Star Trails

Have you wanted to try outer space photography but don’t own or want to invest in a telescope? One very popular and picturesque subject for sky photography is star trails. All you need is a clear night, a good sturdy tripod, a cable release, camera capable of long exposures, and a wide angle lens.

Due to the earth’s rotation, stars appear to rotate about a point directly above the earth’s axis referred to as the celestial axis. In the Northern Hemisphere, the axis of rotation is located by the North Star or Polaris. In the Southern Hemisphere, the point above the celestial axis is located just off the Southern Cross.

The position of the stars relative to the axis of rotation determines both the shape of the star trail and the apparent speed that the star moves through the sky. In the Northern Hemisphere, to record circular star trails, point your camera at the North Star. The location of the North Star in your camera’s viewfinder will determine the center of the circular star trail. If the camera is pointed at stars above the equator, the star trails will appear on film as straight lines. Stars above the equator appear to move faster since they cover a greater distance in the sky than stars close to the North Star.

The technique used to photograph star trails is simple. Since exposures can be very long, it is important to take steps to minimize camera movement during the exposure. Besides a good sturdy tripod and cable release, lock up the mirror prior to exposing the film if your camera has this feature. For long exposures, it is best to use a camera that will not cause battery drain after the shutter is released. The newer electronic cameras, when on the “B” or bulb setting, may drain the camera batteries and prematurely terminate the exposure. Some cameras are equipped with a shutter setting which will not result in battery drain regardless of the exposure time. It is a good idea to check your camera instruction manual to determine camera operation during long exposures. No matter what camera you are using, replace the camera batteries with fresh ones.

I find wide angle lenses to be the best for photographing star trails, since their wide angle of coverage allows you to place an object such as a tree or large rock in the frame for visual interest and still have enough space to record the star trails. A wide angle zoom lens such as a 28 to 70mm lens works very well.

I have had good success using 200 ISO slide film. Be careful when selecting a film to use since some films have a shift in color when exposed for long periods of time. Fuji Provia F is an example of a film that exhibits minimal color shift during long exposures.

The f/stop or aperture will have an effect on the width of the star trail. A wide-open lens will record the star trail with good width but the edges of the star trail will be soft. Stopping the lens down to an aperture of f/5.6 or f/8 will result in a sharper and narrower star trail.

Best results can be achieved if you photograph star trails on a cold clear winter moonless night far away from city lights. Light pollution will result in sky fog on the film reducing contrast. Sky fog is also a function of the exposure time. The longer the exposure, the more sky fog will be recorded on film.

The length of time for the exposure depends upon a number of factors. Star trail length and lens focal length are two factors which determine the exposure time. A shorter focal length lens requires a longer exposure time than a longer focal lens to produce the same length of star trail on film.

The image of the star trails with the tree silhouette, titled "Heaven's Path", was taken with 200 ISO slide film and a 35mm lens set at f/5.6 and exposed for 5 hours. Before dark, I located a tree that would make a nice silhouette, and then selected a location which would record the desired diagonal arc of the star trails. Placing my camera on the tripod, I was then able to position the tree in the viewfinder for my desired composition. Since the selected location was in a remote area, I knew that my tripod would be safe if left unattended until dark. Being careful not to disturb the set-up, I removed the camera from the tripod and left. Returning after dark, I placed the camera on the tripod and used a flashlight to check the composition. When satisfied with the setup, I opened the shutter and left to get some sleep. I made sure to return before the moon rose in the sky.

When we look at the sky on a clear night our eyes see black sky and white stars. In reality, stars emit light that has color. Because of its sensitivity to color, photographic film can record the color of the light emitted by the stars that the human eye can’t see. This is what makes star trail photography so interesting. On the next clear night, grab your camera and head for a location away from the city lights and try star trail photography. You will be amazed at the results!