Vicent,
I was attempting your procedure last night and for some odd reason, when I did the division (i.e. Red/Ha), I only got about six of the brightest stars in the continuum map. The red image has probably got over a 100,000 stars while the H-alpha image as around 20,000-30,000 stars. Although both images could be stretched more, I backed off on the stretching for the preservation of star colors. In addition, the continuum map looked very dark, I really had to stretch it before it looked similar to your background. Working with very wide-field images is proving difficult. Any suggestions would be greatly appreciated.
Thanks,
Wade