Post Processing for Star Trails

Prepping Images for Stacking

Once you've made your exposures, you're ready to post process. If you're using RAW source files or you wish to aesthetically adjust your entire sequence of images, you will need to first import the images into Adobe Lightroom or an equivalent image editor. The key at this point is having the ability to apply adjustments easily across all images. Work on the first image in the sequence. Once you have that image where you want it, copy the image adjustment settings and then apply them to the rest of the images in the sequence. Now export those images out as JPEGs or TIFFs into a folder using an ordered number sequence (i.e. img-001.jpg, img-002.jpg etc.) and you're ready to stack.

Star Trails Software

Star Stax Logo

The two well regarded stacking software apps are StarStax (Mac or Windows) and Startrails.exe (Windows only). They're both free but donations are encouraged. As a Mac devotee (10 years on Apple's Web Marketing team will do that), I had one choice: Star Stax. Fortunately it's available for both platforms and so the following discussion applies to both Mac and PC users.

Working with StarStax

StarStax is very easy to use. Open the app and load the images. They can be dragged and dropped into the app. If you properly numbered your files in sequence, they will load in proper order with the first image at the top.

Star Stax: Images Loaded
Images dropped into Star Stax

With the images loaded, you're ready to blend. On the right side, you should see a preference panel with a "Blending" tab. Choose the Blending Tab and then select the "Blending Mode" options — the two you're likely most interested in are "Lighten" and "Gap Filling". In the evolution of the application, "Lighten" came first. The Lighten mode retains the brightest pixel from the image stack when creating the star trails. I'd start with the Lighten option first and review the results. The "Start Processing" button is 4th in from the upper left (see pic below).

Star Stax: Processing Images
Images in the middle of processing

Once the Lighten stacking is done, you can review the image up to 1:1 size. Because each image of a star is actually a hollowed circle, the resulting star trails will likely contain small gaps. Gap Filling is a feature added in version 0.52 that, like its name says, helps fill the gaps. To try Gap Filling you'll have to restack the images. Once processing completes, Gap Filling works on the final stacked image with options for adjusting both the Threshold and Amount in the side Panel. The tool is nondestructive so you can try different settings and decide if you like the result or not.

Star Stax: Gap Filling Options
Gap Filling can be controlled by Threshold and Amount

Once you have a stacked image you're happy with, you need to save it. There's a small gotcha here. StarStax automatically saves files as JPGs regardless of the original images you loaded. If you loaded a sequence of TIFF images and want the final output image to be a TIFF so you can make aesthetic adjustments to it, then you need to make sure you name the file with the suffix .tiff when saving.

And, that's it. You're done stacking!

Video Options

StarStax provides the option to save each cumulative stacked images which can then be combined to create a star trails video as in this YouTube example:

There are a number of apps that can accomplish this. On the Mac side I used some donationware, Time Lapse Assembler which combines JPG images to create a QuickTime compatible movie that can be edited in iMovie or other movie software.