Raymo, since you work with photoshop one of the best place to learn
how to do is
http://www.imagingdeepsky.com/ and his tutorials movies in
http://www.imagingdeepsky.com/Presentations.html ; from
Ken Crawford.
For mask:
http://www.imagingdeepsky.com/Tutorials/Masks/Masks.htm
note: copy the original photo of tutorials and try to repeat each action of the movies. Try to change the values to understand very well what each movement or action makes with the image.
With those informations I did :
http://astronomia-e-astrofotos.10697...k-140206-A.jpg with ISO 400 5 x 30 sec, 5 x 60 sec, 6 x 240 sec
And
http://astronomia-e-astrofotos.10697...tilayer_k5.jpg with: 11 x 120 s iso 800; 2 x 60 s iso 1600; 4 x 60 s iso 400; 4 x 30 s iso 400
minor time for trapezium, medium time for nebula, large time for border of nebula and periphery