I will be using Adobe Lightroom, but this technique should be possible in most image editing programs. The image on the camera looked like this.
Set the white-balance to daylight. Some stars are hotter (and look blueish) than our sun, others are cooler (and reddish). Daylight is a good starting point and can be fine tuned later. Now is also a good time to increase saturation.
Scroll down and enable lens corrections. This will correct for vignetting which will make an upcoming step easier.
Zoom right in on the image and change noise reduction settings. Try to find a balance between cleaning up the image, and not losing faint stars.
Crop the image to show only sky and no foreground images. Also crop out clouds if they are present. This is just temporary and will help us get relevant information from the histogram.
Set the contrast to 100% and push the exposure until the lower corner of the histogram (circled here) is touching the right.
This next step is what actually removes the light pollution.
Select the coloured point curves (red, green and blue) and drag the black-point to where it touches the left of the histogram. Do this for each of the red, green and blue colours.
Space is dark, there are parts of the sky that should have no light at all. The darkest pixels (left hand side of the histogram) are lit, purely by light pollution. This lets us know how much to remove. Light pollution will add light to all pixels. So in the above steps, we are subtracting that light again. Any remaining light should be from celestial objects.
Now we can undo the cropping of the image.
In this example, there was more light pollution/haze in the lower portion of the image. A simple way to correct for this is to use the gradient tool and slightly lower exposure for that portion of the image. Use an appropriately wide transition so it is blended well. I have also used the temperature and tint sliders to balance the colours to match.
Exit the linear gradient settings and use the ‘Effects’ sliders to emphasise or de-emphasise the texture of the main section of the milky way.
Radial gradients can be used to selectively darken/brighten portions of the image. Tiny changes can be made to colour too.
Other fine adjustments can be made specific to your tastes and the requirements of the image you are editing.
The foreground has been completely lost. The easiest way to fix this is to save this version of the image, create a second version and edit just for the foreground. Don’t forget to apply ‘lens corrections’ to that version too. Else the images won’t line up. Also keep it fairly dark, else it will look out of place.
Images and content copyright belongs to Sam Wighton.
Photos taken at Bombo Headland Quarry Geological Site, Australia
Contact me at firstname.lastname@example.org for enquiries.