Pre-processing and post-processing a deep sky image is very different than processing a daylight image or even night shoots. In particular, pre-processing is a very rigorous process with no room for creativity. At this point, you don’t have an image, you have data.
Here are the steps to produce a Deep-sky image from a stack of “subs”:
Calibrate your subs: dark subtraction and flat field correction
Register (align) your subs
Stack (integrate) your subs. The image generated is called the “master light”.
Crop
Remove gradients
Color balance
Deconvolute your data (sometimes, we won’t cover it)
Delinearize your image
Time to have fun: histogram, curves, contrast, saturation adjustments, etc.
More info on Stacking and Calibration frames