If you enjoy taking pictures of the night sky but live in or near a city then light pollution can really ruin your images. The light pollution usually causes an orange glow. Sometimes it will manifest as a bright grey background instead of a deep black night sky. This light pollution causes the stars and nebulae to be washed out or at worst lost in the image.
Light pollution, or sky glow is caused by lighting from buildings and streetlights at night. It is becoming a worsening problem in urban areas. Many people have never seen the Milky Way due to the amount of sky glow.
This step-by-step guide will demonstrate how to remove the effects of light pollution from your images of the night sky and bring out the faint stars. It uses the free to download software GIMP. This software allows you to manipulate pictures and images much like Photoshop.
How to Take Out the Light Pollution
The method outlined here will use the despeckle filter tool found in GIMP. This tool removes the stars from a copy of your original image. This leaves you with just the light polluted background. If any nebulae or particularly bright stars are still evident on this copy you can remove them using the clone tool. To use the clone tool you select an area of background sky that is clear of any nebulae or stars immediately next to the area you want to remove. Select the clear area and then paint over the area you wish to remove that contains the stars. Once you are happy you have removed all the detail from the copy image you then need to go into the layers dialog box and select mode. From the drop-down box select subtract and before your eyes you will see the light polluted background disappear and all the stars will reappear on a lovely inky black night sky.
The following section provides a step-by-step guide on performing the whole process in GIMP. So load up GIMP and follow the instructions below.
1. File > Open and select the image you want to work on.
2. In the Layer dialog box, right click the image and click duplicate layer.
3. Now click on the menu Filters > enhance > despeckle.
4. You should now be able to see the despeckle box. Uncheck adaptive and make sure recursive is not ticked.
5. Set radius to 30, Set Black to -1 and Set White to 256, and click OK.
6. In the layer box make sure you still have the duplicated layer selected, then click on mode and select subtract.
7. Next select Image > Flatten Image from the menu.
8. Now save your image and show your friends and family your great images of the night sky and the constellations.
If after you despeckle the image you can still see nebulae, galaxies, planets, the Milky Way or bright stars in the image you can remove them using the clone tool. This has been outlined above.
Enjoy your new found clear skies images.