GIMP: Adjust curves to blacken sky

A widefield image of a passenger jet passing by the constellation of Orion. Betelgeuse is clearly visible to the upper right of the jet and M42, the Orion Nebula is apparent as a faint smudge below the belt stars. The photograph was taken through cloud cover under heavy light pollution with an unshielded LED street lamp just below the foot of the constellation which gave the whole image a bright washed out appearance with the clouds showing. A single adjustment of the curve in GIMP removed both cloud and light pollution, showing just how much can be achieved in sub-par skies using free software. I chose to leave the aeroplane light trail in as a point of added interest. The naked eye view of Orion from my location shows the main stars of the constellation with the nebula being just visible.