Hi Massimo,
Everything in your image is fine, focus and colour calibration; well done!
IMHO the subs could be longer to show up more of the faint dust wall backing the horse head.
Bearing in mind the OSC chips show up more noise and need longer exposures this would clearly be a big step forward for your picture.
On the other hand you have calibrate with more darks and bias's to suppress the amount of noise..
Best and clear skies!