Photographing star colors in star trails
If you progressively change the focus during a long star trail exposure you can see the many different colors of the stars. Start with the stars out of focus and progressively improve the focus so that the last one in the sequence is sharp.
This works well with a camera that allows long time exposures, and it should work with stacking separate exposures the way fbonomi has done.
It takes trial and error to find out how much to shift the focus between exposures. Fifteen minutes is a sufficiently long exposure time.
fbonomi, I hope you have a clear night to try this, so you can post the results here!
