First, we use the Photoshop Levels tool to neutralize the background. The sky should be a nice neutral dark gray, with perhaps a tint of blue. As you could see from the first image, we started with an ugly orange/brown color due to the local light pollution. The Levels tool helps to get the background back to a flat dark gray.
As we go through later processing steps, like Curves, we may have to come back to this step and re-neutralize the background.