When people walk into my booth at art shows, the images that usually catch their eye first are the ones of the night sky: Milky Way, northern lights, and star trails. And of those three, the star trails get the most questions. Kids and adults alike ask me, “What causes these circles?”, or “How do you make this happen, by spinning the camera?” And the more skeptical types will ask accusingly, “Did you add this in with Photoshop?”

The fact of the matter is, like with any of my shots, the real work is done on-location, by knowing my subject, knowing my camera, and having a solid grasp of the technique. Many of you who are photographers may have already tried this technique with varying degrees of success, but perhaps you are just getting into night photography, and are looking for a way to create a new type of thought-provoking image that will surely get lots of attention in your portfolio.

For those of you who don’t know, trailing stars in images like this one are caused by the rotation of the earth over time, as the camera stays fixed on a tripod, in a stationary position relative to the earth. The camera’s shutter is left in the open position for an extended period, gathering light the whole time, and as the earth turns, the stars in the sky appear to “move”, even though their actual position in the universe isn’t changing. The moving stars leave trails or streaks across your image, much like the headlights of moving cars or the wing lights of airplanes leave trails in night shots of the city.

Any time you make a shot of the night sky, whether it’s long or relatively short, if there is no light being cast on the ground, either by the moon, or another ambient light source, you will get a dark, silhouetted foreground. This is why I prefer to make my star trail exposures on nights with a crescent moon, or at most a half moon. This type of moon provides enough light to illuminate the foreground, while not completely washing out the sky. If you shoot on a night with too full of a moon, the ground will be nice and bright, but you will not see many stars in the sky.

It’s also essential to avoid areas with light pollution from cities, as this washes out detail in the sky. This is why I make my star trails in the middle of the desert, in high altitude areas, or in remote coastal areas facing nothing but ocean.

A star trail image, then, is simply a long exposure photograph of “moving” stars. Any type of extended exposure creates digital noise, or grain, in your image, which is a product of the sensor generating heat from being active for so long. This noise can generally be reduced in post-processing, within certain limits. I’ve found that with cameras produced in recent years, I can make an exposure at low ISO for up to 10 to 25 minutes without generating too much noise in my image.

Our technique for a basic star trail:

• On a night with a crescent moon. Set up your camera on the tripod with the remote release attached.

• Focus your camera to the infinity distance. Practice doing this in daylight, so you can easily do it in the dark.

• Set your camera’s ISO to a low number such as 100 or 200.

• Set your aperture to the widest value. Hopefully you have a lens that can open at least as wide as f/2.8

• Set your shutter speed to the “bulb” setting. This means that the camera’s shutter will stay open for as long as the shutter button (or remote switch) is pressed.

• Compose your image to include the sky, and as much foreground as you would like in your image, and lock your tripod knobs securely

• Press your remote switch, and lock it open for the desired amount of time. Keep time using an external timer, or use the built-in timer in your switch if it has one.

• Expose for between 10 to 25 minutes, depending on your camera’s long exposure performance, and then finish the shot.

• You should hopefully see an image that has trailing stars in the sky and an illuminated foreground.

So what happens when you want to make an exposure that is longer than that, like an hour long, or even longer, without degrading your image too much? The solution is to shoot a series of shorter exposures and blend them together using software.

To make your basic exposures, just use the same technique as above, but set your programmable remote release to shoot a series of exposures that are 10-25 minutes long, until the desired total amount of time has been reached. Make sure that the interval between shots is as close to zero as possible, so that you will not have a gap between your individual star trails in your final shot. I know many photographers who insist on doing a series of 30-second shots until the desired total time is reached, but for an hour-long star trail shot, this creates an overwhelming number of images that will clutter your memory cards and drives, and take a long, long time to process. When you’re still just starting out with star trails, keep it simple for yourself and do longer, but fewer, exposures.

Blend your shots using software. I have used StarStax for many of my star trail images and find it very easy to use. You could also use Photoshop. Put each individual star trail shot into the same Photoshop file as layers, and blend them using the “Lighten” blending mode. Do your final edits, and then, you’re done!