Sometimes, even with the best of equipment, wind or other problems can cause minor trailing of stars in an image. If you separate the stars from the background and process each on its own layer it is possible to correct for minor guiding errors using an additional star layer. First let's start with a crop of a cluster image showing the trailing.
|Now with the stars and background separated on different layers, duplicate the stellar layer. Call the original star layer "A" and the new upper layer "B". Set the combine mode for B to darken. Using the move tool shift the B layer a few pixels in the direction of the star trailing ans presto the stars become noticably rounder. Now merge the star layers and switch the combine mode of the final merged layer to lighten and merge it with the background to create the final image shown below.|
|Special thanks to Renzo Del Rosso of the Canon Digital Astrophoto Yahoo group for this one.|