Star trail photos can be incredibly compelling and while they take time to produce, they're probably not as difficult as you might think they are. In fact, there are two methods that you can use to capture the night sky with the stars streaking across it: a single long exposure or what effectively amounts to a time-lapse composited into a single image. This is our guide to shooting star trails.
Location
Light pollution can be a pain when you're attempting to shoot a star trail photo. If you're not able to see the stars, your camera won't be able to, either. Should you live in a city, this means looking for a location that's suitably isolated to give you a view of the sky, but isn't so isolated that you make yourself vulnerable. And if you don't live in a city, you still need to be somewhere accessible.
You also want to think about your scene. You might find that having something of interest in the foreground of your shot will improve it. Barns, dilapidated or otherwise, obelisks, and rock formations are all good starting points.
By finding Polaris and focusing on that, you'll produce a circular star trail; point your camera somewhere else in the sky and your trails will be more linear.
Timing
The best time of year for shooting star trails is definitely dependent on personal preference. How long you can manage safely in the cold is probably your primary concern. But you do need to be shooting on a cloudless night with no or little moon.
Setting up
Once you've decided on your location and set up camp with warm clothes, thick boots, and a thermos flask, it's time to set up your camera.
Camera
Set your camera on its tripod; place it in manual mode and switch the lens, preferably a wide-angle one to get as much sky in the shot as possible, to manual focus, too. Frame your shot—ideally with something of interest in the foreground—with the lens focused to infinity.
When it comes to exposure, you need to be in bulb mode, the aperture should be as wide as possible, and try ISO 1,600.
Take a test shot with a exposure time of 30 seconds; if the stars are bright and clear, you're ready to go. If it looks a little dark, adjust the exposure time until you're happy.
Camera trigger
If you're using an intervalometer, you need to set it to record as you would for a time-lapse video, using the exposure time you tested for.
If you're using Triggertrap Mobile with its star trail mode, set the exposure time that you established in testing with a two second interval between frames, and select the number of frames you want to take. You can choose a huge number of frames and stop after half an hour or 45 minutes of shooting if you're not certain how long you need to be out there for.
Hit go!
That should be about it. Hit go and wait for your camera and the universe to work its magic. Do remember to keep warm and safe!
Compilation
When you've accumulated all the images that you need, it's time to compile them into a single image with the help of some software. If you have Photoshop, that's perfect. If you don't, there are other options including the star-trail-specific StarStax.
A stack of images
Transfer your images from the memory card to your harddrive, keeping them in a single folder with their original file numbers. Whichever programme you use, this is important to ensure that the images don't get out-of-synch. The rest of this tutorial uses Photoshop to assemble your star trail shot, but you should be able to extrapolate the process to any other programme.
Import your images
Open Photoshop and import your star trail images using File –> Scripts –> Load Files into Stack. Select your folder of star trails photos, highlight all of the photos, and then select Open followed by OK.
Blending
When all of your photos have made their way into Photoshop, select all of them in the Layers panel, and then in Blending Mode select Lighten. Tah-da! You should have a star trail composite.
You can make adjustments to individual layers if you want, but otherwise, you're done and it's a case of saving. (You might want to save an unflattened PSD file and a flattened JPEG version.)
Much of this, including all the images, is based on the fantastic How to capture a star trail tutorial found on Triggertrap's How-To microsite, and it's reproduced with permission. Triggertrap How-To is full of great content for making the most of your camera. You should take a look.