Method 1: Using the Statistics Script
Now that you have the basic concepts of star trail exposures under your belt, it's time to get started with the mechanics of stacking. A number of standalone programs can be used to stack star trails, but Photoshop CC is probably the best bet for creating high-resolution images.
One of the ways I like to stack star trails in Photoshop CC is to use the Statistics script. In previous versions of Photoshop, you needed the Extended Version to run Statistics, but Photoshop CC makes Statistics available to all Photoshop users.
With Photoshop running, choose File > Scripts > Statistics (see Figure 4) to start the Statistics application.
Figure 4 The Statistics script is loaded from the Photoshop Scripts menu.
The Statistics application is fairly simple (see Figure 5). Using the Browse button, choose the files that you want to stack. Then use the Choose Stack Mode drop-down list to select the stacking method that the Statistics program will use to combine the selected images (see Figure 6).
Figure 5 The Statistics application allows you to select the files to stack and the statistical method that the application will use to combine the files.
You might be confused by the combination options in the Choose Stack Mode drop-down list (see Figure 6). It helps to understand that the Statistics application blends the images in the stack pixel by pixel, using the statistical method that you chose. The most useful statistical method for stacking star trails is Maximum, which chooses the brightest pixel at every point of the blend. By contrast, Minimum chooses the darkest pixel at every possible point.
Figure 6 Some of the statistical methods may seem pretty obscure, but usually you only need Maximum.
After you've selected the files that you want to combine and the statistical method for combining them, click OK to close the Image Statistics dialog box and start combining the files. Depending on the number of files and your computing power, this process may take a while. (Sometimes I have time to go out for lunch!)
When the Statistics application finishes the stacking process, Photoshop displays a Smart Object in the Layers panel (like the one in Figure 7), showing the combined images. Because this is a Smart Object, you can experiment with changing the stacking mode (method of statistical combination) after the fact, without reloading the image files.
Once you've selected the stacking mode that makes your star trails look their best, you should merge down the layers within the Smart Object before finishing the star trail image. If you don't merge down the layers, this image can be hugely resource-consumptive for your computer, slowing your image editing to a crawl.
Figure 7 Using the Statistics application, I combined this stack of 10 files using the Maximum stack mode. The stacked files appear as a Smart Object in the Layers panel in Photoshop CC.