 pixels, would suffice.

Well that eliminates that. It was just the first thing I think of coming from the Astro camera world. As your flats didn’t fix it, it might be the only option is to crop it out. You’ve got a huge FOV so IMO you really aren’t loosing much.

Cameron Stout avatar
Cameron Stout avatar

I guess in the end it isn’t that big of a deal. If I crop it all away and maintain the same aspect ratio I only lose about 12% of the image. Still, it’s a shame and I’d rather keep those pixels 😔

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