Four days ago, I wrote about a star trails photo I shot (on a recently urban exploration trip) near the ferris wheel of the abandoned Six Flags New Orleans amusement park. Immediately after taking that photo, I shot 20 extra exposures, lasting 1 minute each, of the same scene. My intention was to combine the 20 exposures to create a single long exposure photo using a program called StarStaX. The photo can be seen below.
10-22mm, 20 minutes (stacked exposures), f/5.6, ISO 100
The primary benefit of stacking multiple photos to create a single exposure is the clarity you can achieve in the photo. Particular in this situation, the skies were heavy with the haze of 42ºC humidity. Six Flags is located only 20 kilometers (12 miles) east of the downtown core (the famous French Quarter), so the light pollution was also very intense. Stacking allowed me to cut through the haze, so to speak, and achieve a clarity of star trails not seen in my other long exposure shot of the ferris wheel.
As mentioned above, I used a program called StarStaX (written by Markus Enzweiler).
"StarStaX is a fast multi-platform image stacking and blending software, which is developed primarily for Star Trail Photography. Besides star trails, it can be of great use in other settings. StarStaX allows to merge a series of photos into a single image, where the relative motion of the stars creates structures looking like star trails."
One critical issue with stacking photos like this is to minimize the amount of time between exposures. I used my remote sensor to trigger each exposure, which caused a 1-2 second gap between each one. That may seem insignificant, but if you zoom in on the photo, very slight gaps can be seen in each trail. This isn't something most people would see, but it's still a consideration when using this process/technique for star trails.
Overall, I'm quite pleased with the results of my first photo stacking attempt and will definitely try it again!