DBE puts object color in the background even when no samples are on it

[Nocturnal]

Hi,

I thought I understood DBE pretty well but now I'm seeing something that I find a little strange. Last night I was processing M31 and wanted to get rid of some glow in one of the corners. I let DBE put samples on the image and 80% of them were in the right place, around M31 and the other galaxies. I moved the remaining ones so they were 'in space'. To my surprise the extracted image had a clear oval 'glow' in the middle where M31 is. I did not expect that. I thought it would extrapolate between the measuring points around the galaxy which would basically make it the same as the back ground.

Just now I'm working on a Rosette image. Now the extracted image has a red glow in the middle I don't want to subtract any red data from my image so I'm going to discard it.

What am I not understanding or doing wrong?

Thanks!

[Nocturnal]

Correction on the above. I moved samples farther away from the nebula and now the extracted image looks more like what I expected. I'll have to play with M31 some more.