Since we'll be working with a fictional photo to make the idea more clear, you may want to check out some examples of the types of images we're talking about producing from our multiple shots: Click here for link.
So, let's get to the how-to. The idea is to take several shots of the sky with short star trails and then combine them together to get the longer trails that you want in your final image. The illustration below should help you visualize what we'll be doing:
- Open all 3 photos as layers in Gimp. (If you are not accustomed to working with layers in Gimp, please click on the menu on this blog and read up on how to use layers under "Image Editing".)
- Open the "Layers" menu if it does not open automatically. (It's in the "Windows" menu under "Dockable Dialogues" and then "Layers").
- Click on the image above the "Background" layer in your layers dialog box.
- At the top of the "Layers" box, click the down arrow to select the layer "Mode".
- Choose "Lighten only".
- Do the same for all the other layers except the "Background" layer.
- That's it!