Originally posted by micromacro Wow, it is a problem then.((( Is it for any filter, or it depends on filter quality? I'm totally novice, have no idea.
It is a quality issue. I suspect the filter doesn't have an anti reflection coating on the lens side of the glass. The ultra wide angle lens is going to make this sort of thing look worse. It's just the nature of the beast. I'm kind of in awe of the 800 second exposure. That harkens me back to my view camera days when 15 minute exposures were quite normal for me. Funny reflection artifacts can pop up in those types of exposures too.
You were concerned about vignetting, and gatorguy suggested a breakthrough filter might help in that area. i suspect a filter from that brand would help with the reflections as well, but they are very expensive.
There is a company selling filters that mount inside the camera body. I don't recall the name offhand (embarassing, since I posted it here a few days ago). That might be something to look into for use with the 12mm lens.
Also, I just noticed that you had stacked the filters. This might be the culprit. Maybe try some pictures with just one filter on and see if that cures things. If that does, try changing the order of the stack and see if you can make the reflections go away.
Stacking filters is one of those fraught with peril things.
I frequently do it, but I'm not using lenses as wide as you are, and I am generally using filters from Zeiss or Breakthrough that are multicoated on both surfaces.
The things that I see that are giving you grief are the very small aperture, the very long exposures, and, of course, the filter stack.
See if you can reproduce it with just one filter. Your exposure time will be shortened, but I think if it's going to show up, it will probably show up in an exposure of a few minutes the same way it showed up in your 13 minute exposure. The idea is to pin down exactly what caused the problem so as to eliminate it. When doing your sherlocking, only change one lens parameter at a time. Try both filters, but open the aperture a few stops (obviously this will necessitate a shorter exposure, that can't be helped). Remove one filter, but leave the lens stopped down, try with just the other filter, adjusting the exposure time, not the aperture.
Try reversing the filter stack, keeping the aperture small. Try the reversed stack with the aperture opened up a bit.
You won't have to work to hard to figure it out, and then you can eliminate the issue.
Good luck, and don't be afraid to PM me if you want.