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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?

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7  
"it is the earth that moves actually" HERESY! –  Greg Feb 2 '11 at 6:05
    
[Citation needed] –  MikeW Mar 1 '13 at 12:17

4 Answers 4

up vote 9 down vote accepted

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."

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Thanks for the script, I'll try it tomorrow! –  tomm89 Feb 2 '11 at 6:57
    
I've never used a script in Gimp. How would I execute it? From the command line, or from the program. Should I save that as a script? –  tomm89 Feb 2 '11 at 23:12
    
IF I remember correctly, run gimp from the command line with "gimp -b -" and then you'll get either a prompt or a blank line; paste all that in. And then it'll just run. Or something. You may need to experiment or look up some GIMP tutorials. Sorry, it's been about 2 years since I've done this. –  drewbenn Feb 3 '11 at 0:32

If you have Photoshop, you can create an image stack. This automatically aligns the layers, so this works hand-held, too. It's a nifty trick if you're shooting a static scene without a tripod and have some extra memory space.

(I wonder if the auto-alignment would be fooled by star trails, as a significant part of the image will be moving in unison.)

Here's the before/after on a set of 12 pics I took:

The effects of averaging several frames in Photoshop

On top, a scaled down version of the whole thing. On the bottom, a 100% crop of the image. Left is one of the 12 original frames, and the right is the averaged picture.

Although the original images were only 1/250s, making a total exposure time of about 1/20s, the wave motion is almost completely averaged away. You can also see some horizontal blurring in the clouds as they move across the sky. This is because the images were taken with about 1s delays in between.

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+1 : Great example –  Guillaume Feb 2 '11 at 11:41
    
That's not what I would want for stars. Because they'd be averaged with the other frames, making the sky empty! –  tomm89 Feb 2 '11 at 23:04
    
@tomm89 Your stars won't be totally gone with stack mode mean, they'll just be darker than in the original images. If you've increased the ISO on your shorter exposures, though, it should be identical to a longer exposure. –  Evan Krall Feb 8 '11 at 2:30
1  
That is a great idea, because the noise produced in a shorter exposure, will be averaged in the stack. Or I think so... –  tomm89 Feb 8 '11 at 3:56
    
Does this work in Photoshop Elements, or only the full Photoshop? Thanks. –  Kartick Vaddadi Dec 15 '13 at 7:16

There's a free Windows application called Startrails that does exactly what you're looking for.

If you've got Photoshop, there are ways to build Photoshop actions to do the same thing. Basically, you combine images in "Lighten Only" layer blending mode.

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What about ''summing" the images? –  tomm89 Feb 2 '11 at 1:15
    
@tomm89: That's what the multiple Photoshop layers are doing basically. With the blend mode, you're saying produce a final image summarising all the lighter pixels as you go up the stack of layers. Here's an article that describes the process in detail: liquidinplastic.com/2008/06/startrails –  Conor Boyd Feb 2 '11 at 1:24
    
I'd download that program but I'm in Linux. –  tomm89 Feb 2 '11 at 1:31
    
If you have software that supports layers, just follow the manual instructions for OSX users in that article, which replicates what StarTrails does. –  Conor Boyd Feb 2 '11 at 2:08
    
I actually have Gimp, but it migh be easier and less memory consuming to just use imagemagick. –  tomm89 Feb 2 '11 at 2:23

From what I understand, is you want to layer all the stars, but just one sky layer. I did something similar once.

Wanted the streaks of lights from passing cars to all combine in one pic.

I layered them in Photoshop and applied the "lighten" blending mode to them. Worked great. Not sure how it'll work in your case, but it's worth a squirt.

enter image description here

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Is that image subject to copyright, by any chance ;) –  Matt Grum Mar 1 '13 at 13:21

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