NGC 7023 (Iris Nebula or Caldwell 4)

I've tried processing this many times and I was never happy with it. Different combinations of stretching techniques, separating stars and background, etc. Separating the stars left too many star halos in the background, so I chose to just process it without star separation.

I should have framed it better by rotating the camera 90°, but...

Acquisition:

Over 7 nights, with slight Moon and smoke/haze
Bortle 7 skies
Nikon D5600 camera (unmodified)
ISO 200
Astro-Tech AT102ED refractor, 714mm focal length, f/7
HoTech 2" SCA field flattener (1X)
iOptron CEM26 mount
30s x 920 frames (7h 40m)

Processing:

Siril 1.2.4:

Crop to 5000x3500px
Photometric color calibration
Star desaturation
GraXpert denoise
Background extraction
Modified arcsinh stretch (4x)
Black point
Generalized hyperbolic stretch (2x)

RawTherapee 5.9:

L*a*b chrominance/luminance adjustment (to reduce chrominance noise)
Luminance noise reduction

GIMP 2.10:

Luminance curve
Saturation
Green level (slight midpoint reduction)