Back in the days of film cameras, we would take a photo of star-trails by taking a single long time exposure of a dark sky. As long as the sky was dark enough, then we would make the exposure as long as possible. Now in the days of digital cameras it is preferable to take a large number of shorter exposures and stack them all together to form a single image.

The way a star-trail stacking algorithm works is quite simple. First we select one image as the reference and copy that entire image into an empty stack. Then we take the next image and on a pixel by pixel basis, compare the intensities. If the intensity of a pixel on the new image is greater than the intensity of a pixel on the stack image, then swap that pixel to the stack image. We may also include a threshold to exclude the darker areas. We do this comparison for every pixel, then move on to the next image and repeat the process.

Software can be found on the internet to do this, but I have also incorporated this feature into the StarStack software. 

Star-trail stack Star-trail stack Star-trail stack

The series of photos above show the star-trail tracks during processing, as the trails get successively longer. The final image is shown below.

Star-trail stack

Star-trails observed from Boyanup in Western Australia.