astrophotography - How can I simulate a long exposure photo using a set of shorter exposure photos?

I'm shooting towards the sky to capture the stars. The exposure time is 15 seconds, so I can see the stars still, without a trace. These photos are taken continuously one after the other, because I want to make a time-lapse video showing the "movement" of the sky (it is the earth that moves actually).

For that, everything's fine. But I'd also like to do one other thing. If instead of taking all those photos, I would take just one with a exposure time that equals the sums of all those photos photos together (15 sec times the quantity of photos), I would see the trace the stars left in the sky.

Is there anyway to "create" that photo, from all the "short" exposition ones?

Answer

You can do it with a script for The GIMP. I did it a couple years ago, and got pretty good results. Remember to keep the time between exposures as short as possible, otherwise you will get visible gaps in the trails. That's why it's best to take a single dark frame at the end, and subtract that frame from the result (I had intended to incorporate that into the script, but never got around to it).

My notes for the script:

Combined with renaming the first to base.JPG, "gimp -b -" with
(let* ((filelist (cadr (file-glob "IMG*.JPG" 1)))
      (img (car (gimp-file-load RUN-NONINTERACTIVE "base.JPG" "base.JPG"))))
  (while (not (null? filelist))
    (let* ((filename (car filelist))


  (layer (car (gimp-file-load-layer RUN-NONINTERACTIVE img filename)))
  )
      (gimp-image-add-layer img layer 0)
      (gimp-layer-set-mode layer LIGHTEN-ONLY-MODE)
      (gimp-image-merge-visible-layers img CLIP-TO-IMAGE)
      )
    (set! filelist (cdr filelist))
    )
  (gimp-file-save RUN-NONINTERACTIVE img (car (gimp-image-flatten img)) "test2.jpg" "test2.jpg")

)

For subtracting the dark frame, my notes say, "I opened this as a layer on the composite image (the result of my gimp script), and set the dark layer's mode to Difference."