I have conducted the first narrowband imaging test with the Bresser 152mm, f5, 760mm fl, modified Achro. I still have not received the Field Flattener, Bintel was out of stock and I had to order it elsewhere.
The test image on M8 Lagoon Nebula, is made up of 10 x 300" Ha subs stacked in DSS with Darks and Bias Masters.
Final full size image is posted (jpg) on Astrobin here:
http://www.astrobin.com/full/253809/0/
Details page here:
http://www.astrobin.com/253809/
Minimal processing as this was a test image, mainly to check scope performance and narrowband focus, and field curvature.
Thumbnail below is here as a courtesy, it is far to small to tell much, sugges you examine the full size one on the link above.