Photo Stacking and Long Exposures — Part 3: Stack!

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Editor’s Note:  In case you missed them, you may wish to read Part I (Introduction) and Part II (Taking the Photos) before reading this, the last part of David Clark’s Photo Stacking and Long Exposure series.

Welcome to the final installment in my series on photo stacking and long exposures. Throughout the series, I’ve been describing the benefits of photo stacking as a method for taking “star trail” photos: photos which use a long exposure to capture the motion of stars through the sky. In the previous installments, I’ve described the hardware and software you’ll need to take a long exposure star trail photo. I’ve also discussed the details of selecting a location, choosing a night, composing, and actually taking a series of photos. A typical session may involve taking several hundred photos, each of them 30 seconds to 2 minutes long. In this article, I’ll finally be getting down to the nitty gritty details of assembling those photos into a final image.

The theory of photo stacking

I’ll begin by describing the idea of photo stacking, without referring to specific software. I’ll discuss specific software used for stacking photos in the next section. Photo stacking requires a series of photos with a few key features:

  • Each photo must be taken from exactly the same location, pointing in the same direction, with the same focal length.
  • Each photo should have as close to the same exposure as possible. This is somewhat flexible, as the stacking process “evens out” exposure problems — but seriously different exposures can cause trouble.
  • The photos should be taken in sequence during a specific period of time, so that any moving objects (such as stars) move gradually from frame to frame.
If you have a bunch of photos that fit these requirements — such as photos you might have taken after reading the previous articles — then you’re ready to begin. The fundamental idea of photo stacking is to “stack” a series of photos on top of each other, allowing the brightest parts of each image to show through.
Here’s an example based on one of my own star trail images, of one of my favorite subjects: the Quincy Mine. I’ll step through a few stages of stacking to demonstrate the results.  Note that this isn’t the stacking process which I would normally use — it is simply a demonstration to illustrate the concepts.

Begin with the first photo. It should be fully visible. In the image below, you should be able to see a few stars which moved slightly during the 30-second exposure.

The Base Image

Place the second photo directly on top of the first photo. Compare every pixel with the pixel directly beneath it. Always let the brighter pixel show through, whether it is in the top or bottom image. In this way, only bright, moving objects (such as stars) will appear, while darker areas will be the same as the original image. In the photo below, notice how the star trails are slightly longer.

Image 2 layered on top of the base image.

Add the third photo directly above the first two. Again, only allow the pixels which are brighter than those below them to show, and leave all other parts out. Repeat this process until all photos are “stacked”. The image below shows an example of the results after 10 images have been stacked. Notice the long star trails, including some disappearing behind the building, or appearing from behind it.

A stacked image with 10 layers

The result should have “star trails” clearly visible, as the moving stars will be the brightest parts of each image (much brighter than the dark sky). All other portions of the image should be essentially the same as in the first image. You might notice tiny gaps in the star trails in the final image above. This is due to the motion of the stars during the 2 or 3 seconds that the shutter is closed between each exposure. It’s amazing how fast stars move!

Note that I achieved this “stacking” in Gimp, a free photo editor. To do so, I placed each photo on a new layer and set the layer’s mode to “Lighten Only”. This is also possible in Photoshop, with the “Lighten” mode. If you’re into ultra-manual editing, this is the way to do it.

Stacking with software: Startrails

As mentioned in Part 1, you’ll need some sort of image editing software in order to “stack” your photos. This part will describe how to use one of my favorite pieces of stacking software, Startrails. Startrails is a one-trick pony: it makes star trail photos. Looking at other software, you will almost always find additional features designed for astrophotography, HDR, special alignment or exposure adjustments, and so on. Startrails does nothing but stack photos directly on top of each other, producing a final image or video. For this purpose, it does an excellent job. Startrails is free, but it is unfortunately only available for Windows. It also hasn’t been updated in many years — but it does what it does so well, that it really doesn’t need to be updated. Using Startrails is quite easy. After downloading the Startrails package from the link above, open up the program. You’ll see a blank window with a few buttons. Two of the buttons look like a folder being opened — click the left one, which has no black box on it:

The "Open" button in Startrails

This will open a file chooser. Navigate to the folder containing your stackable photos, and select all of them at once. This usually involves shift-clicking to select all of the photos at once. Click the Open button.

Selecting files in Startrails

After clicking Open, a list of all of your files should appear in the top left frame (labeled “Files”). You can click each individual file to preview it. If you want to eliminate any, you can uncheck the checkbox next to them — you may want to do this if a person with a light, or a car, crossed a frame. When you’re happy with your images, click the “Startrails” button to begin stacking:

The Star trails button: click to begin stacking

The program will show you the full progress of stacking, as each image is stacked on top of the previous ones. It’s a lot of fun to watch it happen! Stacking may take a while, depending on the number of frames you have taken. When stacking is done, click the “Save” button (with a disk icon) to save your result. That’s it! You’ve successfully created your first star trail photo.

Stars over the Quincy Mine -- the final result

Don’t forget that most photos will still need more editing, even after the stacking process is finished. To really bring out the stars, use the Levels or Curves command in Gimp or Photoshop. Bring up the hilights, which will cause the bright stars to show up more, while leaving the rest of the dark image unchanged.

Startrails has a few other options as well:

  • Dark frames: the “Open” button with a black box (to the right of the normal Open button) lets you select darkframes. These are images taken with the lens cap on. If your camera adds noise or “halos” to long exposures, they should appear in these dark frames. Startrails will automatically subtract these frames from all other frames, ideally resulting in a cleaner image.
  • Averaging: The “Sigma” button (between Save and Star trails) creates a single image which is the average of all images you took. This can be significantly different from a stacked photo, as each resulting pixel is the average of that pixel in each image. This can be useful if you want to remove moving objects (which appear in only a few images), such as in a cityscape. Used on star trail photos, you would get a blank sky.
  • Star videos: Instead of creating a single star trail photo, you can create a movie of the stars moving! This can be quite interesting, as stars wheel overhead and any other moving objects zip past. An (admittedly non-star-related) example: the Portage Lake Lift Bridge at Sunset.

Wrap up

Star trail photos are beautiful and surprising photos, showing an aspect of nature that humans can’t see with the naked eye. After this series of articles, you should be ready to plan and execute a nighttime star trail photo shoot, and edit the results. The process of setting up and photographing star trails can look technical and complicated, but don’t let it scare you. My final two pieces of advice are: be creative and keep trying! You’ll be amazed at what you can do.

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About Author

David Clark is a mathematician and teacher from the Upper Peninsula of Michigan. Outside of math, David is an improviser, explorer, and photographer. His photo blog documents the industrial heritage and natural beauty of Michigan's Copper Country.

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