Semi-Circular artefacts?

[SJP]

Since starting-out in astrophotography I've noticed at least one semi-circular artefact in the corner of my DSLR images, but usually it's cropped-out.  This week I noticed three in this capture.  I've (over) processed to bring them out, does anyone have any idea where I might look for the culprit?

I'm using an unmodified Canon EOS 6D attached via a T-Mount to a Celestron 6" Newtonian. I usually use a 2" LP filter (which I thought may be causing the artefact (through internal reflection), but for this capture I didn't so I guess that precludes the filter as the cause!

NGC7000 by S.J.P, on Flickr

[Ruud]

This looks like reflections inside the tube (maybe the main tube or the focuser). Flocking might help.

6 and 8" SCTs have this effect from reflections inside the primary baffle tube, especially when a focal reducer is used. Flocking the baffle removes the effect.

[SJP]

Thanks for the suggestions guys.

I'm not convinced the artefacts are entirely 'scope related though, as they're equidistant from the rectangular camera sensor corners.  If they were entirely 'scope reflections I would expect them to be in a square pattern

[Alveprinsen]

Thanks for the suggestions guys.

I'm not convinced the artefacts are entirely 'scope related though, as they're equidistant from the rectangular camera sensor corners.  If they were entirely 'scope reflections I would expect them to be in a square pattern

Those are Ferengi battleships uncloaking... no doubt about it...

[Ruud]

Here and here are two halo examples originating from the inside of shiny SCT baffle tubes. The baffle tubes are actually blackened, but just not dull enough.

See how similar those halos look to what you've imaged? Except that in the SCT the effect can be much stronger due to the length of the baffle. Your focuser drawtube and whatever is behind it is probably much shorter.

[jamespels]

Those are Ferengi battleships uncloaking... no doubt about it...

Either that, or spectacular and surprisingly clear examples of gravitational lensing - must be a couple of black holes we weren't aware of...