Horsehead and Flame Nebulas

This is a picture of the Horsehead area made with my modified Canon 1100D piggybacked onto the C8. I decided to desaturate to get a b/w image.
At the end of the processing a dark narrow band appeared on the photo. I have erased it with the retouch tool... I must understand what it was.

Horsehead and flame nebulae

Modded Canon EOS 1100D piggybacked on a GE mount (no guide). 300mm macro lens, f/5.6, ISO 
800, 90 x 60 sec RAW frames , 30 dark frames, processed with DSS and Gimp.

Here the color version: