Merging the whole set of layers works of course (thanks for the ALT-Merging, forgot that one meanwhile) Saving the nebula's mask and applying to the merged render did help a bit - the opacity in the lower half (that yellow spot) is vanishing, though. I am really missing the WhatYouSeeIsWhatYouGet-Effect when merging parts of the layers. Always having the problem is a bit of a bad tradeoff in my opinion. Regards, Volker Harun