If the tracking of a mount is not perfect and long exposures are used, the stars in an image will appear as short ‘sausages’ rather than circular images. This is termed ‘Star Trailing’.
Providing that is the stellar streaks are not too long, this is easy to correct in Adobe PhotoShop or Infinity Photo (see article in the Digest about these two editing programs).
The image is duplicated to give two layers and the blending mode is set to ‘Darken’. One should zoom into a small region (as seen above) of the image to see the trailing in detail. If the ‘Move Tool’ is selected and the mouse clicked on the top layer (important), then, as the upper layer is moved over the lower, using the four arrow keys with each press moving the upper layer by one pixel, it will be seen that, with the appropriate movement, the trailing is reduced to leave circular stellar images.
These diagrams, produced by my colleague Andy Verwer, show how the trailing is reduced.
The final result will, hopefully, then look like this.