Fixing Slight Star Trails

Ever have a shot that came out great except for a very slight star trail? I know, it never happened to me either, but if it does here is how to fix it. Here I'm using Images Plus which has a great tool for doing this processing automatically, but this layered approach should work in any layer capable processor including Photoshop. First have a look at the central crop of a M44 shot taken from my driveway. Here I bumped the north, south, east and west buttons to cause a slight diagonal star trail.

The idea here is to use a masked darken combine with a shifted version of the image to remove the trailing.

  1. First duplicate the image (A) as a second layer (B).
  2. Make a star mask to limit the effect to the stars only (C).
  3. Slightly blur the mask to blend the effect into the background.
  4. Now change the blend mode of the top layer to darken.
  5. Shift the top layer a few pixels in the direction of the trail to remove it (D).
  6. Apply the mask to the top layer to blend the effect into the final image (E)

The darken combine covers over some of the bright trailed stars in (B) with the darker area from the shifted top layer (A) to make nice round stars. The picture below will blink the trailed and fixed image so you can see the effect.