This one is HII molecular hydrogen cloud in Perseus arm of our Galaxy. It’s Sharpless number is Sh2-173, and can be found in Cassiopeia close to NGC103 open cluster (at the frame bottom). Pretty faint target, I collected 8 hours with hydrogen alpha filter and the outcome is moderate. This nebula nickname is Phantom Mask.
Subframes were shot on September 25 and 26. All made with Photoline 130mm refractor and Atik383 camera on EQ6 mount under suburban sky. This region is filled with interstellar matter, so the background is not flat actually in any place of this frame. The fact hydrogen is everywhere is not surprising, but since this hydrogen shines in red, it must be excited. So there must be some hot stars nearby, that radiates in ultraviolet band.